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Articles about stars and galaxies from Wikipedia and the Alvarez hypothesis.
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PDF generated using the open source mwlib toolkit. See http://code.pediapress.com/ for more information. PDF generated at: Fri, 25 Jan 2013 01:31:25 UTC Space Traveler 1st WikiBook!
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Page 1: Space Traveler

PDF generated using the open source mwlib toolkit. See http://code.pediapress.com/ for more information.PDF generated at: Fri, 25 Jan 2013 01:31:25 UTC

Space Traveler1st WikiBook!

Page 2: Space Traveler

ContentsArticles

Centaurus A 1Andromeda Galaxy 7Pleiades 20Orion (constellation) 26Orion Nebula 37Eta Carinae 47Comet Hale–Bopp 55Alvarez hypothesis 64

ReferencesArticle Sources and Contributors 67Image Sources, Licenses and Contributors 69

Article LicensesLicense 71

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Centaurus A 1

Centaurus A

Centaurus A

Centaurus A (NGC 5128)Observation data (J2000 epoch)

Constellation Centaurus

Right ascension 13h 25m 27.6s[1]

Declination -43° 01′ 09″[1]

Redshift 547 ± 5 km/s[1]

Distance 10-16 Mly (3-5 Mpc)[2][1][3][4][5]

Type S0 pec[1] or Ep[6]

Apparent dimensions (V) 25′.7 × 20′.0[1]

Apparent magnitude (V) 6.84[7][8]

Notable features Unusual dust lane

Other designations

NGC 5128,[1] Arp 153,[1] PGC 46957,[1] 4U 1322-42,[9] Caldwell 77

Centaurus A (also known as NGC 5128 or Caldwell 77) is a prominent galaxy in the constellation of Centaurus.There is considerable debate in the literature regarding the galaxy's fundamental properties such as its Hubble type(lenticular galaxy or a giant elliptical galaxy)[6] and distance (10-16 million light-years).[2][1][3][4][5] NGC 5128 isone of the closest radio galaxies to Earth, so its active galactic nucleus has been extensively studied by professionalastronomers.[10] The galaxy is also the fifth brightest in the sky,[10] making it an ideal amateur astronomy target,[11]

although the galaxy is only visible from low northern latitudes and the southern hemisphere.The center of the galaxy contains a supermassive black hole weighing in at 55 million solar masses,[12] which ejectsa relativistic jet that is responsible for emissions in the X-ray and radio wavelengths. By taking radio observations ofthe jet separated by a decade, astronomers have determined that the inner parts of the jet are moving at about onehalf of the speed of light. X-rays are produced farther out as the jet collides with surrounding gases resulting in thecreation of highly energetic particles. The radio jets of Centaurus A are over a million light years long.[13]

Like other starburst galaxies, a collision is suspected to be responsible for the intense burst of star formation. SpitzerSpace Telescope studies have confirmed that Centaurus A is colliding with and devouring a smaller spiral galaxy.

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Centaurus A 2

MorphologyCentaurus A may be described as having a peculiar morphology. As seen from Earth, the galaxy looks like alenticular or elliptical galaxy with a superimposed dust lane.[14] The peculiarity of this galaxy was first identified in1847 by John Herschel, and the galaxy was included in the Atlas of Peculiar Galaxies (published in 1966) as one ofthe best examples of a "disturbed" galaxy with dust absorption.[15] The galaxy's strange morphology is generallyrecognized as the result of a merger between two smaller galaxies.[16]

The bulge of this galaxy is composed mainly of evolved red stars.[14] The dusty disk, however, has been the site ofmore recent star formation;[10] over 100 star formation regions have been identified in the disk.[17]

SupernovaeOne supernova has been detected in Centaurus A.[18] The supernova, named SN 1986G, was discovered within thedark dust lane of the galaxy by R. Evans in 1986.[19] It was later identified as a type Ia supernova,[20] which formswhen a white dwarf's mass grows large enough to ignite carbon fusion in its center, touching off a runawaythermonuclear reaction, as may happen when a white dwarf in a binary star system strips gas away from the otherstar. SN 1986G was used to demonstrate that the spectra of type Ia supernovae are not all identical, and that type Iasupernovae may differ in the way that they change in brightness over time.[20]

DistanceDistance estimates to NGC 5128 established since the 1980s typically range between 3-5 Mpc.[2][1][3][4][5][21]

Classical Cepheids discovered in the heavily-obscured dust lane of NGC 5128 yield a distance between ~3-3.5 Mpc,depending on the nature of the extinction law adopted and other considerations.[3][4] Mira variables[21] and Type IICepheids[3][4] were also discovered in NGC 5128, the latter being rarely detected beyond local group.[22] Thedistance to NGC 5128 established from several indicators such as Mira variables and planetary nebulae favour amore distant value of ~3.8 Mpc.[6][5]

Nearby galaxies and galaxy group informationCentaurus A is at the center of one of two subgroups within the Centaurus A/M83 Group, a nearby group ofgalaxies.[23] Messier 83 (the Southern Pinwheel Galaxy) is at the center of the other subgroup. These two groups aresometimes identified as one group[24][25] and sometimes identified as two groups.[26] However, the galaxies aroundCentaurus A and the galaxies around M83 are physically close to each other, and both subgroups appear not to bemoving relative to each other.[27] The Centaurus A/M83 Group is located in the Virgo Supercluster.

Amateur astronomy informationCentaurus A is located approximately 4° north of Omega Centauri (a globular cluster visible with the naked eye).[11]

Because the galaxy has a high surface brightness and relatively large angular size, it is an ideal target for amateurastronomy observations. The bright central bulge and dark dust lane are visible even in finderscopes and largebinoculars,[11] and additional structure may be seen in larger telescopes.[11] Centaurus A is visible to the naked eyeunder exceptionally good conditions.[28]

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Centaurus A 3

Gallery

The radio galaxy CentaurusA, as seen by ALMA

Image taken by the WideField Imager attached to the

MPG/ESO 2.2-metertelescope at the La Silla

Observatory.

"Hubble's panchromaticvision... reveals the vibrant

glow of young, blue starclusters..."[29]

A Hubble Space Telescope(HST) image of the dust disk in

front of the nucleus of CentaurusA. Credit: HST/NASA/ESA.

This image of thecentral parts of

Centaurus A revealsthe

parallelogram-shapedremains of a smaller

galaxy that wasabsorbed about 200to 700 million years

ago.

The heavily-obscured inner(barred?) spiral disk at 24 μm as

shown by the Spitzer IRtelescope

Chandra X-Ray view ofCen A in X Rays

showing one relativisticjet from the central black

hole

Page 6: Space Traveler

Centaurus A 4

Video about Centaurus A jets. "False-colour imageof the nearby radio

galaxy Centaurus A,showing radio (red),

24-micrometreinfrared (green) and

0.5-5 keV X-rayemission (blue). The

jet can be seen to emitsynchrotron emission

in all threewavebands. The lobesonly emit in the radiofrequency range, and

so appear red. Gas anddust in the galaxy

emits thermalradiation in the

infrared. ThermalX-ray radiation from

hot gas andnon-thermal emission

from relativisticelectrons can be seen

in the blue 'shells'around the lobes,particularly to the

south (bottom)."[30]

References[1] "Distance Results for NGC 5128" (http:/ / nedwww. ipac. caltech. edu/ cgi-bin/ nDistance?name=NGC+ 5128). NASA/IPAC Extragalactic

Database. . Retrieved 2010-04-26.[2] J. L. Tonry, A. Dressler, J. P. Blakeslee, E. A. Ajhar, A. B. Fletcher, G. A. Luppino, M. R. Metzger, C. B. Moore (2001). "The SBF Survey of

Galaxy Distances. IV. SBF Magnitudes, Colors, and Distances". Astrophysical Journal 546 (2): 681–693. arXiv:astro-ph/0011223.Bibcode 2001ApJ...546..681T. doi:10.1086/318301.

[3] Ferrarese Laura, Mould Jeremy R., Stetson Peter B., Tonry John L., Blakeslee John P., Ajhar Edward A. (2007). "The Discovery of Cepheidsand a Distance to NGC 5128". The Astrophysical Journal 654: 186. arXiv:astro-ph/0605707. Bibcode 2007ApJ...654..186F.doi:10.1086/506612.

[4] Majaess, D. (2010). "The Cepheids of Centaurus A (NGC 5128) and Implications for H0". Acta Astronomica 60: 121. arXiv:1006.2458.Bibcode 2010AcA....60..121M.

[5] Harris, Gretchen L. H.; Rejkuba, Marina; Harris, William E. (2010). "The Distance to NGC 5128 (Centaurus A)". Publications of theAstronomical Society of Australia 27 (4): 457–462. arXiv:0911.3180. Bibcode 2010PASA...27..457H. doi:10.1071/AS09061.

[6] Harris, Gretchen L. H. (2010). "NGC 5128: The Giant Beneath". Publications of the Astronomical Society of Australia 27 (4): 475.arXiv:1004.4907. Bibcode 2010PASA...27..475H. doi:10.1071/AS09063.

[7] "SIMBAD-A" (http:/ / simbad. u-strasbg. fr/ simbad/ sim-id?Ident=Centaurus+ A). SIMBAD Astronomical Database. . Retrieved2009-11-29.

[8] Armando, Gil de Paz; Boissier; Madore; Seibert; Boselli et al. (2007). "The GALEX Ultraviolet Atlas of Nearby Galaxies". AstrophysicalJournal Supplement 173 (2): 185–255. arXiv:astro-ph/0606440. Bibcode 2007ApJS..173..185G. doi:10.1086/516636.

[9] 4U catalog browse version. (http:/ / heasarc. gsfc. nasa. gov/ W3Browse/ uhuru/ uhuru4. html)

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Centaurus A 5

[10] F. P. Israel (1998). "Centaurus A - NGC 5128". Astronomy and Astrophysics Review 8 (4): 237–278. arXiv:astro-ph/9811051.Bibcode 1998A&ARv...8..237I. doi:10.1007/s001590050011.

[11] D. J. Eicher (1988). The Universe from Your Backyard. Cambridge: Cambridge University Press. ISBN 0-521-36299-7.[12] "Radio Telescopes Capture Best-Ever Snapshot of Black Hole Jets" (http:/ / www. nasa. gov/ topics/ universe/ features/ radio-particle-jets.

html). NASA. . Retrieved 2012-10-02.[13] "Astronomy Picture of the Day - Centaurus Radio Jets Rising" (http:/ / apod. nasa. gov/ apod/ ap110413. html). NASA. 2011-04-13. .

Retrieved 2011-04-16.[14] A. Sandage, J. Bedke (1994). Carnegie Atlas of Galaxies. Washington, D.C.: Carnegie Institution of Washington. ISBN 0-87279-667-1.[15] H. Arp (1966). "Atlas of Peculiar Galaxies". Astrophysical Journal Supplement 14: 1–20. Bibcode 1966ApJS...14....1A.

doi:10.1086/190147.[16] W. Baade, R. Minkowski (1954). "On the Identification of Radio Sources". Astrophysical Journal 119: 215–231.

Bibcode 1954ApJ...119..215B. doi:10.1086/145813.[17] P. W. Hodge, R. C. Kennicutt Jr. (1982). "An atlas of H II regions in 125 galaxies". Astrophysical Journal 88: 296–328.

Bibcode 1983AJ.....88..296H. doi:10.1086/113318.[18] "NASA/IPAC Extragalactic Database" (http:/ / nedwww. ipac. caltech. edu/ ). Results for extended name search on Centaurus A. . Retrieved

2007-03-07.[19] R. Evans, R. H. McNaught, C. Humphries; McNaught; Humphries (1986). "Supernova 1986G in NGC 5128". IAU Circular 4208: 1.

Bibcode 1986IAUC.4208....1E.[20] M. M. Phillips, A. C. Phillips, S. R. Heathcote, V. M. Blanco, D. Geisler, D. Hamilton, N. B. Suntzeff, F. J. Jablonski, J. E. Steiner, A. P.

Cowley, P. Schmidtke, S. Wyckoff, J. B. Hutchings, J. Tonry, M. A. Strauss, J. R. Thorstensen, W. Honey, J. Maza, M. T. Ruiz, A. U.Landolt, A. Uomoto, R. M. Rich, J. E. Grindlay, H. Cohn, H. A. Smith, J. H. Lutz, R. J. Lavery, A. Saha (1987). "The type 1a supernova1986G in NGC 5128 - Optical photometry and spectra". Publications of the Astronomical Society of the Pacific 99: 592–605.Bibcode 1987PASP...99..592P. doi:10.1086/132020.

[21] Rejkuba, M. (2004). "The distance to the giant elliptical galaxy NGC 5128". Astronomy and Astrophysics 413 (3): 903.arXiv:astro-ph/0310639. Bibcode 2004A&A...413..903R. doi:10.1051/0004-6361:20034031.

[22] Majaess, D.; Turner; Lane (2009). "Type II Cepheids as Extragalactic Distance Candles". Acta Astronomica 59: 403. arXiv:0909.0181.Bibcode 2009AcA....59..403M.

[23] I. D. Karachentsev, M. E. Sharina, A. E. Dolphin, E. K. Grebel, D. Geisler, P. Guhathakurta, P. W. Hodge, V. E. Karachetseva, A.Sarajedini, P. Seitzer (2002). "New distances to galaxies in the Centaurus A group". Astronomy and Astrophysics 385 (1): 21–31.Bibcode 2002A&A...385...21K. doi:10.1051/0004-6361:20020042.

[24] R. B. Tully (1988). Nearby Galaxies Catalog. Cambridge: Cambridge University Press. ISBN 0-521-35299-1.[25] P. Fouque, E. Gourgoulhon, P. Chamaraux, G. Paturel; Gourgoulhon; Chamaraux; Paturel (1992). "Groups of galaxies within 80 Mpc. II -

The catalogue of groups and group members". Astronomy and Astrophysics Supplement 93: 211–233. Bibcode 1992A&AS...93..211F.[26] A. Garcia (1993). "General study of group membership. II - Determination of nearby groups". Astronomy and Astrophysics Supplement 100:

47–90. Bibcode 1993A&AS..100...47G.[27] I. D. Karachentsev (2005). "The Local Group and Other Neighboring Galaxy Groups". Astronomical Journal 129 (1): 178–188.

arXiv:astro-ph/0410065. Bibcode 2005AJ....129..178K. doi:10.1086/426368.[28] http:/ / astronomy-mall. com/ Adventures. In. Deep. Space/ aintno. htm[29] "Firestorm of Star Birth in Galaxy Centaurus A" (http:/ / www. nasa. gov/ multimedia/ imagegallery/ image_feature_2192. html). NASA. .

Retrieved 27 September 2012.[30] Wikipedia editors. "Radio Galaxy" (https:/ / en. wikipedia. org/ wiki/ Radio_galaxy). English Wikipedia. Wikimedia Foundation. . Retrieved

25 November 2012.

• STScI. Hubble Provides Multiple Views of How to Feed a Black Hole (http:/ / hubblesite. org/ newscenter/newsdesk/ archive/ releases/ 1998/ 14/ text/ ). Press release: Space Telescope Science Institute. March 14, 1998.

• Chandra X-Ray Observatory Photo Album Centaurus A Jet (http:/ / chandra. harvard. edu/ photo/ 2003/ cenajet/ )

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Centaurus A 6

External links• Helmut Steinle Centaurus A project (http:/ / www. mpe. mpg. de/ Cen-A/ )• SEDS: Peculiar Galaxy NGC 5128 (http:/ / www. seds. org/ messier/ xtra/ ngc/ n5128. html)• ESA/Hubble images of Centaurus A (http:/ / www. spacetelescope. org/ images/ archive/ freesearch/ 5128/

viewall/ 1)• NASA's APOD: The Galaxy Within Centaurus A (3/4/06) (http:/ / apod. gsfc. nasa. gov/ apod/ ap060304. html)• NASA's APOD: X-Rays from an Active Galaxy (7/5/03) (http:/ / apod. gsfc. nasa. gov/ apod/ ap030705. html)• High-resolution image of Centaurus A (http:/ / www. universetoday. com/ wp-content/ uploads/ 2006/ 07/

ngc5128. jpg) showing the discrete elements of galactic core• Centaurus A (http:/ / www. universetoday. com/ ?s=Centaurus) at UniverseToday.com• NGC5128 Centaurus A (http:/ / www. dsi-astronomie. de/ Centaurus. html)• NGC 5128 (http:/ / www. docdb. net/ show_object. php?id=ngc_5128) at DOCdb (Deep Sky Observer's

Companion)• Centaurus A on WikiSky (http:/ / www. wikisky. org/ ?object=Centaurus+ A)

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Andromeda Galaxy 7

Andromeda Galaxy

Andromeda Galaxy

The Andromeda GalaxyObservation data (J2000 epoch)

Pronunciation pron.: /ænˈdrɒmɪdə/

Constellation Andromeda

Right ascension 00h 42m 44.3s[1]

Declination +41° 16′ 9″[1]

Redshift z = −0.001(minus signindicates blueshift)

Helio radial velocity −301 ± 1 km/s[2]

Distance 2.54 ± 0.06 Mly(778 ± 17 kpc)[2][3][4][5][6][7]

Type SA(s)b[1]

Mass ~1×1012[2][8] M☉

Number of stars 1 trillion (1012)[9]

Apparent dimensions (V) 190′ × 60′[1]

Apparent magnitude (V) 3.44[10][11]

Absolute magnitude (V) −20.0[12][4]

Other designations

M31, NGC 224, UGC 454, PGC 2557, 2C 56 (Core),[1] LEDA 2557

The Andromeda Galaxy (/ænˈdrɒmɪdə/) is a spiral galaxy approximately 2.5 million light-years (2.4×1019 km)from Earth[4] in the Andromeda constellation. Also known as Messier 31, M31, or NGC 224, it is often referred to asthe Great Andromeda Nebula in older texts. The Andromeda Galaxy is the nearest spiral galaxy to our Milky Waygalaxy, but not the closest galaxy overall. It gets its name from the area of the sky in which it appears, theconstellation of Andromeda, which was named after the mythological princess Andromeda. The Andromeda Galaxyis the largest galaxy of the Local Group, which also contains the Milky Way, the Triangulum Galaxy, and about 30other smaller galaxies. Although the largest, the Andromeda Galaxy may not be the most massive, as recent findingssuggest that the Milky Way contains more dark matter and could be the most massive in the grouping.[13] The 2006observations by the Spitzer Space Telescope revealed that M31 contains one trillion (1012) stars:[9] at least twice thenumber of stars in the Milky Way galaxy, which is estimated to be 200–400 billion.[14]

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Andromeda Galaxy 8

The Andromeda Galaxy is estimated to be 7.1×1011 solar masses.[2] In comparison a 2009 study estimated that theMilky Way and M31 are about equal in mass,[15] while a 2006 study put the mass of the Milky Way at ~80% of themass of the Andromeda Galaxy. The two galaxies are expected to collide in 3.75 billion years, eventually merging toform a giant elliptical galaxy.[16]

At an apparent magnitude of 3.4, the Andromeda Galaxy is notable for being one of the brightest Messier objects,[17]

making it visible to the naked eye on moonless nights even when viewed from areas with moderate light pollution.Although it appears more than six times as wide as the full Moon when photographed through a larger telescope,only the brighter central region is visible to the naked eye or when viewed using binoculars or a small telescope.

Observation history

Great Andromeda Nebula by Isaac Roberts

The Persian astronomer Abd al-Rahman al-Sufi wrote a tantalizing lineabout the chained constellation in his Book of Fixed Stars around 964,describing it as a "small cloud".[18][19] Star charts of that period have itlabeled as the Little Cloud.[19] The first description of the object basedon telescopic observation was given by German astronomer SimonMarius on December 15, 1612.[20] Charles Messier catalogued it asobject M31 in 1764 and incorrectly credited Marius as the discoverer,unaware of Al Sufi's earlier work. In 1785, the astronomer WilliamHerschel noted a faint reddish hue in the core region of the M31. Hebelieved it to be the nearest of all the "great nebulae" and based on thecolour and magnitude of the nebula, he incorrectly guessed that it was no more than 2,000 times the distance ofSirius.[21]

William Huggins in 1864 observed the spectrum of M31 and noted that it differed from a gaseous nebula.[22] Thespectra of M31 displayed a continuum of frequencies, superimposed with dark absorption lines that help identify thechemical composition of an object. The Andromeda nebula was very similar to the spectra of individual stars, andfrom this it was deduced that M31 had a stellar nature. In 1885, a supernova (known as S Andromedae) was seen inM31, the first and so far only one observed in that galaxy. At the time M31 was considered to be a nearby object, sothe cause was thought to be a much less luminous and unrelated event called a nova, and was named accordingly"Nova 1885".[23]

The first photographs of M31 were taken in 1887 by Isaac Roberts from his private observatory in Sussex, England.The long-duration exposure allowed the spiral structure of the galaxy to be seen for the first time.[24] However, at thetime this object was still commonly believed to be a nebula within our galaxy, and Roberts mistakenly believed thatM31 and similar spiral nebulae were actually solar systems being formed, with the satellites nascent planets. Theradial velocity of this object with respect to our solar system was measured in 1912 by Vesto Slipher at the LowellObservatory, using spectroscopy. The result was the largest velocity recorded at that time, at 300 kilometres persecond (190 mi/s), moving in the direction of the Sun.[25]

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Andromeda Galaxy 9

Island universe

Location of M31 in the Andromeda constellation

In 1917, American astronomer Heber Curtis observed a nova withinM31. Searching the photographic record, 11 more novae werediscovered. Curtis noticed that these novae were, on average, 10magnitudes fainter than those that occurred elsewhere in the sky. As aresult he was able to come up with a distance estimate of 500,000light-years (3.2×1010 AU). He became a proponent of the so-called"island universes" hypothesis, which held that spiral nebulae wereactually independent galaxies.[26]

In 1920, the Great Debate between Harlow Shapley and Curtis tookplace, concerning the nature of the Milky Way, spiral nebulae, and thedimensions of the universe. To support his claim that the Great Andromeda Nebula (M31) was an external galaxy,Curtis also noted the appearance of dark lanes resembling the dust clouds in our own Galaxy, as well as thesignificant Doppler shift. In 1922 Ernst Öpik presented a very elegant and simple astrophysical method to estimatethe distance of M31. His result put the Andromeda Nebula far outside our Galaxy at a distance of about 450,000parsec, which is about 1,500,000 ly.[27] Edwin Hubble settled the debate in 1925 when he identified extragalacticCepheid variable stars for the first time on astronomical photos of M31. These were made using the 2.5-metre(100-in) Hooker telescope, and they enabled the distance of Great Andromeda Nebula to be determined. Hismeasurement demonstrated conclusively that this feature was not a cluster of stars and gas within our Galaxy, but anentirely separate galaxy located a significant distance from our own.[28]

Stars in the Andromeda Galaxy's disc[29]

M31 plays an important role in galactic studies, since it is the nearestspiral galaxy (although not the nearest galaxy). In 1943 Walter Baadewas the first person to resolve stars in the central region of theAndromeda Galaxy. Based on his observations of this galaxy, he wasable to discern two distinct populations of stars based on theirmetallicity, naming the young, high velocity stars in the disk Type Iand the older, red stars in the bulge Type II. This nomenclature wassubsequently adopted for stars within the Milky Way, and elsewhere.(The existence of two distinct populations had been noted earlier byJan Oort.)[30] Dr. Baade also discovered that there were two types ofCepheid variables, which resulted in a doubling of the distanceestimate to M31, as well as the remainder of the Universe.[31]

Radio emission from the Andromeda Galaxy was first detected byHanbury Brown and Cyril Hazard at Jodrell Bank Observatory using

the 218-ft Transit Telescope, and was announced in 1950[32] (Earlier observations were made by radio astronomypioneer Grote Reber in 1940, but were inconclusive, and were later shown to be an order of magnitude too high).The first radio maps of the galaxy were made in the 1950s by John Baldwin and collaborators at the CambridgeRadio Astronomy Group.[33] The core of the Andromeda Galaxy is called 2C 56 in the 2C radio astronomycatalogue. In 2009, the first planet may have been discovered in the Andromeda Galaxy. This candidate was detectedusing a technique called microlensing, which is caused by the deflection of light by a massive object.[34]

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Andromeda Galaxy 10

General

The Andromeda Galaxy as seen by NASA'sWide-field Infrared Survey Explorer

The measured distance to the Andromeda Galaxy was doubled in 1953when it was discovered that there is another, dimmer type of Cepheid.In the 1990s, measurements of both standard red giants as well as redclump stars from the Hipparcos satellite measurements were used tocalibrate the Cepheid distances.[35][36]

Formation and History

According to a team of astronomers reporting in 2010, M31 wasformed out of the collision of two smaller galaxies between 5 and 9billion years ago.[37]

A paper published in 2012[38] has outlined M31's basic history since itsbirth. According to it, Andromeda was born roughly 10 billion yearsago from the merger of many smaller protogalaxies, leading to agalaxy smaller than the one we see today.

The most important event in M31's past history was the merger mentioned above that took place 8 billion years ago.This violent collision formed most of its (metal-rich) galactic halo and extended disk and during that epochAndromeda's star formation would be very high, to the point of becoming a Luminous infrared galaxy for roughly100 millon years.

2–4 billion years ago, M31 and the Triangulum Galaxy (M33) had a very close passage. This event produced highlevels of star formation across the Andromeda Galaxy's disk - even some globular clusters - and disturbed M33'souter disk.While there has been activity during the last 2 billion years, this has been much lower than during the past. Therehave been interactions with satellite galaxies like M32, M110, or others that have already disappeared absorbed byM31 that have formed structures like Andromeda's Giant Stellar Stream and a merger roughly 100 million years agothat is behind a counter-rotating disk of gas found in the center of M31 as well as the presence there of a relativelyyoung (100 million years) stellar population. During this epoch, star formation through M31's disk decreased to thepoint of nearly shutting down to increasing again relatively recently.

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Andromeda Galaxy 11

Recent distance estimateAt least four distinct techniques have been used to measure distances to the Andromeda Galaxy.In 2003, using the infrared surface brightness fluctuations (I-SBF) and adjusting for the new period-luminosity valueof Freedman et al. 2001 and using a metallicity correction of −0.2 mag dex−1 in (O/H), an estimate of 2.57 ± 0.06megalight-years (790 ± 18 kpc) was derived.

The Andromeda Galaxy pictured in ultravioletlight by GALEX

Using the Cepheid variable method, an estimate of 2.51 ± 0.13 Mly(770 ± 40 kpc) was reported in 2004.[2][3]

In 2005 Ignasi Ribas (CSIC, Institute for Space Studies of Catalonia(IEEC)) and colleagues announced the discovery of an eclipsing binarystar in the Andromeda Galaxy. The binary star, designatedM31VJ00443799+4129236,[39] has two luminous and hot blue stars oftypes O and B. By studying the eclipses of the stars, which occur every3.54969 days, the astronomers were able to measure their sizes.Knowing the sizes and temperatures of the stars, they were able tomeasure the absolute magnitude of the stars. When the visual andabsolute magnitudes are known, the distance to the star can bemeasured. The stars lie at a distance of 2.52 ± 0.14 Mly (770 ± 43 kpc)and the whole Andromeda Galaxy at about 2.5 Mly (770 kpc).[4] Thisnew value is in excellent agreement with the previous, independent

Cepheid-based distance value.

M31 is close enough that the Tip of the Red Giant Branch (TRGB) method may also be used to estimate its distance.The estimated distance to M31 using this technique in 2005 yielded 2.56 ± 0.08 Mly (780 ± 25 kpc).[5]

Averaged together, all these distance measurements give a combined distance estimate of 2.54 ± 0.06 Mly(780 ± 18 kpc).[7] Based upon the above distance, the diameter of M31 at the widest point is estimated to be141 ± 3 kly (43,000 ± 920 pc).[40] Applying trigonometry (arctangent), that figures to extending at an apparent 3.18°angle in the sky.

Mass and luminosity estimatesMass estimates for the Andromeda Galaxy's halo (including dark matter) give a value of approximately1.23×1012 M☉

[8] (or 1.2 trillion solar masses) compared to 1.9×1012 M☉ for the Milky Way. Thus M31 may be lessmassive than our own galaxy, although the error range is still too large to say for certain. Even so, the masses of theMilky Way and M31 are comparable, and M31's spheroid actually has a higher stellar density than that of the MilkyWay.[41]

In particular, M31 appears to have significantly more common stars than the Milky Way, and the estimatedluminosity of M31, ~2.6×1010 L☉, is about 25% higher than that of our own galaxy.[42] However the rate of starformation in the Milky Way is much higher, with M31 producing only about one solar mass per year compared to3–5 solar masses for the Milky Way. The rate of supernovae in the Milky Way is also double that of M31.[43] Thissuggests that M31 once experienced a great star formation phase, but is now in a relative state of quiescence,whereas the Milky Way is experiencing more active star formation.[42] Should this continue, the luminosity in theMilky Way may eventually overtake that of M31.

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Andromeda Galaxy 12

Local group (clickable map)

Structure

The Andromeda Galaxy seen in infrared by theSpitzer Space Telescope, one of NASA's four

Great Space Observatories

Image of the Andromeda Galaxy taken by Spitzerin infrared, 24 micrometres

(Credit:NASA/JPL–Caltech/K. Gordon,University of Arizona)

A Swift Tour of Andromeda Galaxy

Based on its appearance in visible light, the Andromeda Galaxy isclassified as an SA(s)b galaxy in the de Vaucouleurs–Sandageextended classification system of spiral galaxies.[1] However, data fromthe 2MASS survey showed that the bulge of M31 has a box-likeappearance, which implies that the galaxy is actually a barred spiralgalaxy like the Milky Way, with the Andromeda Galaxy's bar viewedalmost directly along its long axis.[44]

In 2005, astronomers used the Keck telescopes to show that thetenuous sprinkle of stars extending outward from the galaxy is actuallypart of the main disk itself.[45] This means that the spiral disk of starsin M31 is three times larger in diameter than previously estimated.This constitutes evidence that there is a vast, extended stellar disk thatmakes the galaxy more than 220,000 light-years (67,000 pc) indiameter. Previously, estimates of the Andromeda Galaxy's size rangedfrom 70000 to 120000 light-years (21,000 to 37,000 pc) across.

The galaxy is inclined an estimated 77° relative to the Earth (where anangle of 90° would be viewed directly from the side). Analysis of thecross-sectional shape of the galaxy appears to demonstrate apronounced, S-shaped warp, rather than just a flat disk.[46] A possiblecause of such a warp could be gravitational interaction with thesatellite galaxies near M31. The galaxy M33 could be responsible forsome warp in M31's arms, though more precise distances and radialvelocities are required.

Spectroscopic studies have provided detailed measurements of therotational velocity of M31 at various radii from the core. In the vicinityof the core, the rotational velocity climbs to a peak of 225 kilometres per second (140 mi/s) at a radius of 1,300light-years (82,000,000 AU), then descends to a minimum at 7,000 light-years (440,000,000 AU) where the rotationvelocity may be as low as 50 kilometres per second (31 mi/s). Thereafter the velocity steadily climbs again out to aradius of 33,000 light-years (2.1×109 AU), where it reaches a peak of 250 kilometres per second (160 mi/s). Thevelocities slowly decline beyond that distance, dropping to around 200 kilometres per second (120 mi/s) at 80,000light-years (5.1×109 AU). These velocity measurements imply a concentrated mass of about 6×109 M☉ in thenucleus. The total mass of the galaxy increases linearly out to 45,000 light-years (2.8×109 AU), then more slowlybeyond that radius.[47]

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Andromeda Galaxy 13

The spiral arms of M31 are outlined by a series of H II regions that Baade described as resembling "beads on astring". They appear to be tightly wound, although they are more widely spaced than in our galaxy.[48] Rectifiedimages of the galaxy show a fairly normal spiral galaxy with the arms wound up in a clockwise direction. There aretwo continuous trailing arms that are separated from each other by a minimum of about 13,000 light-years(820,000,000 AU). These can be followed outward from a distance of roughly 1,600 light-years (100,000,000 AU)from the core. The most likely cause of the spiral pattern is thought to be interaction with galaxy M32. This can beseen by the displacement of the neutral hydrogen clouds from the stars.[49]

In 1998, images from the European Space Agency's Infrared Space Observatory demonstrated that the overall formof the Andromeda Galaxy may be transitioning into a ring galaxy. The gas and dust within M31 is generally formedinto several overlapping rings, with a particularly prominent ring formed at a radius of 32,000 light-years (2.0×109

AU) from the core.[50] This ring is hidden from visible light images of the galaxy because it is composed primarily ofcold dust.Close examination of the inner region of M31 showed a smaller dust ring that is believed to have been caused by theinteraction with M32 more than 200 million years ago. Simulations show that the smaller galaxy passed through thedisk of the galaxy in Andromeda along the latter's polar axis. This collision stripped more than half the mass fromthe smaller M32 and created the ring structures in M31.[51]

Studies of the extended halo of M31 show that it is roughly comparable to that of the Milky Way, with stars in thehalo being generally "metal-poor", and increasingly so with greater distance.[41] This evidence indicates that the twogalaxies have followed similar evolutionary paths. They are likely to have accreted and assimilated about 1–200low-mass galaxies during the past 12 billion years.[52] The stars in the extended halos of M31 and the Milky Waymay extend nearly one-third the distance separating the two galaxies.

Nucleus

HST image of the Andromeda Galaxy coreshowing possible double structure.

NASA/ESA photo

M31 is known to harbor a dense and compact star cluster at its verycenter. In a large telescope it creates a visual impression of a starembedded in the more diffuse surrounding bulge. The luminosity of thenucleus is in excess of the most luminous globular clusters.

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Andromeda Galaxy 14

Chandra X-ray telescope image of the center of M31. A number of X-ray sources,likely X-ray binary stars, within Andromeda's central region appear as yellowish

dots. The blue source at the center is at the position of the supermassive black hole.

In 1991 Tod R. Lauer used WFPC, then onboard the Hubble Space Telescope, to imageM31's inner nucleus. The nucleus consists oftwo concentrations separated by 1.5 parsecs(4.9 ly). The brighter concentration,designated as P1, is offset from the center ofthe galaxy. The dimmer concentration, P2,falls at the true center of the galaxy andcontains a black hole measured at 3–5 × 107

M☉ in 1993,[53] and at 1.1–2.3 × 108 M☉ in2005.[54] The velocity dispersion of materialaround it is measured to be ≈ 160 km/s.[55]

Scott Tremaine has proposed that theobserved double nucleus could be explainedif P1 is the projection of a disk of stars in aneccentric orbit around the central blackhole.[56] The eccentricity is such that starslinger at the orbital apocenter, creating aconcentration of stars. P2 also contains acompact disk of hot, spectral class A stars. The A stars are not evident in redder filters, but in blue and ultravioletlight they dominate the nucleus, causing P2 to appear more prominent than P1.[57]

While at the initial time of its discovery it was hypothesized that the brighter portion of the double nucleus was theremnant of a small galaxy "cannibalized" by M31,[58] this is no longer considered a viable explanation, largelybecause such a nucleus would have an exceedingly short lifetime due to tidal disruption by the central black hole.While this could be partially resolved if P1 had its own black hole to stabilize it, the distribution of stars in P1 doesnot suggest that there is a black hole at its center.[56]

Discrete sources

Artist's concept of the Andromeda Galaxy coreshowing a view across a disk of young, blue stars

encircling a supermassive black hole.NASA/ESA photo

Apparently, by late 1968, no X-rays had been detected from theAndromeda Galaxy.[59] A balloon flight on October 20, 1970, set anupper limit for detectable hard X-rays from M31.[60]

Multiple X-ray sources have since been detected in the AndromedaGalaxy, using observations from the ESA's XMM-Newton orbitingobservatory. Robin Barnard et al. hypothesized that these are candidateblack holes or neutron stars, which are heating incoming gas tomillions of kelvins and emitting X-rays. The spectrum of the neutronstars is the same as the hypothesized black holes, but can bedistinguished by their masses.[61]

There are approximately 460 globular clusters associated with theAndromeda Galaxy.[62] The most massive of these clusters, identifiedas Mayall II, nicknamed Globular One, has a greater luminosity thanany other known globular cluster in the local group of galaxies.[63] It contains several million stars, and is abouttwice as luminous as Omega Centauri, the brightest known globular cluster in the Milky Way. Globular One (or G1)

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Andromeda Galaxy 15

has several stellar populations and a structure too massive for an ordinary globular. As a result, some consider G1 tobe the remnant core of a dwarf galaxy that was consumed by M31 in the distant past.[64] The globular with thegreatest apparent brightness is G76 which is located in the south-west arm's eastern half.[19] Another massiveglobular cluster -named 037-B327-, discovered in 2006 as is heavily reddened by the Andromeda Galaxy'sinterstellar dust, was thought to be more massive than G1 and the largest cluster of the Local Group;[65] howeverother studies have shown is actually similar in properties to G1.[66]

Unlike the globular clusters of the Milky Way, that show a relatively low age dispersion, Andromeda's globularclusters have a much larger range of ages: from systems as old as the galaxy itself to much younger systems, withages between a few hundred million years to five billion years[67]

In 2005, astronomers discovered a completely new type of star cluster in M31. The new-found clusters containhundreds of thousands of stars, a similar number of stars that can be found in globular clusters. What distinguishesthem from the globular clusters is that they are much larger–several hundred light-years across–and hundreds oftimes less dense. The distances between the stars are, therefore, much greater within the newly discovered extendedclusters.[68]

In the year 2012, a microquasar, a radio burst emanating from a smaller black hole, was detected in the AndromedaGalaxy. The progenitor black hole was located near the galactic center and had about 10 . Discovered through adata collected by the ESA's XMM-Newton probe, and subsequently observed by NASA's Swift and Chandra, theVery Large Array, and the Very Long Baseline Array, the microquasar was the first observed within the AndromedaGalaxy and the first outside of the Milky Way Galaxy.[69]

SatellitesLike the Milky Way, the Andromeda Galaxy has satellite galaxies, consisting of 14 known dwarf galaxies. The bestknown and most readily observed satellite galaxies are M32 and M110. Based on current evidence, it appears thatM32 underwent a close encounter with M31 (Andromeda) in the past. M32 may once have been a larger galaxy thathad its stellar disk removed by M31, and underwent a sharp increase of star formation in the core region, whichlasted until the relatively recent past.[70]

M110 also appears to be interacting with M31, and astronomers have found in the halo of M31 a stream ofmetal-rich stars that appear to have been stripped from these satellite galaxies.[71] M110 does contain a dusty lane,which may indicate recent or ongoing star formation.[72]

In 2006 it was discovered that nine of these galaxies lie along a plane that intersects the core of the AndromedaGalaxy, rather than being randomly arranged as would be expected from independent interactions. This may indicatea common tidal origin for the satellites.[73]

Future collision with the Milky WayThe Andromeda Galaxy is approaching the Milky Way at about 100 to 140 kilometres per second (62 to 87 mi/s)which is about 1.96 billion to 2.74 billion miles per year,[74] making it one of the few blueshifted galaxies. TheAndromeda Galaxy and the Milky Way are thus expected to collide in about 4.5 billion years, although the detailsare uncertain since Andromeda's tangential velocity with respect to the Milky Way is known to only within about afactor of two.[75] A likely outcome of the collision is that the galaxies will merge to form a giant elliptical galaxy.[76]

Such events are frequent among the galaxies in galaxy groups. The fate of the Earth and the Solar System in theevent of a collision is currently unknown. If the galaxies do not merge, there is a small chance that the Solar Systemcould be ejected from the Milky Way or join M31.[77]

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[64] Meylan, G. et al. (2001). "G1 in M31 – Giant Globular Cluster or Core of a Dwarf Elliptical Galaxy?". Astronomical Journal 122 (2):830–841. arXiv:astro-ph/0105013. Bibcode 2001AJ....122..830M. doi:10.1086/321166.

[65] Ma, J.; de Grijs, R.; Yang, Y.; Zhou, X.; Chen, J.; Jiang, Z.; Wu, Z.; Wu, J. (2006). "A `super' star cluster grown old: the most massive starcluster in the Local Group" (http:/ / adsabs. harvard. edu/ abs/ 2006MNRAS. 368. 1443M). Monthly Notices of the Royal AstronomicalSociety. .

[66] Cohen, Judith G. (2006). "The Not So Extraordinary Globular Cluster 037-B327 in M31" (http:/ / adsabs. harvard. edu/ abs/ 2006ApJ. . .653L. . 21C). The Astrophysical Journal 653: L21-L23. .

[67] Burstein, David; Li, Yong; Freeman, Kenneth C.; Norris, John E.; Bessell, Michael S.; Bland-Hawthorn, Joss; Gibson, Brad K.; Beasley,Michael A.; Lee, Hyun-chul; Barbuy, Beatriz; Huchra, John P.; Brodie, Jean P.; Forbes, Duncan A. (2004). "Globular Cluster and GalaxyFormation: M31, the Milky Way, and Implications for Globular Cluster Systems of Spiral Galaxies" (http:/ / adsabs. harvard. edu/ abs/2004ApJ. . . 614. . 158B). Astrophysical Journal 614: 158–166. .

[68] Huxor, A. P. et al. (2005). "A new population of extended, luminous, star clusters in the halo of M31". Monthly Notices of the RoyalAstronomical Society 360 (3): 993–1006. arXiv:astro-ph/0412223. Bibcode 2005MNRAS.360.1007H. doi:10.1111/j.1365-2966.2005.09086.x.

[69] Prostak, Sergio (2012-12-14). "Microquasar in Andromeda Galaxy Amazes Astronomers" (http:/ / www. sci-news. com/ astronomy/article00779. html). Sci-News.com. .

[70] Bekki, K. et al. (2001). "A New Formation Model for M32: A Threshed Early-type Spiral?". Astrophysical Journal Letters 557 (1):L39–L42. arXiv:astro-ph/0107117. Bibcode 2001ApJ...557L..39B. doi:10.1086/323075.

[71] Ibata, R. et al. (2001). "A giant stream of metal-rich stars in the halo of the galaxy M31". Nature 412 (6842): 49–52. doi:10.1038/35083506.PMID 11452300.

[72] Young, L. M. (2000). "Properties of the Molecular Clouds in NGC 205". Astronomical Journal 120 (5): 2460–2470.arXiv:astro-ph/0007169. Bibcode 2000AJ....120.2460Y. doi:10.1086/316806.

[73] Koch, Andreas; Grebel, Eva K. (March 2006). "The Anisotropic Distribution of M31 Satellite Galaxies: A Polar Great Plane of Early-typeCompanions". Astronomical Journal 131 (3): 1405–1415. arXiv:astro-ph/0509258. Bibcode 2005astro.ph..9258K. doi:10.1086/499534.

[74] Malik, T. (May 7, 2002). "Crash Course: Simulating the Fate of Our Milky Way" (http:/ / www. space. com/ scienceastronomy/ astronomy/galaxy_collides_020507-1. html). Space.com. . Retrieved 2006-09-18.

[75] "The Grand Collision". The Sky At Night. November 5, 2007.[76] Cox, T. J.; Loeb, A. (2008). "The collision between the Milky Way and Andromeda". Monthly Notices of the Royal Astronomical Society

386 (1): 461–474. Bibcode 2008MNRAS.tmp..333C. doi:10.1111/j.1365-2966.2008.13048.x.[77] Cain, F. (2007). "When Our Galaxy Smashes Into Andromeda, What Happens to the Sun?" (http:/ / www. universetoday. com/ 2007/ 05/ 10/

when-our-galaxy-smashes-into-andromeda-what-happens-to-the-sun/ ). Universe Today. Archived (http:/ / web. archive. org/ web/20070517021426/ http:/ / www. universetoday. com/ 2007/ 05/ 10/ when-our-galaxy-smashes-into-andromeda-what-happens-to-the-sun/ )from the original on 17 May 2007. . Retrieved 2007-05-16.

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Andromeda Galaxy 19

References

External links• StarDate: M31 Fact Sheet (http:/ / blackholes. stardate. org/ directory/ factsheet. php?p=M31)• Simbad data on M31 (http:/ / simbad. u-strasbg. fr/ sim-id. pl?Ident=M+ 31)• Messier 31, SEDS Messier pages (http:/ / www. seds. org/ messier/ m/ m031. html)•• Astronomy Picture of the Day

• A Giant Globular Cluster in M31 (http:/ / antwrp. gsfc. nasa. gov/ apod/ ap981017. html) 1998 October 17.• M31: The Andromeda Galaxy (http:/ / antwrp. gsfc. nasa. gov/ apod/ ap040718. html) 2004 July 18.• Andromeda Island Universe (http:/ / antwrp. gsfc. nasa. gov/ apod/ ap051222. html) 2005 December 22.• Andromeda Island Universe (http:/ / apod. nasa. gov/ apod/ ap100109. html) 2010 January 9.• WISE Infrared Andromeda (http:/ / antwrp. gsfc. nasa. gov/ apod/ ap100219. html) 2010 February 19

• M31 and its central Nuclear Spiral (http:/ / www. beskeen. com/ gallery/ galaxy/ m31/ m31. shtml)• Amateur photography – M31 (http:/ / www. starpointing. com/ ccd/ m31. html)• Globular Clusters in M31 (http:/ / astro. neutral. org/ imagehtml/ 20050903_m31. html) at The Curdridge

Observatory• First direct distance to Andromeda (http:/ / www. astronomy. com/ asy/ default. aspx?c=a& id=3640) −

Astronomy magazine article• Andromeda Galaxy (http:/ / www. solstation. com/ x-objects/ andromeda. htm) at SolStation.com• Andromeda Galaxy at The Encyclopedia of Astrobiology, Astronomy, & Spaceflight (http:/ / www. daviddarling.

info/ encyclopedia/ A/ Andromeda. html)• M31, the Andromeda Galaxy (http:/ / www. nightskyinfo. com/ archive/ m31_galaxy) at NightSkyInfo.com• Than, Ker (January 23, 2006). "Strange Setup: Andromeda's Satellite Galaxies All Lined Up" (http:/ / space. com/

scienceastronomy/ 060123_andromeda_plane. html). Space.com.• The Andromeda Galaxy on WikiSky (http:/ / www. wikisky. org/ ?object=Andromeda+ Galaxy)• Hubble Finds Mysterious Disk of Blue Stars Around Black Hole (http:/ / hubblesite. org/ newscenter/ archive/

releases/ 2005/ 26/ full/ ) Hubble observations (Sep 20 2005) put the mass of the Andromeda core black hole at140 million solar masses

• M31 (Apparent) Novae Page (http:/ / cbat. eps. harvard. edu/ CBAT_M31. html) (IAU)• Multi-wavelength composite (http:/ / herschel. esac. esa. int/ Images/ 2011/ M31_COMPO_A. jpg)• Andromeda Project (http:/ / www. andromedaproject. org) (crowd-source)

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Pleiades

Pleiades

A color-composite image of the Pleiades from the Digitized Sky SurveyCredit: NASA/ESA/AURA/CaltechObservation data (J2000 epoch)

Constellation Taurus

Right ascension 3h 47m 24s[1]

Declination +24° 7′[1]

Distance 390–460 ly (120–140 pc[2][3][4][5])

Apparent magnitude (V) 1.6[6]

Apparent dimensions (V) 110' (arcmin.)[6]

Physical characteristics

Other designations M45,[1] Seven Sisters,[1] Melotte 22[1]

In astronomy, the Pleiades (pron.: /ˈplaɪ.ədiːz/ or /ˈpliː.ədiːz/), or Seven Sisters (Messier object 45 or M45), is anopen star cluster containing middle-aged hot B-type stars located in the constellation of Taurus. It is among thenearest star clusters to Earth and is the cluster most obvious to the naked eye in the night sky. The name Pleiadescomes from Greek mythology; it has several meanings in different cultures and traditions.The cluster is dominated by hot blue and extremely luminous stars that have formed within the last 100 millionyears. Dust that forms a faint reflection nebulosity around the brightest stars was thought at first to be left over fromthe formation of the cluster (hence the alternate name Maia Nebula after the star Maia), but is now known to be anunrelated dust cloud in the interstellar medium that the stars are currently passing through. Computer simulationshave shown that the Pleiades was probably formed from a compact configuration that resembled the Orion Nebula.[7]

Astronomers estimate that the cluster will survive for about another 250 million years, after which it will dispersedue to gravitational interactions with its galactic neighborhood.

Observational historyThe Pleiades are a prominent sight in winter in the Northern Hemisphere and in summer in the SouthernHemisphere, and have been known since antiquity to cultures all around the world, including the Māori, AboriginalAustralians, the Persians, the Chinese, the Japanese, the Maya, the Aztec, and the Sioux and Cherokee. In Tamilculture this star cluster is attributed to Lord Murugan (Lord Murugan raised by the six sisters known as theKārththikai Pengal and thus came to be known as Kārtikeyan), in Sanskrit he is noted as Skanda.

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Pleiades 21

The Nebra sky disk, dated c. 1600 BC. Thecluster of dots near the upper right portion of the

disk is believed to be the Pleiades.

The Babylonian star catalogues name them MUL.MUL or "star ofstars", and they head the list of stars along the ecliptic, reflecting thefact that they were close to the point of vernal equinox around the 23rdcentury BC. The earliest known depiction of the Pleiades is likely abronze age artifact known as the Nebra sky disk, dated toapproximately 1600 BC. Some Greek astronomers considered them tobe a distinct constellation, and they are mentioned by Hesiod, and inHomer's Iliad and Odyssey. They are also mentioned three times in theBible (Job 9:9 and 38:31, as well as Amos 5:8). The Pleiades (Krittika)are particularly revered in Hindu mythology as the six mothers of thewar god Murugan, who developed six faces, one for each of them.Some scholars of Islam suggested that the Pleiades (Ats-tsuraiya) arethe Star in Najm, which is mentioned in the Quran.

In Japan, the constellation is mentioned under the name Mutsuraboshi("six stars") in the 8th century Kojiki and Manyosyu documents. The constellation is also known in Japan as Subaru(“unite”) and is depicted in the logo of the Subaru automobile company. The Persian equivalent is Nahid(pronounced "Naheed").

The rising of the Pleiades is mentioned in the Ancient Greek text Geoponica.[8] The Greeks oriented theHecatompedon temple of 1150 BC and the Parthenon of 438 BC to their rising.[9]

A Spitzer image of the Pleiades ininfrared, showing the associated dust

(Merope Nebula). Credit:NASA/JPL-Caltech

They have long been known to be a physically related group of stars rather thanany chance alignment. The Reverend John Michell calculated in 1767 that theprobability of a chance alignment of so many bright stars was only 1 in 500,000,and so correctly surmised that the Pleiades and many other clusters of stars mustbe physically related.[10] When studies were first made of the stars' propermotions, it was found that they are all moving in the same direction across thesky, at the same rate, further demonstrating that they were related.

Charles Messier measured the position of the cluster and included it as M45 inhis catalogue of comet-like objects, published in 1771. Along with the OrionNebula and the Praesepe cluster, Messier's inclusion of the Pleiades has beennoted as curious, as most of Messier's objects were much fainter and more easilyconfused with comets—something that seems scarcely possible for the Pleiades.One possibility is that Messier simply wanted to have a larger catalogue than hisscientific rival Lacaille, whose 1755 catalogue contained 42 objects, and so he added some bright, well-knownobjects to boost his list.[11]

Edme-Sébastien Jeaurat then drew in 1782 a map of 64 stars of the Pleiades from his observations in 1779, which hepublished in 1786.[12][13][14]

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Pleiades 22

Distance

Comet Machholz appears to pass near thePleiades in early 2005

The distance to the Pleiades can be used as an important first step tocalibrate the cosmic distance ladder. As the cluster is so close to theEarth, its distance is relatively easy to measure and has been estimatedby many methods. Accurate knowledge of the distance allowsastronomers to plot a Hertzsprung-Russell diagram for the cluster,which, when compared to those plotted for clusters whose distance isnot known, allows their distances to be estimated. Other methods canthen extend the distance scale from open clusters to galaxies andclusters of galaxies, and a cosmic distance ladder can be constructed. Ultimately astronomers' understanding of theage and future evolution of the universe is influenced by their knowledge of the distance to the Pleiades. Yet someauthors argue that the controversy over the distance to the Pleiades discussed below is a red herring, since the cosmicdistance ladder can (presently) rely on a suite of other nearby clusters where consensus exists regarding the distancesas established by Hipparcos and independent means (e.g., the Hyades, Coma Berenices cluster, etc.).[3]

Measurements of the distance have elicited much controversy. Results prior to the launch of the Hipparcos satellitegenerally found that the Pleiades were about 135 parsecs away from Earth. Data from Hipparcos yielded a surprisingresult, namely a distance of only 118 parsecs by measuring the parallax of stars in the cluster—a technique thatshould yield the most direct and accurate results. Later work consistently argued that the Hipparcos distancemeasurement for the Pleiades was erroneous.[3][4][5][15][16] In particular, distances derived to the cluster via theHubble Space Telescope and infrared color-magnitude diagram fitting favor a distance between 135–140 pc.[3][15]

However, the author of the 2007–2009 catalog of revised Hipparcos parallaxes reasserted that the distance to thePleiades is ~120 pc, and challenged the dissenting evidence.[2] Recently, Francis and Anderson[17] proposed that asystematic effect on Hipparcos parallax errors for stars in clusters biases calculation using the weighted mean, andgave a Hipparcos parallax distance of 126 pc, and photometric distance 132 pc based on stars in the AB Doradus,Tucana-Horologium moving group and Beta Pictoris moving groups, which are similar in age and composition to thePleiades. Those authors note that the difference between these results can be attributed to random error.

Composition

X-ray images of the Pleiades reveal the stars withthe hottest atmospheres. Green squares indicate

the seven optically brightest stars.

The cluster core radius is about 8 light years and tidal radius is about43 light years. The cluster contains over 1,000 statistically confirmedmembers, although this figure excludes unresolved binary stars.[18] It isdominated by young, hot blue stars, up to 14 of which can be seen withthe naked eye depending on local observing conditions. Thearrangement of the brightest stars is somewhat similar to Ursa Majorand Ursa Minor. The total mass contained in the cluster is estimated tobe about 800 solar masses.[18]

The cluster contains many brown dwarfs, which are objects with lessthan about 8% of the Sun's mass, not heavy enough for nuclear fusionreactions to start in their cores and become proper stars. They mayconstitute up to 25% of the total population of the cluster, althoughthey contribute less than 2% of the total mass.[19] Astronomers havemade great efforts to find and analyse brown dwarfs in the Pleiadesand other young clusters, because they are still relatively bright andobservable, while brown dwarfs in older clusters have faded and are much more difficult to study.

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Age and future evolutionAges for star clusters can be estimated by comparing the Hertzsprung-Russell diagram for the cluster with theoreticalmodels of stellar evolution. Using this technique, ages for the Pleiades of between 75 and 150 million years havebeen estimated. The wide spread in estimated ages is a result of uncertainties in stellar evolution models, whichinclude factors such as convective overshoot, in which a convective zone within a star penetrates an otherwisenon-convective zone, resulting in higher apparent ages.Another way of estimating the age of the cluster is by looking at the lowest-mass objects. In normal main sequencestars, lithium is rapidly destroyed in nuclear fusion reactions. Brown dwarfs can retain their lithium, however. Due tolithium's very low ignition temperature of 2.5 million kelvin, the highest-mass brown dwarfs will burn it eventually,and so determining the highest mass of brown dwarfs still containing lithium in the cluster can give an idea of itsage. Applying this technique to the Pleiades gives an age of about 115 million years.[20][21]

The cluster is slowly moving in the direction of the feet of what is currently the constellation of Orion. Like mostopen clusters, the Pleiades will not stay gravitationally bound forever. Some component stars will be ejected afterclose encounters with other stars; others will be stripped by tidal gravitational fields. Calculations suggest that thecluster will take about 250 million years to disperse, with gravitational interactions with giant molecular clouds andthe spiral arms of our galaxy also hastening its demise.

Reflection nebulosity

Hubble Space Telescope image ofreflection nebulosity near Merope

(IC 349)

Under ideal observing conditions, some hint of nebulosity may be seen aroundthe cluster, and this shows up in long-exposure photographs. It is a reflectionnebula, caused by dust reflecting the blue light of the hot, young stars.

It was formerly thought that the dust was left over from the formation of thecluster, but at the age of about 100 million years generally accepted for thecluster, almost all the dust originally present would have been dispersed byradiation pressure. Instead, it seems that the cluster is simply passing through aparticularly dusty region of the interstellar medium.

Studies show that the dust responsible for the nebulosity is not uniformlydistributed, but is concentrated mainly in two layers along the line of sight to thecluster. These layers may have been formed by deceleration due to radiationpressure as the dust has moved towards the stars.[22]

Brightest stars

The nine brightest stars of the Pleiades are named for the Seven Sisters of Greek mythology: Sterope, Merope,Electra, Maia, Taygeta, Celaeno, and Alcyone, along with their parents Atlas and Pleione. As daughters of Atlas, theHyades were sisters of the Pleiades. The English name of the cluster itself is of Greek origin, though of uncertainetymology. Suggested derivations include: from πλεîν pleîn, to sail, making the Pleiades the "sailing ones"; frompleos, full or many; or from peleiades, flock of doves. The following table gives details of the brightest stars in thecluster:

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Pleiades 24

A map of the PleiadesThe location of the Pleiades on the constellation

Taurus.

Pleiades Bright Stars

Name Pronunciation (IPA & respelling) Designation Apparent magnitude Stellar classification

Alcyone /ælˈsaɪ.əniː/ al-SY-ə-nee Eta (25) Tauri 2.86 B7IIIe

Atlas /ˈætləs/ AT-ləs 27 Tauri 3.62 B8III

Electra /ɨˈlɛktrə/ i-LEK-trə 17 Tauri 3.70 B6IIIe

Maia /ˈmeɪə/, /ˈmaɪə/ MAY-ə, MY-ə 20 Tauri 3.86 B7III

Merope /ˈmɛrəpiː/ MERR-ə-pee 23 Tauri 4.17 B6IVev

Taygeta /teɪˈɪdʒɨtə/ tay-IJ-i-tə 19 Tauri 4.29 B6V

Pleione /ˈplaɪ.əniː/ PLY-ə-nee 28 (BU) Tauri 5.09 (var.) B8IVpe

Celaeno /sɨˈliːnoʊ/ sə-LEE-noh 16 Tauri 5.44 B7IV

Sterope, Asterope /ˈstɛrɵpiː/, /əˈstɛrɵpiː/ (ə)-STERR-ə-pee 21 and 22 Tauri 5.64;6.41 B8Ve/B9V

— — 18 Tauri 5.65 B8V

Possible planetsAnalyzing deep-infrared images obtained by the Spitzer Space Telescope and Gemini North telescope, astronomersdiscovered that one of cluster's star - HD 23514, which has a mass and luminosity a bit greater than those of the sun,is surrounded by an extraordinary number of hot dust particles. This could be an evidence for planets formationaround HD 23514.[23]

References[1] "SIMBAD Astronomical Database" (http:/ / simbad. u-strasbg. fr/ Simbad). Results for M45. . Retrieved 2007-04-20.[2] van Leeuwen, F. "Parallaxes and proper motions for 20 open clusters as based on the new Hipparcos catalogue" (http:/ / adsabs. harvard. edu/

abs/ 2009A& A. . . 497. . 209V), A\&A, 2009[3] Majaess, D.; Turner, D.; Lane, D.; Krajci, T. "Deep Infrared ZAMS Fits to Benchmark Open Clusters Hosting delta Scuti Stars" (http:/ /

adsabs. harvard. edu/ cgi-bin/ bib_query?arXiv:1102. 1705), JAAVSO, 2011[4] Percival, S. M.; Salaris, M.; Groenewegen, M. A. T. (2005). "The distance to the Pleiades. Main sequence fitting in the near infrared".

Astronomy and Astrophysics 429: 887. arXiv:astro-ph/0409362. Bibcode 2005A&A...429..887P. doi:10.1051/0004-6361:20041694.[5] Zwahlen, N.; North, P.; Debernardi, Y.; Eyer, L.; Galland, F.; Groenewegen, M. A. T.; Hummel, C. A. (2004). "A purely geometric distance

to the binary star Atlas, a member of the Pleiades". Astronomy and Astrophysics Letters 425: L45. arXiv:astro-ph/0408430.Bibcode 2004A&A...425L..45Z. doi:10.1051/0004-6361:200400062.

[6] Messier 45 (http:/ / www. seds. org/ messier/ m/ m045. html)[7] Kroupa, P., Aarseth, S.J., Hurley, J. 2001, MNRAS, 321, 699, "The formation of a bound star cluster: from the Orion nebula cluster to the

Pleiades" (http:/ / adsabs. harvard. edu/ abs/ 2001MNRAS. 321. . 699K)[8] http:/ / www. ancientlibrary. com/ geoponica/ 0028. html

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[9] On the Rising of the Pleiades (http:/ / evols. library. manoa. hawaii. edu/ bitstream/ handle/ 10524/ 105/ JL27179. pdf?sequence=2)[10] Michell J. (1767). "An Inquiry into the probable Parallax, and Magnitude, of the Fixed Stars, from the Quantity of Light which they afford

us, and the particular Circumstances of their Situation". Philosophical Transactions 57: 234–264. Bibcode 1767RSPT...57..234M.doi:10.1098/rstl.1767.0028.

[11] Frommert, Hartmut (1998). "Messier Questions & Answers" (http:/ / www. seds. org/ messier/ m-q& a. html#why_M42-45). . Retrieved2005-03-01.

[12] A New review: with literary curiosities and literary intelligence, page 326, Paul Henry Maty, Printed for the author, 1783.[13] Mémoires de l'Acadêmie des sciences de l'Institut de France, page 289, Didot frères, fils et cie, 1786.[14] Edme-Sébastien Jeaurat, Carte des 64 Principales Etoiles des Playades par M. Jeaurat, pour le 1.er Janvier 1786.[15] Soderblom D. R., Nelan E., Benedict G. F., McArthur B., Ramirez I., Spiesman W., Jones B. F. (2005). "Confirmation of Errors in

Hipparcos Parallaxes from Hubble Space Telescope Fine Guidance Sensor Astrometry of the Pleiades". Astronomical Journal 129:1616–1624. arXiv:astro-ph/0412093. Bibcode 2005AJ....129.1616S. doi:10.1086/427860.

[16] Turner, D. G. (1979). "A reddening-free main sequence for the Pleiades cluster". Publications of the Astronomical Society of the Pacific 91:642–647. Bibcode 1979PASP...91..642T. doi:10.1086/130556.

[17] Francis C., Anderson E., (2012). "XHIP II: clusters and associations". Astronomy Letters. arXiv:1203.4945. Bibcode 2012arXiv1203.4945F.[18] Adams, Joseph D.; Stauffer, John R.; Monet, David G.; Skrutskie, Michael F.; Beichman, Charles A. (2001). "The Mass and Structure of the

Pleiades Star Cluster from 2MASS". Astronomical Journal 121: 2053. arXiv:astro-ph/0101139. Bibcode 2001AJ....121.2053A.doi:10.1086/319965.

[19] Moraux, E.; Bouvier, J.; Stauffer, J. R.; Cuillandre, J.-C. (2003). "Brown in the Pleiades cluster: Clues to the substellar mass function".Astronomy and Astrophysics 400: 891. arXiv:astro-ph/0212571. Bibcode 2003A&A...400..891M. doi:10.1051/0004-6361:20021903.

[20] Basri G., Marcy G. W., Graham J. R. (1996). "Lithium in Brown Dwarf Candidates: The Mass and Age of the Faintest Pleiades Stars".Astrophysical Journal 458: 600. Bibcode 1996ApJ...458..600B. doi:10.1086/176842.

[21] Ushomirsky, G., Matzner, C., Brown, E., Bildsten, L., Hilliard, V., Schroeder, P. (1998). "Light-Element Depletion in Contracting BrownDwarfs and Pre-Main-Sequence Stars". Astrophysical Journal 497: 253. arXiv:astro-ph/9711099. Bibcode 1998ApJ...497..253U.doi:10.1086/305457.

[22] Gibson, Steven J.; Nordsieck, Kenneth H. (2003). "The Pleiades Reflection Nebula. II. Simple Model Constraints on Dust Properties andScattering Geometry". Astrophysical Journal 589: 362. Bibcode 2003ApJ...589..362G. doi:10.1086/374590.

[23] ScienceDaily (2007). "Planets Forming In Pleiades Star Cluster, Astronomers Report" (http:/ / www. sciencedaily. com/ releases/ 2007/ 11/071114203718. htm). . Retrieved 2012-11-15.

External links• The Pleiades (M45) At the astro-photography site of Mr. T. Yoshida. (http:/ / ryutao. main. jp/ english/ stl_m45.

html)• Photos and information on the Pleiades from the University of Calgary (http:/ / www. ras. ucalgary. ca/ ~gibson/

pleiades/ )• Information on the Pleiades from SEDS (http:/ / www. seds. org/ messier/ m/ m045. html)• Information and images from the Anglo-Australian Observatory (http:/ / www. aao. gov. au/ images. html/

captions/ uks018. html)• NightSkyInfo.com: The Pleiades (http:/ / www. nightskyinfo. com/ archive/ pleiades/ )• Maya Astronomy (http:/ / www. authenticmaya. com/ maya_astronomy. htm)• Doppler Imaging: Results (http:/ / www. aip. de/ groups/ activity/ DI/ results. html) first Doppler image of a

Pleiades solar-type G dwarf – HII314, Strassmeier & Rice 2001, A&A 377, 264• Dark Atmospheres Photography (deep nebulosity exposure) (http:/ / www. darkatmospheres. com/ astro/ gallery/

nebulae/ enlarge. php?fileBase=nebulae_1)• not available november 24, 2012: Pleiades in Hungarian and Middle Asian culture (http:/ / www. youtube. com/

watch?v=0lyvX7KUIA8& feature=related)• The Pleiades on WikiSky (http:/ / www. wikisky. org/ ?object=Pleiades)• WEBDA open cluster database webpage on Pleiades cluster (http:/ / www. univie. ac. at/ webda/ cgi-bin/

ocl_page. cgi?dirname=mel022) – E. Pauzen (Univ. Vienna)

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Orion (constellation) 26

Orion (constellation)

Orion

Constellation

List of stars in OrionAbbreviation Ori

Genitive Orionis

Pronunciation /ɒˈraɪ.ən/

Symbolism Orion, the Hunter

Right ascension 5 h

Declination +5°

Quadrant NQ1

Area 594 sq. deg. (26th)

Main stars 7

Bayer/Flamsteedstars

81

Stars with planets 8

Stars brighter than 3.00m 8

Stars within 10.00 pc (32.62 ly) 8

Brightest star Rigel (β Ori) (0.12m)

Nearest star GJ 3379(17.51 ly, 5.37 pc)

Messier objects 3

Meteor showers OrionidsChi Orionids

Borderingconstellations

GeminiTaurusEridanusLepusMonoceros

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Orion (constellation) 27

Visible at latitudes between +85° and −75°.Best visible at 21:00 (9 p.m.) during the month of January.

Orion, sometimes subtitled The Hunter, is a prominent constellation located on the celestial equator and visiblethroughout the world. It is one of the most conspicuous, and most recognizable constellations in the night sky.[1] Itsname refers to Orion, a hunter in Greek mythology. Its brightest stars are Beta (Rigel) and Alpha (Betelgeuse), ablue-white and red supergiant respectively. Many other of the brightest stars in the constellation are hot bluesupergiant stars.

Characteristics

Orion as depicted in Urania's Mirror [2], a set ofconstellation cards published in London c.1825

In artistic renderings, the surrounding constellations aresometimes related to Orion: he is depicted standing next to theriver Eridanus with his two hunting dogs Canis Major and CanisMinor, fighting Taurus the bull. He is sometimes depictedhunting Lepus the hare. He also sometimes is depicted to have alion's hide in his hand.

There are alternative ways to visualise Orion. From the SouthernHemisphere, Orion is oriented south-upward, and the belt andsword are sometimes called the saucepan or pot in Australia andNew Zealand. Orion's Belt is called Drie Konings (Three Kings)or the Drie Susters (Three Sisters) by Afrikaans speakers inSouth Africa[3] and are referred to as les Trois Rois (the ThreeKings) in Daudet's Lettres de Mon Moulin (1866). Theappellation Driekoningen (the Three Kings) is also often foundin 17th- and 18th-century Dutch star charts and seaman's guides.The same three stars are known in Spain and Latin America as"Las Tres Marías".

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VisibilityOrion can be easily seen in the night sky from November to February of each year – late autumn to winter in theNorthern Hemisphere, late spring to summer in the Southern Hemisphere. In the tropics (less than about 8° from theequator) the constellation transits at the zenith.In the period May–July (summer in the Northern Hemisphere, winter in the Southern Hemisphere) Orion is in thedaytime sky and thus not visible at most latitudes. However for much of Antarctica in the Southern Hemisphere'swinter months, the Sun is below the horizon even at midday. Stars (and thus Orion) are then visible at twilight for afew hours around local noon, low in the North. At the same time of day at the South Pole itself (Amundsen–ScottSouth Pole Station), Rigel is only 8° above the horizon and the Belt sweeps just along it. In the SouthernHemisphere's summer months, when Orion is normally visible in the night sky, the constellation is actually notvisible in Antarctica because the sun does not set at that time of year south of the Antarctic Circle.[4][5]

In countries close to the equator (e.g. Kenya, Indonesia, Colombia, Ecuador) Orion appears overhead in Decemberaround midnight and in the February evening sky.

Navigational aid

Using Orion to find stars in neighbor constellations

Orion is very useful as an aid to locating other stars. Byextending the line of the Belt southeastward, Sirius (α CMa)can be found; northwestward, Aldebaran (α Tau). A lineeastward across the two shoulders indicates the direction ofProcyon (α CMi). A line from Rigel through Betelgeusepoints to Castor and Pollux (α Gem and β Gem). Additionally,Rigel is part of the Winter Circle. Sirius and Procyon, whichmay be located from Orion by following imaginary lines (seemap), also are points in both the Winter Triangle and theCircle.[6]

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Notable features

The constellation Orion as it can be seen by the naked eye.

Orion's seven brightest stars form a distinctivehourglass-shaped asterism, or pattern, in the night sky. Fourstars—Rigel, Betelgeuse, Bellatrix and Saiph—form a largeroughly rectangular shape, in the centre of which lie the threestars of Orion's Belt—Alnitak, Alnilam and Mintaka.Descending from the 'belt' is a smaller line of three stars (themiddle of which is in fact not a star but the Orion Nebula),known as the hunter's 'sword'.

Many of the stars are luminous hot blue supergiants, with thestars of the belt and sword forming the Orion OB1Association. Standing out by its red hue, Betelgeuse maynevertheless be a runaway member of the same group.

Stars

• Betelgeuse, known alternatively by its Bayer designationAlpha Orionis, is a massive M-type red supergiant starnearing the end of its life. When it explodes it will even bevisible during the day. It is the second brightest star inOrion, and is a semiregular variable star.[7] It serves as the"right shoulder" of the hunter it represents (assuming that

he is facing the observer), and is the eighth brightest star in the night sky.[8]

• Rigel, which is also known as Beta Orionis, is a B-type blue supergiant that is the sixth brightest star in the nightsky. Similar to Betelgeuse, Rigel is fusing heavy elements in its core and will pass its supergiant stage soon (on anastronomical timescale), either collapsing in the case of a supernova or shedding its outer layers and turning into awhite dwarf. It serves as the left foot of Orion, the hunter.[9]

• Bellatrix was designated Gamma Orionis by Johann Bayer, but is known colloquially as the "Amazon Star". It isthe twenty-seventh brightest star in the night sky.[10] Bellatrix is considered a B-type blue giant, though it is toosmall to explode in a supernova. Bellatrix's luminosity is derived from its high temperature rather than itsradius,[11] a factor that defines Betelgeuse.[7] Bellatrix serves as Orion's left shoulder.[11]

• Mintaka garnered the name Delta Orionis from Bayer, even though it is the faintest of the three stars in Orion'sBelt. It is a multiple star system, composed of a large B-type blue giant and a more massive O-type white star.The Mintaka system constitutes an eclipsing binary variable star, where the eclipse of one star over the othercreates a dip in brightness. Mintaka is the westernmost of the three stars of Orion's Belt.[12]

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Orion Constellation Map

• Alnilam was named Epsilon Orionis, a consequence of Bayer's wishto name the three stars in Orion's Belt (from north to south) inalphabetical order. Alnilam is a B-type blue supergiant; despitebeing nearly twice as far from the Sun as Mintaka and Alnitak, theother two belt stars, its luminosity makes it nearly equal inmagnitude. Alnilam is losing mass quickly, a consequence of itssize; it is approximately four million years old.[13]

• Alnitak was designated Zeta Orionis by Bayer, and is theeasternmost star in Orion's Belt. It is a triple star some 800 lightyears distant, with the primary star being a hot blue supergiant andthe brightest class O star in the night sky.

• Saiph was designated Kappa Orionis by Bayer, and serves asOrion's right foot. It is of a similar distance and size to Rigel, butappears much fainter, as its hot surface temperature (46,000°F or26,000°C) causes it to emit most of its light in the ultraviolet regionof the spectrum.

Of the lesser stars, Hatsya (or Iota Orionis) forms the tip of Orion's sword, whilst Meissa (or Lambda Orionis) formsOrion's head. In common with many other bright stars, the names Betelgeuse, Rigel, Saiph, Alnitak, Mintaka,Alnilam, Hatsya, and Meissa originate from the Arabic language.

ProperName

Solar Radii ApparentMagnitude

~Distance(L Yrs)

  Betelgeuse 667 0.43 643

  Rigel 78 0.18 772

  Bellatrix 7.0 1.62 243

  Mintaka ? 2.23 (3.2/3.3) / 6.85 / 14.0 900

  Alnilam 26 1.68 1359

  Alnitak ? 1.70/~4/4.21 800

  Saiph 11 2.06 724

Belt

Orion's Belt

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Closeup Image of Orion's Belt

Orion's Belt or The Belt of Orion is an asterism within the constellation. It consists of the three bright stars ζ Ori(Alnitak), ε Ori (Alnilam), and δ Ori (Mintaka). Alnitak is approximately 800 light years away from earth and,including ultraviolet radiation, which the human eye cannot see, Alnitak is 100,000 times more luminous than theSun.[14] Alnilam is approximately 1340 light years away from Earth, shines with magnitude 1.70, and withultraviolet light is 375,000 times more luminous than the Sun.[13] Mintaka is 915 light years away and shines withmagnitude 2.21. It is 90,000 times more luminous than the Sun and is a double star: the two orbit each other every5.73 days.[12] Looking for Orion's Belt in the night sky is the easiest way to locate the constellation. In the NorthernHemisphere, Orion's Belt is best visible in the night sky during the month of January around 9:00 pm, when it isapproximately around the local meridian.[1]

Just southwest of Alnitak lies Sigma Orionis, a multiple star system composed of five stars which have a combinedapparent magnitude of 3.7, and lying 1150 light years distant. Southwest of Mintaka lies the quadruple star EtaOrionis.

HeadThree stars compose a small triangle which mark the head. The apex is marked by Meissa (Lambda Orionis), a hotblue giant of spectral type O8 III and apparent magnitude 3.54 which lies some 1100 light years distant. Phi-1 andPhi-2 Orionis make up the base. Also nearby is the very young star FU Orionis.

ClubStretching north from Betelgeuse are the stars that make up Orion's club. Mu Orionis marks the elbow, Nu and Ximark the handle of the club, and Chi1 and Chi2 mark the end of the club. Just east of Chi1 is the Mira-type variablered gaint U Orionis.

ShieldWest from Bellatrix lie six stars all designated Pi Orionis (π1 Ori,π2 Ori, π3 Ori, π4 Ori, π5 Ori and π6 Ori) whichmake up Orion's shield.

Meteor showersAround 20 October each year the Orionid meteor shower (Orionids) reaches its peak. Coming from the border withthe constellation Gemini as many as 20 meteors per hour can be seen. The shower's parent body is Halley'sComet.[15]

Deep-sky objectsHanging from Orion's belt is his sword, consisting of the multiple stars θ1 and θ2 Orionis, called the Trapezium andthe Orion Nebula (M42). This is a spectacular object that can be clearly identified with the naked eye as somethingother than a star. Using binoculars, its clouds of nascent stars, luminous gas, and dust can be observed. TheTrapezium cluster has many newborn stars, including several brown dwarfs, all of which are at an approximatedistance of 1,500 light-years. Named for the four bright stars that form a trapezoid, it is largely illuminated by thebrightest stars, which are only a few hundred thousand years old. Observations by the Chandra X-ray Observatoryshow both the extreme temperatures of the main stars—up to 60,000 Kelvin—and the star forming regions stillextant in the surrounding nebula.[16]

M78 (NGC 2068) is a nebula in Orion. With an overall magnitude of 8.0, it is significantly dimmer than the Great Orion Nebula that lies to its south; however, it is at approximately the same distance, at 1600 light-years from Earth. It can easily be mistaken for a comet in the eyepiece of a telescope. M78 is associated with the variable star V351

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Orionis, whose magnitude changes are visible in very short periods of time.[17] Another fairly bright nebula in Orionis NGC 1999, also close to the Great Orion Nebula. It has an integrated magnitude of 10.5 and is 1500 light-yearsfrom Earth. The variable star V380 Orionis is embedded in NGC 1999.[18]

Another famous nebula is IC 434, the Horsehead Nebula, near ζ Orionis. It contains a dark dust cloud whose shapegives the nebula its name.Besides these nebulae, surveying Orion with a small telescope will reveal a wealth of interesting deep-sky objects,including M43, M78, as well as multiple stars including Iota Orionis and Sigma Orionis. A larger telescope mayreveal objects such as Barnard's Loop and the Flame Nebula (NGC 2024), as well as fainter and tighter multiple starsand nebulae.All of these nebulae are part of the larger Orion Molecular Cloud Complex, which is located approximately 1,500light-years away and is hundreds of light-years across. It is one of the most intense regions of stellar formationvisible in our galaxy.

Cultural significance

Star formation in the constellation Orion as photographed in infraredby NASA's Spitzer Space Telescope.

The distinctive pattern of Orion has been recognized innumerous cultures around the world, and many mythshave been associated with it. It has also been used as asymbol in the modern world.

Mediterranean

Ancient Near East

The Babylonian star catalogues of the Late Bronze Agename Orion MULSIPA.ZI.AN.NA,[19] "The HeavenlyShepherd" or "True Shepherd of Anu" - Anu being thechief god of the heavenly realms.[20] The Babylonian constellation was sacred to Papshukal and Ninshubur, bothminor gods fulfilling the role of 'messenger to the gods'. Papshukal was closely associated with the figure of awalking bird on Babylonian boundary stones, and on the star map the figure of the Rooster was located below andbehind the figure of the True Shepherd—both constellations represent the herald of the gods, in his bird and humanforms respectively.[21]

The stars of Orion were associated with Osiris, the sun-god of rebirth and afterlife, by the ancientEgyptians.[22][23][24]

Orion has also been identified with the Egyptian Pharaoh of the Fifth Dynasty called Unas who, according to thePyramid Texts, became great by eating the flesh of his mortal enemies and then slaying and devouring the godsthemselves. This was based on a belief in contiguous magic whereby consuming the flesh of great people wouldbring inheritance of their power.[23] After devouring the gods and absorbing their spirits and powers, Unas journeysthrough the day and night sky to become the star Sahu, or Orion.[22] The Pyramid Texts also show that the deadPharaoh was identified with the god Osiris, whose form in the stars was often said to be the constellation Orion.[22]

The Armenians identified their forefather Hayk with Orion. Hayk is also the name of the Orion constellation in theArmenian translation of the Bible.[25]

The Bible mentions Orion three times, naming it "Kesil" (כסיל, literally - fool). Though, this name perhaps is etymologically connected with "Kislev", the name for the ninth month of the Hebrew calendar (i.e. November–December), which, in turn, may derive from the Hebrew root K-S-L as in the words "kesel, kisla" (כֵּסֶל, hope, positiveness), i.e. hope for winter rains.): Job 9:9 ("He is the maker of the Bear and Orion"), Job 38:31 ,כִּסְלָה

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("Can you loosen Orion`s belt?"), and Amos 5:8 ("He who made the Pleiades and Orion"). In ancient Aram, theconstellation was known as Nephîlā′, the Nephilim may have been Orion's descendants.[26]

Greco-Roman antiquity

Orion in the 9th century Leiden Aratea.

Orion's current name derives from Greek mythology, in which Orionwas a gigantic hunter of primordial times.[27] Some of these mythsrelate to the constellation; one story tells that Orion was killed by agiant scorpion; the gods raised him and the Scorpion to the skies, asScorpio/Scorpius. Yet other stories say Orion was chasing thePleiades.[28]

The constellation is mentioned in Horace's Odes (Ode 3.27.18),Homer's Odyssey (Book 5, line 283) and Iliad, and Virgil's Aeneid(Book 1, line 535)

Africa

In ancient Egypt, the constellation of Orion was known to representOsiris, who, after being killed by his evil brother Set, was revived byhis wife Isis to live immortal among the stars.[29]

Middle EastIn medieval Muslim astronomy, Orion was known as al-jabbar "the giant".

Asian antiquity

In China, Orion was one of the 28 lunar mansions Sieu (Xiu) ( 宿 ). It is known as Shen ( 參 ), literallymeaning "three", for the stars of Orion's Belt. (See Chinese constellations)The Chinese character 參 (pinyin shēn) originally meant the constellation Orion (Chinese: 參 宿; pinyin: shēnxiù);its Shang dynasty version, over three millennia old, contains at the top a representation of the three stars of Orion'sbelt atop a man's head (the bottom portion representing the sound of the word was added later).[30]

The Rig Veda refers to the Orion Constellation as Mriga (The Deer).[31]

The Malay called Orion' Belt Bintang Tiga Beradik (the "Three Brother Star").

European folkloreIn old Hungarian tradition, "Orion" is known as (magic) Archer (Íjász), or Reaper (Kaszás). In recently rediscoveredmyths he is called Nimrod (Hungarian "Nimród"), the greatest hunter, father of the twins "Hunor" and "Magor").The "π" and "o" stars (on upper right) form together the reflex bow or the lifted scythe. In other Hungarian traditions,"Orion's belt" is known as "Judge's stick" (Bírópálca).[32]

In Scandinavian tradition, "Orion's belt" was known as Frigg's Distaff (Friggerock) or Freyja's distaff.[33]

The Finns call the Orion's belt and the stars below it as Väinämöisen viikate (Väinämöinen's scythe).[34] Anothername for the asterism of Alnilam, Alnitak and Minkata is Väinämöisen vyö' (Väinämöinen's Belt) and the stars"hanging" from the belt as Kalevanmiekka (Kaleva's sword).

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New WorldThe Seri people of northwestern Mexico call the three stars in the belt of this constellation Hapj (a name denoting ahunter) which consists of three stars: Hap (mule deer), Haamoja (pronghorn), and Mojet (bighorn sheep). Hap is inthe middle and has been shot by the hunter; its blood has dripped onto Tiburón Island.[35]

The same three stars are known in Latin America as "The Three Marys."[36]

The Ojibwa (Chippewa) Native Americans call this constellation Kabibona'kan, the Winter Maker, as its presence inthe night sky heralds winter.To the Lakota Native Americans, Tayamnicankhu (Orion’s Belt) is the spine of a bison. The great rectangle of Orionare the bison's ribs; Orion's belt forms the bison's spine; The Pleiades star cluster in nearby Taurus is the bison’s headand Sirius in Canis Major, known as Tayamnisinte, is its tail.

Contemporary symbolismThe imagery of the belt and sword has found its way into popular western culture, for example in the form of theshoulder insignia of the 27th Infantry Division of the United States Army during both World Wars, probably owingto a pun on the name of the division's first commander, Major General John F. O'Ryan.The defunct film distribution company Orion Pictures used the constellation as its logo.

In fiction

In J. R. R. Tolkien's mythology surrounding Middle-earth, Orion is known as Menelvagor, which is Sindarin for"The Swordsman in the Sky."[37]

In J.K. Rowling's Harry Potter series, one of the main Death Eater characters, Bellatrix Lestrange, is named after thegamma star in Orion.

FutureOrion is located on the celestial equator, but it will not always be so located due to the effects of precession of theEarth's axis. Orion lies well south of the ecliptic, and it only happens to lie on the celestial equator because the pointon the ecliptic that corresponds to the June solstice is close to the border of Gemini and Taurus, to the north ofOrion. Precession will eventually carry Orion further south, and by AD 14000 Orion will be far enough south that itwill become invisible from the latitude of Great Britain.[38]

Further in the future, Orion's stars will gradually move away from the constellation due to proper motion. However,Orion's brightest stars all lie at a large distance from the Earth on an astronomical scale—much farther away thanSirius, for example. Orion will still be recognizable long after most of the other constellations—composed ofrelatively nearby stars—have distorted into new configurations, with the exception of a few of its stars eventuallyexploding as supernovae, for example Betelgeuse, which is predicted to explode sometime in the next millionyears.[39]

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ReferencesExplanatory notes[1] Dolan, Chris. "Orion" (http:/ / www. astro. wisc. edu/ ~dolan/ constellations/ constellations/ Orion. html). Archived (http:/ / www.

webcitation. org/ 63XCardIx) from the original on 2011-11-28. . Retrieved 2011-11-28.[2] http:/ / www. ianridpath. com/ atlases/ urania. htm[3] Three Kings and the Cape Clouds at psychohistorian.org (http:/ / www. psychohistorian. org/ astronomy/ ethnoastronomy/

three_kings_cape_clouds. php)[4] A Beginner's Guide to the Heavens in the Southern Hemisphere (http:/ / www. dibonsmith. com/ downunder. htm)[5] The Evening Sky Map Southern Hemisphere Edition (http:/ / skymaps. com/ downloads. html)[6] Orion Constellation (http:/ / www. donnayoung. org/ science/ stars/ orion. htm)[7] "Variable Star of the Month, Alpha Ori" (http:/ / www. aavso. org/ vstar/ vsots/ 1200. shtml). Variable Star of the Season. American

Association of Variable Star Observers. 2000. . Retrieved 2009-02-26.[8] "Betelgeuse" (http:/ / www. astro. wisc. edu/ ~dolan/ constellations/ hr/ 2061. html). Chris Dolan's Constellations. University of Wisconsin.

2009. . Retrieved 2009-02-26.[9] "Rigel" (http:/ / www. astro. uiuc. edu/ ~jkaler/ sow/ rigel. html). Jim Kaler's Stars. University of Illinois, Urbana-Champaign Campus. 2009.

. Retrieved 2009-02-26.[10] "Bellatrix" (http:/ / www. astro. wisc. edu/ ~dolan/ constellations/ hr/ 1790. html). Chris Dolan's Constellations. University of Wisconsin.

2009. . Retrieved 2009-02-26.[11] "Bellatrix" (http:/ / www. astro. uiuc. edu/ ~jkaler/ sow/ bellatrix. html). Jim Kaler's Stars. University of Illinois, Urbana-Champaign

Campus. 2009. . Retrieved 2009-02-26.[12] "Mintaka" (http:/ / stars. astro. illinois. edu/ sow/ mintaka. html). Jim Kaler's Stars. University of Illinois, Urbana-Champaign Campus.

2009. Archived (http:/ / www. webcitation. org/ 63XCA2Co1) from the original on 2011-11-28. . Retrieved 2011-11-28.[13] "Alnilam" (http:/ / stars. astro. illinois. edu/ sow/ alnilam. html). Jim Kaler's Stars. University of Illinois, Urbana-Champaign Campus. 2009.

Archived (http:/ / www. webcitation. org/ 63XBTECPT) from the original on 2011-11-28. . Retrieved 2011-11-28.[14] "Alnitak" (http:/ / stars. astro. illinois. edu/ sow/ alnitak. html). Stars.astro.illinois.edu. . Retrieved 2012-05-16.[15] Jenniskens, Peter (September 2012). "Mapping Meteoroid Orbits: New Meteor Showers Discovered". Sky & Telescope: 22.[16] Wilkins, Jamie; Dunn, Robert (2006). 300 Astronomical Objects: A Visual Reference to the Universe (1st ed.). Buffalo, New York: Firefly

Books. ISBN 978-1-55407-175-3.[17] Levy 2005, pp. 99–100[18][18] Levy 2005, p. 107

[19] The determiner glyph for "constellation" or "star" in these lists is MUL (ሂ�). See Babylonian star catalogues.[20] John H. Rogers, " Origins of the ancient contellations: I. The Mesopotamian traditions (http:/ / adsabs. harvard. edu/ abs/ 1998JBAA. . 108. .

. . 9R)", Journal of the British Astronomical Association 108 (1998) 9–28[21] Babylonian Star-lore by Gavin White, Solaria Pubs, 2008, page 218ff & 170[22][22] The Oxford Guide: Essential Guide to Egyptian Mythology, Edited by Donald B. Redford, p302-307, Berkley, 2003, ISBN 0-425-19096-X[23] Mackenzie, Donald A. (1907). "Triumph of the Sun God" (http:/ / www. sacred-texts. com/ egy/ eml/ eml15. htm). Egyptian Myth and

Legend. Gresham Pub. Co.. pp. 167–168. ISBN 0-517-25912-5. .[24] http:/ / www. coldwaterschools. org/ lms/ planetarium/ myth/ orion. html; http:/ / www. constellationsofwords. com/ Constellations/ Orion.

html[25] Vahan Kurkjian, "History of Armenia," Michigan, 1968 (http:/ / penelope. uchicago. edu/ Thayer/ E/ Gazetteer/ Places/ Asia/ Armenia/

_Texts/ KURARM/ 8*. html)[26] Peake's commentary on the Bible, 1962, page 260 section 221f.[27] Star Tales – Orion (http:/ / www. ianridpath. com/ startales/ orion. htm)[28] Chandra :: Photo Album :: Constellation Orion (http:/ / chandra. harvard. edu/ photo/ constellations/ orion. html)[29][29] Mystery of the Sphinx. Documentary 2005. Morningstar Entertainment.[30] 漢 語 大 字 典 Hànyǔ Dàzìdiǎn (in Chinese), 1992 (p.163). 湖 北 辭 書 出 版 社 和 四 川 辭 書 出 版 社 Húbĕi Cishu

Chūbǎnshè and Sìchuān Cishu Chūbǎnshè, re-published in traditional character form by 建 宏 出 版 社 Jiànhóng Publ. in Taipei, Taiwan;ISBN 957-813-478-9

[31] Holay, P. V.. "Vedic astronomers". Bulletin of the Astronomical Society of India 26: 91–106. Bibcode 1998BASI...26...91H.[32][32] Toroczkai-Wigand Ede : Öreg csillagok ("Old stars"), Hungary (1915) reedited with Műszaki Könyvkiadó METRUM (1988).[33] Schön, Ebbe. (2004). Asa-Tors hammare, Gudar och jättar i tro och tradition. Fält & Hässler, Värnamo. p. 228.[34] http:/ / www. ursa. fi/ yhd/ uranus/ luennot%202004/ perusteet/ tahdet. htm[35] Moser, Mary B.; Stephen A. Marlett (2005) (in Spanish and English). Comcáac quih yaza quih hant ihíip hac: Diccionario

seri-español-inglés (http:/ / lengamer. org/ admin/ language_folders/ seri/ user_uploaded_files/ links/ File/ DiccionarioSeri2005. pdf).Hermosillo, Sonora and Mexico City: Universidad de Sonora and Plaza y Valdés Editores. .

[36] Lenda de Órion e as Três Marias (http:/ / www. sobrenatural. org/ site\Lendas\Lendas_Descricao. asp?Str_ID=4143)[37] "Encyclopedia of Arda: Swordsman of the Sky" (http:/ / www. glyphweb. com/ arda/ s/ swordsmanofthesky. html). Glyphweb.com.

1999-11-27. . Retrieved 2012-05-16.

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[38] "Precession" (http:/ / myweb. tiscali. co. uk/ moonkmft/ Articles/ Precession. html). Myweb.tiscali.co.uk. . Retrieved 2012-05-16.[39] Wilkins, Alasdair. "Earth may soon have a second sun" (http:/ / io9. com/ 5738542/ earth-may-soon-have-a-second-sun). io9. Space Porn. .

CitationsBibliography• Levy, David H. (2005). Deep Sky Objects. Prometheus Books. ISBN 1-59102-361-0.• Ian Ridpath and Wil Tirion (2007). Stars and Planets Guide, Collins, London. ISBN 978-0-00-725120-9.

Princeton Universitl Press, Princeton. ISBN 978-0-691-13556-4.

External links• The Deep Photographic Guide to the Constellations: Orion (http:/ / www. allthesky. com/ constellations/ orion/ )• Melbourne Planetarium: Orion Sky Tour (http:/ / museumvictoria. com. au/ DiscoveryCentre/ Infosheets/ Planets/

Sky-Tour-for-Binoculars---Orion/ )• Views of Orion from other places in our Galaxy (http:/ / old. orionsarm. com/ galactography/ views_of_Orion.

html)• The clickable Orion (http:/ / 160. 114. 99. 91/ astrojan/ orion. htm)• Star Tales – Orion (http:/ / www. ianridpath. com/ startales/ orion. htm)• Deep Widefield image of Orion (http:/ / blog. deepskycolors. com/ archive/ 2010/ 10/ 22/

orion-from-Head-to-Toes. html)• Constellations of Words. Orion (http:/ / www. constellationsofwords. com/ Constellations/ Orion. html)

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Orion Nebula

Orion Nebula

The entire Orion Nebula in visible light.Credit: NASA/ESA

Observation data: J2000 epoch

Type Reflection and Emission[1]

Right ascension 05h 35m 17.3s[2]

Declination -05° 23′ 28″[2]

Distance 1,344±20 ly (412 pc)[3]

Apparent magnitude (V) +4.0[4]

Apparent dimensions (V) 65×60 arcmins[5]

Constellation Orion

Physical characteristics

Radius 12 ly[a]

Absolute magnitude (V) —

Notable features Trapezium cluster

Other designations NGC 1976, M42,LBN 974, Sharpless 281

The Orion Nebula (also known as Messier 42, M42, or NGC 1976) is a diffuse nebula situated south[b] of Orion'sBelt in the constellation of Orion. It is one of the brightest nebulae, and is visible to the naked eye in the night sky.M42 is located at a distance of 1,344 ± 20 light years[3][6] and is the closest region of massive star formation toEarth. The M42 nebula is estimated to be 24 light years across. It has a mass of about 2000 times the mass of theSun. Older texts frequently refer to the Orion Nebula as the Great Nebula in Orion or the Great Orion Nebula.[7]

The Orion Nebula is one of the most scrutinized and photographed objects in the night sky, and is among the mostintensely studied celestial features.[8] The nebula has revealed much about the process of how stars and planetarysystems are formed from collapsing clouds of gas and dust. Astronomers have directly observed protoplanetarydisks, brown dwarfs, intense and turbulent motions of the gas, and the photo-ionizing effects of massive nearby starsin the nebula. There are also supersonic "bullets" of gas piercing the hydrogen clouds of the Orion Nebula. Eachbullet is ten times the diameter of Pluto's orbit and tipped with iron atoms glowing bright blue. They were probablyformed one thousand years ago from an unknown violent event.[9]

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General information

Discussing the location of the Orion Nebula, what we see within the star-formationregion, and the effects of interstellar winds in shaping the nebula.

The constellation of Orion with the OrionNebula (lower middle).

The nebula is visible with the naked eyeeven from areas affected by some lightpollution. It is seen as the middle "star" inthe sword of Orion, which are the three starslocated south of Orion's Belt. The starappears fuzzy to sharp-eyed observers, andthe nebulosity is obvious through binocularsor a small telescope.

The Orion Nebula contains a very youngopen cluster, known as the Trapezium due tothe asterism of its primary four stars. Two ofthese can be resolved into their componentbinary systems on nights with good seeing,giving a total of six stars. The stars of theTrapezium, along with many other stars, arestill in their early years. The Trapezium maybe a component of the much-larger OrionNebula Cluster, an association of about2,000 stars within a diameter of 20 lightyears. Two million years ago this clustermay have been the home of the runawaystars AE Aurigae, 53 Arietis, and MuColumbae, which are currently movingaway from the nebula at velocities greaterthan 100 km/s.[10]

Coloration

Observers have long noted a distinctivegreenish tint to the nebula, in addition toregions of red and areas of blue-violet. Thered hue is well-understood to be caused byHα recombination line radiation at awavelength of 656.3 nm. The blue-violetcoloration is the reflected radiation from themassive O-class stars at the core of thenebula.

The green hue was a puzzle for astronomersin the early part of the 20th century becausenone of the known spectral lines at that timecould explain it. There was somespeculation that the lines were caused by a

new element, and the name "nebulium" was coined for this mysterious material. With better understanding of atomic

physics, however, it was later determined that the green spectra was caused by a low-probability electron transition in doubly ionized oxygen, a so-called "forbidden transition". This radiation was all but impossible to reproduce in

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the laboratory because it depended on the quiescent and nearly collision-free environment found in deep space.[11]

History

Messier's drawing of the Orion Nebula in his1771 memoir, Mémoires de l'Académie Royale.

There has been speculation that the Mayans of Central America mayhave described the nebula within their "Three Hearthstones" creationmyth; if so, the three would correspond to two stars at the base ofOrion, Rigel and Saiph, and another, Alnitak at the tip of the "belt" ofthe imagined hunter, the vertices of a nearly perfect triangle withOrion's Sword (including the Orion Nebula) in the middle seen as thesmudge of smoke from copal incense in a modern myth, or, in (thetranslation it suggests of) an ancient one, the literal or figurativeembers of a fiery creation.[12][13]

Neither Ptolemy's Almagest nor Al Sufi's Book of Fixed Stars notedthis nebula, even though they both listed patches of nebulosityelsewhere in the night sky; nor did Galileo mention it, even though healso made telescopic observations surrounding it in 1610 and 1617.[14]

This has led to some speculation that a flare-up of the illuminatingstars may have increased the brightness of the nebula.[15]

The first discovery of the diffuse nebulous nature of the Orion Nebula is generally credited to French astronomerNicolas-Claude Fabri de Peiresc, on 26 November 1610 when he made a record of observing it with a refractingtelescope purchased by his patron Guillaume du Vair.[14]

The first published observation of the nebula was by the Jesuit mathematician and astronomer Johann Baptist Cysatof Lucerne in his 1619 monograph on the comets (describing observations of the nebula that may date back to1611).[16] He made comparisons between it and a bright comet seen in 1618 and described how the nebula appearedthrough his telescope as:

"one sees how in like manner some stars are compressed into a very narrow space and how round about andbetween the stars a white light like that of a white cloud is poured out"[17]

His description of the center stars as different from a comets head in that they were a "rectangle" may have been anearly description of the Trapezium Cluster[14][17][18] (The first detection of three of the four stars of this cluster iscredited to Galileo Galilei in a February 4, 1617 although he did not notice the surrounding nebula — possibly dueto the narrow field of vision of his early telescope.[19])The nebula was independently discovered by several other prominent astronomers in the following years, including,in 1656, Christiaan Huygens (whose sketch was the first published, in 1659).Charles Messier first noted the nebula on March 4, 1769, and he also noted three of the stars in Trapezium. CharlesMessier published the first edition of his catalog of deep sky objects in 1774 (completed in 1771).[20] As the OrionNebula was the 42nd object in his list, it became identified as M42.

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Henry Draper's 1880 photograph of the Orion Nebula,the first ever taken.

One of Andrew Ainslie Common's 1883 photograph ofthe Orion Nebula, the first to show that a long exposure

could record new stars and nebulae invisible to thehuman eye.

In 1865 English amateur astronomer William Huggins used hisvisual spectroscopy method to examine the nebula showing it, likeother nebula he had examined, was made up of "luminous gas".[21]

On September 30, 1880 Henry Draper used the new dry platephotographic process with an 11-inch (28 cm) refracting telescopeto make a 51-minute exposure of the Orion Nebula, the firstinstance of astrophotography of a nebula in history. Another set ofphotographs of the nebula in 1883 saw breakthrough inastronomical photography when amateur astronomer AndrewAinslie Common used the dry plate process to record severalimages in exposures up to 60 minutes with a 36-inch (91 cm)reflecting telescope that he constructed in the backyard of hishome in Ealing, outside London. These images for the first timeshowed stars and nebula detail too faint to be seen by the humaneye.[22]

In 1902, Vogel and Eberhard discovered differing velocities withinthe nebula and by 1914 astronomers at Marseilles had used theinterferometer to detect rotation and irregular motions. Campbelland Moore confirmed these results using the spectrograph,demonstrating turbulence within the nebula.[23]

In 1931, Robert J. Trumpler noted that the fainter stars near theTrapezium formed a cluster, and he was the first to name them theTrapezium cluster. Based on their magnitudes and spectral types,he derived a distance estimate of 1,800 light years. This was threetimes further than the commonly accepted distance estimate of theperiod but was much closer to the modern value.[24]

In 1993, the Hubble Space Telescope first observed the OrionNebula. Since then, the nebula has been a frequent target for HSTstudies. The images have been used to build a detailed model of the nebula in three dimensions. Protoplanetary diskshave been observed around most of the newly formed stars in the nebula, and the destructive effects of high levels ofultraviolet energy from the most massive stars have been studied.[25]

In 2005, the Advanced Camera for Surveys instrument of the Hubble Space Telescope finished capturing the mostdetailed image of the nebula yet taken. The image was taken through 104 orbits of the telescope, capturing over3,000 stars down to the 23rd magnitude, including infant brown dwarfs and possible brown dwarf binary stars.[26] Ayear later, scientists working with the HST announced the first ever masses of a pair of eclipsing binary browndwarfs, 2MASS J05352184–0546085. The pair are located in the Orion Nebula and have approximate masses of0.054 M☉ and 0.034 M☉ respectively, with an orbital period of 9.8 days. Surprisingly, the more massive of the twoalso turned out to be the less luminous.[27]

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Structure

Optical images reveal clouds of gas and dust inthe Orion Nebula; an infrared image (right)

reveals the new stars shining within. Credit: C. R.O'Dell-Vanderbilt University, NASA, and ESA.

The entirety of the Orion Nebula extends across a 1° region of the sky,and includes neutral clouds of gas and dust, associations of stars,ionized volumes of gas, and reflection nebulae.

The Nebula is part of a much larger nebula that is known as the OrionMolecular Cloud Complex. The Orion Molecular Cloud Complexextends throughout the constellation of Orion and includes Barnard'sLoop, the Horsehead Nebula, M43, M78, and the Flame Nebula. Starsare forming throughout the Orion Nebula, and due to thisheat-intensive process the region is particularly prominent in theinfrared.

The nebula forms a roughly spherical cloud that peaks in density near the core.[28] The cloud has a temperatureranging up to 10,000 K, but this temperature falls dramatically near the edge of the nebula.[28] Unlike the densitydistribution, the cloud displays a range of velocities and turbulence, particularly around the core region. Relativemovements are up to 10 km/s (22,000 mi/h), with local variations of up to 50 km/s and possibly more.The current astronomical model for the nebula consists of an ionized region roughly centered on Theta1 Orionis C,the star responsible for most of the ultraviolet ionizing radiation. (It emits 3-4 times as much photoionizing light asthe next brightest star, Theta2 Orionis A.)[29] This is surrounded by an irregular, concave bay of more neutral,high-density cloud, with clumps of neutral gas lying outside the bay area. This in turn lies on the perimeter of theOrion Molecular Cloud.Observers have given names to various features in the Orion Nebula. The dark lane that extends from the northtoward the bright region is called the "Fish's Mouth". The illuminated regions to both sides are called the "Wings".Other features include "The Sword", "The Thrust", and "The Sail".[30]

Stellar formation

View of several proplyds within the OrionNebula taken by the Hubble Space Telescope.

Credit:NASA.

The Orion Nebula is an example of a stellar nursery where new starsare being born. Observations of the nebula have revealedapproximately 700 stars in various stages of formation within thenebula.

Recent observations with the Hubble Space Telescope have yielded themajor discovery of protoplanetary disks within the Orion Nebula,which have been dubbed proplyds.[31] HST has revealed more than 150of these within the nebula, and they are considered to be systems in theearliest stages of solar system formation. The sheer numbers of themhave been used as evidence that the formation of star systems is fairlycommon in our universe.

Stars form when clumps of hydrogen and other gases in an H II regioncontract under their own gravity. As the gas collapses, the central clump grows stronger and the gas heats to extremetemperatures by converting gravitational potential energy to thermal energy. If the temperature gets high enough,nuclear fusion will ignite and form a protostar. The protostar is 'born' when it begins to emit enough radiative energyto balance out its gravity and halt gravitational collapse.

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Star Formation Fireworks in Orion.Credit:ESA/Hubble & NASA.

Typically, a cloud of material remains a substantial distance from thestar before the fusion reaction ignites. This remnant cloud is theprotostar's protoplanetary disk, where planets may form. Recentinfrared observations show that dust grains in these protoplanetarydisks are growing, beginning on the path towards formingplanetesimals.[32]

Once the protostar enters into its main sequence phase, it is classifiedas a star. Even though most planetary disks can form planets,observations show that intense stellar radiation should have destroyedany proplyds that formed near the Trapezium group, if the group is asold as the low mass stars in the cluster.[25] Since proplyds are foundvery close to the Trapezium group, it can be argued that those stars aremuch younger than the rest of the cluster members.[c]

Stellar wind and effectsOnce formed, the stars within the nebula emit a stream of charged particles known as a stellar wind. Massive starsand young stars have much stronger stellar winds than the Sun.[33] The wind forms shock waves or hydrodynamicalinstabilities when it encounters the gas in the nebula, which then shapes the gas clouds. The shock waves from stellarwind also play a large part in stellar formation by compacting the gas clouds, creating density inhomogeneities thatlead to gravitational collapse of the cloud.

Herbig-Haro 47 seen with a bow shock and aseries of jet-driven shocks.[34]

There are three different kinds of shocks in the Orion Nebula. Manyare featured in Herbig-Haro objects:[35]

• Bow shocks are stationary and are formed when two particlestreams collide with each other. They are present near the hotteststars in the nebula where the stellar wind speed is estimated to bethousands of kilometers per second and in the outer parts of thenebula where the speeds are tens of kilometers per second. Bowshocks can also form at the front end of stellar jets when the jet hitsinterstellar particles.

• Jet-driven shocks are formed from jets of material sprouting off newborn T Tauri stars. These narrow streams aretraveling at hundreds of kilometers per second, and become shocks when they encounter relatively stationarygases.

View of the ripples (Kelvin–Helmholtzinstability) formed by the action of stellar winds

on the cloud. Credit:ESO/J. Emerson/VISTA.

•• Warped shocks appear bow-like to an observer. They are producedwhen a jet-driven shock encounters gas moving in a cross-current.

• The interaction of the stellar wind with the surrounding cloud alsoforms "waves" which are believed to be due to the hydrodynamicalKelvin-Helmholtz instability.[36]

The dynamic gas motions in M42 are complex, but are trending outthrough the opening in the bay and toward the Earth.[28] The largeneutral area behind the ionized region is currently contracting under itsown gravity.

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Evolution

Panoramic image of the center of the nebula,taken by the Hubble Telescope. This view is

about 2.5 light years across. The Trapezium is atcenter left. Credit:NASA/ESA.

Interstellar clouds like the Orion Nebula are found throughout galaxiessuch as the Milky Way. They begin as gravitationally bound blobs ofcold, neutral hydrogen, intermixed with traces of other elements. Thecloud can contain hundreds of thousands of solar masses and extendfor hundreds of light years. The tiny force of gravity that could compelthe cloud to collapse is counterbalanced by the very faint pressure ofthe gas in the cloud.

Whether due to collisions with a spiral arm, or through the shock waveemitted from supernovae, the atoms are precipitated into heaviermolecules and the result is a molecular cloud. This presages theformation of stars within the cloud, usually thought to be within aperiod of 10-30 million years, as regions pass the Jeans mass and thedestabilized volumes collapse into disks. The disk concentrates at thecore to form a star, which may be surrounded by a protoplanetary disk.This is the current stage of evolution of the nebula, with additionalstars still forming from the collapsing molecular cloud. The youngest

and brightest stars we now see in the Orion Nebula are thought to be less than 300,000 years old,[37] and the brightestmay be only 10,000 years in age.

Some of these collapsing stars can be particularly massive, and can emit large quantities of ionizing ultravioletradiation. An example of this is seen with the Trapezium cluster. Over time the ultraviolet light from the massivestars at the center of the nebula will push away the surrounding gas and dust in a process called photo evaporation.This process is responsible for creating the interior cavity of the nebula, allowing the stars at the core to be viewedfrom Earth.[8] The largest of these stars have short life spans and will evolve to become supernovae.Within about 100,000 years, most of the gas and dust will be ejected. The remains will form a young open cluster, acluster of bright, young stars surrounded by wispy filaments from the former cloud.[38] The Pleiades is a famousexample of such a cluster.

Gallery

Orion Nebula wascaptured using theWide Field Imager

camera on theMPG/ESO2.2-metretelescope.

Orion Nebula Complexincluding M42, M43,Running Man Nebula(NGC 1973, 1975, and1977) and much of thesurrounding nebulosity.

Panoramic image of theOrion Nebula, taken by

Ioannidis Panos with an 8Inch Newtonian telescopeand a Nikon D70 camera.

Infant stars, image from NASA'sSpitzer Space Telescope.

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The Orion Nebula imaged withthe 2.2m ESO/MPG telescope.

Credit: ESO

The central part of the OrionNebula. Credit ESO

This wide-field view ofthe Orion Nebula (Messier

42), was taken with theVISTA infrared survey

telescope at ESO’s ParanalObservatory in Chile.

Credit ESO/J.Emerson/VISTA.

Orion bySpitzer.

The Orion Nebula's biggest stars. An infrared image showingfledgling stars located in the

Orion Nebula.

Notesa. 1,270 × tan( 66′ / 2 ) = 12 ly. radiusb.b. From temperate zones in the Northern Hemisphere, the nebula appears below the Belt of Orion; from temperate

zones in the Southern Hemisphere the nebula appears above the Belt.c.c. C. Robert O'Dell commented about this Wikipedia article, "The only egregious error is the last sentence in the

Stellar Formation section. It should actually read 'Even though most planetary disks can form planets,observations show that intense stellar radiation should have destroyed any proplyds that formed near theTrapezium group, if the group is as old as the low mass stars in the cluster. Since proplyds are found very close tothe Trapezium group, it can be argued that those stars are much younger than the rest of the cluster members.'"

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Publications of the Astronomical Society of Japan 59 (5): 897–903. arXiv:0705.3792. Bibcode 2007PASJ...59..897H.[7] "Nebula". The American Cyclopædia. 1879.[8] Press release, " Astronomers Spot The Great Orion Nebula's Successor (http:/ / cfa-www. harvard. edu/ press/ pr0605. html)",

Harvard-Smithsonian Center for Astrophysics, 2006.[9] "Gemini's Laser Vision Reveals Striking New Details in Orion Nebula" (http:/ / www. gemini. edu/ index. php?q=node/ 226). Gemini

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doi:10.1038/120473a0.[12] Davíd Carrasco, ed., The Oxford encyclopedia of Mesoamerican cultures: the civilizations of Mexico and Central America, Oxford

University Press, 2001, [entry by E.C. Krupp], page 165 (http:/ / books. google. com/ books?id=9I3rAAAAMAAJ& q="Rift+ �+ was+ the+road+ to+ Xibalba,+ the+ otherworld. + The"& dq="Rift+ �+ was+ the+ road+ to+ Xibalba,+ the+ otherworld. + The")

[13] American astronomer and author Ed Krupp, Krupp, Edward C. (February 1999). "Igniting the Hearth" (http:/ / web. archive. org/ web/20070930222028/ http:/ / pqasb. pqarchiver. com/ skyandtelescope/ access/ 886319051. html?dids=886319051:886319051& FMT=CITE&FMTS=CITE:PAGE& date=Feb+ 1999& author=E+ C+ Krupp& desc=Igniting+ the+ Hearth). Sky & Telescope: 94. Archived from theoriginal (http:/ / pqasb. pqarchiver. com/ skyandtelescope/ access/ 886319051. html?dids=886319051:886319051& FMT=CITE&FMTS=CITE:PAGE& date=Feb+ 1999& author=E+ C+ Krupp& desc=Igniting+ the+ Hearth) on September 30, 2007. . Retrieved2006-10-19.

[14] James, Andrew (June 27, 2012). "The Great Orion Nebula: M42 &amp M43" (http:/ / www. southastrodel. com/ Page204. htm). SouthernAstronomical Delights. . Retrieved 2012-06-27.

[15] Tibor Herczeg, Norman (January 22, 1999). "The Orion Nebula: A chapter of early nebular studies" (http:/ / www. astro. uni-bonn. de/~pbrosche/ aa/ acta/ vol03/ acta03_246. html). History of Astronomy. . Retrieved 2006-10-27.

[16] Scientific American, Volume 114, Munn & Company, 1916, page 615 (http:/ / books. google. com/ books?id=98sxAQAAMAAJ&pg=PA615& dq=Cysatus+ of+ Lucerne+ orion+ nebula& hl=en& sa=X& ei=RAPrT6D0BYT-6gG07b2-BQ&ved=0CEMQ6AEwAw#v=onepage& q=Cysatus of Lucerne orion nebula& f=false)

[17] Popular astronomy, Volume 12, Goodsell Observatory of Carleton College, 1904, page 101 (http:/ / books. google. com/books?id=_hTyAAAAMAAJ& pg=PA101& dq=Cysatus+ + orion+ nebula& hl=en& sa=X& ei=8QPrT8O6GpSK6QG2hNDcBQ&ved=0CDMQ6AEwAA#v=onepage& q=Cysatus orion nebula& f=false)

[18] Harrison, Thomas G. (1984). "The Orion Nebula: Where in History is it?" (http:/ / adsabs. harvard. edu/ abs/ 1984QJRAS. . 25. . . 65H).Quarterly Journal of the Royal Astronomical Society 25: 71. Bibcode 1984QJRAS..25...65H. .

[19] Galileo Galilei: Siderius Nuncius, Venice, 1610. English Translation published at Bard College, Hudson NY" October 9, 2003 EnglishTranslation (http:/ / www. bard. edu/ admission/ forms/ pdfs/ galileo. pdf) Original Latin version (http:/ / www. liberliber. it/ biblioteca/ g/galilei/ sidereus_nuncius/ html/ sidereus. htm)

[20] Messier, Charles (1774). "Catalogue des Nébuleuses & des amas d'Étoiles, que l'on découvre parmi les Étoiles fixes sur l'horizon de Paris;observées à l'Observatoire de la Marine, avec différens instruments" (http:/ / seds. org/ messier/ Xtra/ history/ m-cat71. html). Mémoires del'Académie Royale des Sciences (Paris). .

[21] Barbara J. Becker, Eclecticism, Opportunism, and the Evolution of a New Research Agenda: William and Margaret Huggins and the Originsof Astrophysics, CHAPTER 2—PART 3 (https:/ / eee. uci. edu/ clients/ bjbecker/ huggins/ ch2c. html#_ftn45)

[22] J. B. Hearnshaw,The measurement of starlight: two centuries of astronomical photometry, page 122 (http:/ / books. google. com/books?id=Kp7G4IqK7woC& pg=PA122& dq=common+ orion+ nebula+ "dry+ plate"& cd=6#v=onepage& q=common orion nebula "dryplate"& f=false)

[23] Campbell, W. W.; Moore, J. H. (1917). "On the Radial Velocities of the Orion Nebula". Publications of the Astronomical Society of thePacific 29 (169): 143. Bibcode 1917PASP...29..143C. doi:10.1086/122612.

[24] Trumpler, Robert Julius (1931). "The Distance of the Orion Nebula". Publications of the Astronomical Society of the Pacific 43 (254): 255.Bibcode 1931PASP...43..255T. doi:10.1086/124134.

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[25] David F. Salisbury, 2001, " Latest investigations of Orion Nebula reduce odds of planet formation (http:/ / exploration. vanderbilt. edu/news/ news_orion. htm)".

[26] Robberto, M.; O'Dell; Hillenbrand; Simon; Soderblom; Feigelson; Krist; McCullough et al.; et al. (2005). "An overview of the HSTTreasury Program on the Orion Nebula". Bulletin of the American Astronomical Society 37: 1404. Bibcode 2005AAS...20714601R. See alsosee the NASA press release (http:/ / hubblesite. org/ newscenter/ newsdesk/ archive/ releases/ 2006/ 01/ text/ ).

[27] K.G. Stassun, R.D. Mathieu and J.A. Valenti (2006). "Discovery of two young brown dwarfs in an eclipsing binary system". Nature 440(7082): 311–314. Bibcode 2006Natur.440..311S. doi:10.1038/nature04570.

[28] Balick, B.; Gammon, R. H.; Hjellming, R. M. (1974). "The structure of the Orion nebula". Publications of the Astronomical Society of thePacific 86: 616. Bibcode 1974PASP...86..616B. doi:10.1086/129654.

[29] O'Dell, C. R. (2001). "Structure of the Orion Nebula". Publications of the Astronomical Society of the Pacific 113 (779): 29.Bibcode 2001PASP..113...29O. doi:10.1086/317982.

[30] " M-42 (http:/ / www. seds. org/ messier/ m/ m042. html)", Students for the Exploration and Development of Space, April 12, 2006.[31] McCaughrean, Mark J.; O'dell, C. Robert. (1996). "Direct Imaging of Circumstellar Disks in the Orion Nebula". Astronomical Journal 111:

1977. Bibcode 1996AJ....111.1977M. doi:10.1086/117934.[32] Kassis, Marc; et al. (2006). "Mid-Infrared Emission at Photodissociation Regions in the Orion Nebula". Astrophysical Journal 637 (2):

823–837. Bibcode 2006ApJ...637..823K. doi:10.1086/498404. See also the press release (http:/ / www. keckobservatory. org/ news/ science/060109_orion/ index. html)

[33] Ker Than, 11 January 2006, " The Splendor of Orion: A Star Factory Unveiled (http:/ / www. space. com/ scienceastronomy/060111_orion_news. html)", Space.com

[34] Patrick Hartigan. Herbig-Haro Jet Movies from HST (http:/ / sparky. rice. edu/ ~hartigan/ movies. html), Physics and Astronomy Dept., RiceUniversity

[35] " Mapping Orion's Winds (http:/ / www. vanderbilt. edu/ news/ releases?id=23966)", January 16, 2006, Vanderbilt News Service[36] Denise Chow. Young stars blamed for space cloud ripples (http:/ / www. msnbc. msn. com/ id/ 38757647/ ns/

technology_and_science-space/ ), MSNBC.com[37] " Detail of the Orion Nebula (http:/ / www. seds. org/ hst/ OrionFull. html)", HST image and text.[38] Kroupa, P., Aarseth, S.J., Hurley, J. 2001, MNRAS, 321, 699, "The formation of a bound star cluster: from the Orion nebula cluster to the

Pleiades" (http:/ / adsabs. harvard. edu/ abs/ 2001MNRAS. 321. . 699K)

External links• Animated tour of the Orion Nebula (http:/ / alienworlds. glam. ac. uk/ OrionNebula. html), University of

Glamorgan• Orion Nebula observed by Chandra/HST (http:/ / chandra. harvard. edu/ photo/ 2007/ orion/ )• Orion Nebula observed by Gemini Observatory (http:/ / www. gemini. edu/ index. php?option=content&

task=view& id=226)• Orion Nebula at ESA/Hubble (http:/ / www. spacetelescope. org/ images/ archive/ freesearch/ orion+ nebula/

viewall/ 1)• Messier 42, SEDS Messier pages (http:/ / www. seds. org/ messier/ m/ m042. html) and specifically NGC 1976

(http:/ / www. seds. org/ ~spider/ ngc/ ngc. cgi?m42).• January 2006 Hubble Space Telescope image of the Orion Nebula (http:/ / hubblesite. org/ newscenter/ newsdesk/

archive/ releases/ 2006/ 01/ image/ a)• January 2006 Hubble Space Telescope image of the Trapezium cluster (http:/ / hubblesite. org/ newscenter/

newsdesk/ archive/ releases/ 2006/ 01/ image/ e)• Orion Nebula M42, Hubble Images (http:/ / www. seds. org/ messier/ more/ m042_hst2. html)• Remarkable new views captured of Orion Nebula (http:/ / spaceflightnow. com/ news/ n0101/ 19orion/ ),

SpaceFlight Now, 2001.• NightSkyInfo.com - The Great Orion Nebula (http:/ / www. nightskyinfo. com/ archive/ orion_nebula)• Computer visualization of Orion Nebula. (http:/ / vis. sdsc. edu/ research/ orion. html) Data gathered from the

Hubble Space Telescope and ground-based sensors were combined to form a 3D volume visualization of thenebula. Narration of the planetarium-like flythrough describes notable features and views from angles notpossible from Earth. Link contains downloadable MPEG and QuickTime movies of flythrough.

•• Astronomy Picture of the Day• Spitzer's Orion (http:/ / apod. nasa. gov/ apod/ ap100410. html) 2010 April 10

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• Planetary Systems Now Forming in Orion (http:/ / apod. nasa. gov/ apod/ ap091222. html) 2009 December 22• Great Orion Nebulae (http:/ / apod. nasa. gov/ apod/ ap081023. html) 2008 October 23

• The Orion Nebula on WikiSky (http:/ / www. wikisky. org/ ?object=Orion+ Nebula)

Eta Carinae

Eta Carinae

Hubble Space Telescope image showing Eta Carinae and the bipolar Homunculus Nebula which surrounds the star. The Homunculuswas partly created in an eruption of Eta Carinae, the light from which reached Earth in 1843. Eta Carinae itself appears as the white

patch near the center of the image, where the two lobes of the Homunculus touch.Observation data

Epoch J2000      Equinox J2000

Constellation Carina

Right ascension 10h 45m 03.591s[1]

Declination −59° 41′ 04.26″[1]

Apparent magnitude (V) −0.8 to 7.9[2] (4.6 February 2012)[3]

Characteristics

Spectral type BIae-0 / OI[4]

U−B color index -0.45

B−V color index 0.61

Variable type LBV[2] & binary

Astrometry

Radial velocity (Rv) −25.0[1] km/s

Proper motion (μ) RA: −7.6[1] mas/yrDec.: 1.0[1] mas/yr

Absolute magnitude (MV

) -7 (current)

Details

Mass 120 / 30[5] M☉

Radius ~240[6][7] / 24[4] R☉

Luminosity 5,000,000 / 1,000,000[4] L☉

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Temperature ~15,000[8] / 37,200[4] K

Age ~ <3 × 106 years

Other designations

Foramen, Tseen She, 231 G. Carinae,[9] HR 4210, CD−59°2620, HD 93308, SAO 238429, WDS 10451-5941, IRAS 10431-5925,GC 14799, CCDM J10451-5941

Eta Carinae (η Carinae or η Car) is a stellar system in the constellation Carina, about 7,500 to 8,000 light-yearsfrom the Sun. The system contains at least two stars, of which the primary is a luminous blue variable (LBV) thathad an initial mass of around 150 solar masses, of which it has lost at least 30 since. A hot supergiant ofapproximately 30 solar masses is in orbit around the primary, although an enormous thick red nebula surroundingEta Carinae makes it impossible to see this companion optically. The Eta Carinae system is enclosed in theHomunculus Nebula, itself part of the much larger Carina Nebula, and currently has a combined bolometricluminosity of over five million times that of the Sun.[5] It is not visible north of latitude 30°N and is circumpolarsouth of latitude 30°S. Because of its mass and the stage of life, it is expected to explode in a supernova orhypernova in the astronomically near future.

Observational historyWhen Eta Carinae was first catalogued in 1677 by Edmond Halley, it was of the 4th magnitude, but by 1730,observers noticed it had brightened considerably and was, at that point, one of the brightest stars in Carina. In themiddle of the 18th century, Nicolas Louis de Lacaille mapped and gave the stars of Argo Navis Bayer designations.He assigned the then second-magnitude star the Greek letter Eta.[10]

Subsequently Eta Carinae dimmed, and by 1782 it appeared to have reverted to its former magnitude. In 1820, it wasobserved to be growing in brightness again. By 1827, it had brightened more than tenfold and reached its greatestapparent brightness in April 1843. With a magnitude of −0.8, it was the second brightest star in the night-time sky(after Sirius at 8.6 light years away), despite its enormous distance. (To put the relationship in perspective, therelative brightness would be like comparing a candle (representing Sirius) at 14.5 meters (48 feet) to another lightsource (Eta Carinae) about 10 kilometers (6.2 mi) away, which would appear almost as bright as the candle.)Eta Carinae sometimes has large outbursts, the last of which appeared in 1841, at around the time of its maximumbrightness. The reason for these outbursts is not known. The most likely explanation is that they are caused by abuild-up of radiation pressure caused by the star's enormous luminosity. After 1843, Eta Carinae faded yet again andbetween about 1900 and 1940 it was only 8th magnitude, invisible to the naked eye.[11] A sudden and unexpecteddoubling of brightness was observed in 1998–1999. In 2007, at magnitude 5, Eta Carinae could once again be seenwith the naked eye.[3]

In 2008, the formerly clockwork regularity of the dimming was upset.[12] Following its 5.52-year cycle, the starwould normally have started its next dimming in January 2009, but the pattern was noticed starting early in July2008 by the southern Gemini Observatory near La Serena, Chile. Spectrographic measurements showed an increasein blue light from superheated helium, which was formerly assumed to occur with the wind shock. However, if thecause is a binary star, it would be located too far away at this point in time for the wind to interact in so significant afashion. There is some debate about the cause of the recent event.[12]

In 2011, light echoes from the 19th century Great Eruption of Eta Carinae were detected using the U.S. NationalOptical Astronomy Observatory's Blanco 4-meter telescope at the Cerro Tololo Inter-American Observatory.Analysis of the reflected spectra indicated the light was emitted when Eta Carinae was a 5000 K G2-to-G5supergiant, some 2000 K cooler than other supernova impostor events.[13]

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Eta Carinae 49

System and propertiesThis stellar system is currently one of the most massive that can be studied in great detail. Until recently, Eta Carinaewas thought to be the most massive single star, but in 2005 it was proved to be a binary system.[14] The mostmassive star in the Eta Carinae multiple star system probably has more than 100 times the mass of the Sun.[15] Otherknown massive stars are more luminous and more massive.Stars in the mass class of Eta Carinae produce more than a million times as much light as the Sun. They are quiterare — only a few dozen are present in a galaxy the size the Milky Way. They are assumed to approach (orpotentially exceed) the Eddington limit, i.e., the outward pressure of their radiation is almost strong enough tocounteract gravity. Stars that are more than 120 solar masses exceed the theoretical Eddington limit, and their gravityis barely strong enough to hold in their radiation and gas.Eta Carinae's chief significance for astrophysics is based on its giant eruption or supernova impostor event, whichwas observed around 1843. In a few years, Eta Carinae produced almost as much visible light as a supernovaexplosion, but it survived. Other supernova impostors have been seen in other galaxies, for example the possiblefalse supernovae SN 1961v in NGC 1058[16] and SN 2006jc in UGC 4904,[17] which produced a false supernova,noted in October 2004. Significantly, SN 2006jc was destroyed in a supernova explosion two years later, observedon October 9, 2006.[18] The supernova impostor phenomenon may represent a surface instability[19] or a failedsupernova. Eta Carinae's giant eruption was the prototype for this phenomenon.One remarkable aspect of Eta Carinae is its changing brightness. It is currently classified as a luminous blue variable(LBV) binary star due to peculiarities in its pattern of brightening and dimming.

SurroundingsEta Carinae is surrounded by the Homunculus Nebula, an emission nebula which is itself a part of the much largerCarina Nebula.

X-ray source

Classified as a peculiar star, Eta Carinae exhibits a superstar at its center as seen inthis image from Chandra. The new X-ray observation shows three distinct

structures: an outer, horseshoe-shaped ring about 2 light years in diameter, a hotinner core about 3 light-months in diameter, and a hot central source less than 1

light-month in diameter which may contain the superstar that is responsible for theHomunculus nebula. The outer ring provides evidence of another large explosionthat occurred over 1,000 years ago. Credit: Chandra Science Center and NASA.

4U 1037–60 (A 1044–59) is Eta Carinae.Three structures around Eta Carinae arethought to represent shock waves producedby matter rushing away from the superstar atsupersonic speeds. The temperature of theshock-heated gas ranges from 60 MK in thecentral regions to 3 MK on thehorseshoe-shaped outer structure. "TheChandra image contains some puzzles forexisting ideas of how a star can producesuch hot and intense X-rays," says Prof. KrisDavidson of the University of Minnesota.[20]

A "spectroscopic minimum", or "X-rayeclipse", occurred in July and August2003.[21] Astronomers organized a largeobserving campaign which included everyavailable ground-based (e.g. CCD opticalphotometry[3]) and space observatory, including major observations with the Hubble Space Telescope, the Chandra

X-ray Observatory, the INTEGRAL Gamma-ray space observatory, and the Very Large Telescope. Primary goals of these observations were to determine if, in fact, Eta Carinae is a binary star and, if so, to identify its companion star;

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Eta Carinae 50

to determine the physical mechanism behind the "spectroscopic minima"; and to understand their relation (if any) tothe large-scale eruptions of the 19th century.There is good agreement between the X-ray light curve and the evolution of a wind-wind collision zone of a binarysystem. These results were complemented by new tests on radio wavelengths.[22]

Spectrographic monitoring of Eta Carinae[23] showed that some emission lines faded every 5.52 years, and that thisperiod was stable for decades. The star's radio emission,[24] along with its X-ray brightness,[25] also dropprecipitously during these "events" as well. These variations, along with ultra-violet variations, suggest a highprobability that Eta Carinae is actually a binary star in which a hot, lower-mass star revolves around η Carinae in a5.52-year, highly eccentric elliptical orbit.[14]

The ionizing radiation emitted by the secondary star in Eta Carinae is the major radiation source of the system. Muchof this radiation is absorbed by the primary stellar wind, mainly after it encounters the secondary wind and passesthrough a shock wave. The amount of absorption depends on the compression factor of the primary wind in theshock wave. The compression factor is limited by the magnetic pressure in the primary wind.[26] The variation of theabsorption by the post-shock primary wind with orbital phase changes the ionization structure of the circumbinarygas, and can account for the radio light curve of Eta Carinae. Fast variations near periastron passage are attributed tothe onset of the accretion phase.

Future prospects

Carina nebula (NGC 3372) containing Eta Carinae. Eta Carinae and its surroundingHomunculus Nebula are the small, saturated (white) blob centered vertically in theimage and approximately 1/5 of the distance from left to right. Photograph by the

Hubble Space Telescope.

Because of their disproportionately highluminosities, very large stars such asEta Carinae use up their fuel veryquickly. Eta Carinae is expected toexplode as a supernova or hypernovasome time within the next million yearsor so. As its current age andevolutionary path are uncertain,however, it could explode within thenext several millennia or even in thenext few years. LBVs such as EtaCarinae may be a stage in the evolutionof the most massive stars; the prevailingtheory now holds that they will exhibitextreme mass loss and become Wolf-Rayet stars before they go supernova, if they are unable to hold their mass toexplode as a hypernova.[27]

More recently, another possible Eta Carinae analogue was observed: SN 2006jc, some 77 million light years away inUGC 4904, in the constellation of Lynx.[28] Its brightened appearance was noted on 20 October 2004, and wasreported by amateur astronomer Koichi Itagaki as a supernova. However, although it had indeed exploded, hurling0.01 solar masses (~20 Jupiters) of material into space, it had survived, before finally exploding nearly two yearslater as a Mag 13.8 type Ib supernova, seen on 9 October 2006. Its earlier brightening was a supernova impostorevent.

The similarity between Eta Carinae and SN 2006jc has led Stefan Immler of NASA's Goddard Space Flight Centerto suggest that Eta Carinae could explode in our lifetime, or even in the next few years. However, Stanford Woosleyof the University of California in Santa Cruz disagrees with Immler’s suggestion, and says it is likely that EtaCarinae is at an earlier stage of evolution, and that there are still several stages of nuclear burning to go before thestar runs out of fuel.

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In NGC 1260, a spiral galaxy in the constellation of Perseus some 238 million light years from earth, anotheranalogue star explosion, supernova SN 2006gy, was observed on September 18, 2006. A number of astronomersmodelling supernova events have suggested that the explosion mechanism for SN 2006gy may be very similar to thefate that awaits Eta Carinae.

Possible effects on Earth

One theory of Eta Carinae's ultimate fate.Drawing of a massive star collapsing to form a black hole. Energy released as jets along

the axis of rotation forms the gamma ray bursts.Credit: Nicolle Rager Fuller/NSF

It is possible that the Eta Carinaehypernova or supernova, when itoccurs, could affect Earth, about 7,500light years away. It is unlikely,however, to affect terrestrial lifeformsdirectly, as they will be protected fromgamma rays by the atmosphere, andfrom some other cosmic rays by themagnetosphere. The damage wouldlikely be restricted to the upperatmosphere, the ozone layer, spacecraft,including satellites, and any astronautsin space, although a certain few [29]claim that radiation damage to the upperatmosphere would have catastrophiceffects as well. At least one scientist hasclaimed that when the star explodes, "itwould be so bright that you would see itduring the day, and you could even read a book by its light at night".[30] A supernova or hypernova produced by EtaCarinae would probably eject a gamma ray burst (GRB) out from both polar areas of its rotational axis. Calculationsshow that the deposited energy of such a GRB striking the Earth's atmosphere would be equivalent to one kiloton ofTNT per square kilometer over the entire hemisphere facing the star, with ionizing radiation depositing ten times thelethal whole body dose to the surface.[31] This catastrophic burst would probably not hit Earth, though, because therotation axis does not currently point towards our solar system. If Eta Carinae is a binary system, this may affect thefuture intensity and orientation of the supernova explosion that it produces, depending on the circumstances.[14]

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Eta Carinae 52

The luminous blue variable Eta Carinae taken with the NACOnear-infrared adaptive optics instrument on ESOs Very Large

Telescope. Credit ESO

Cultural significance

In traditional Chinese astronomy, Eta Carinae has thenames Tseen She (from the Chinese 天 社 [Mandarin:tiānshè] "Heaven's altar") and Foramen.[32] It is alsoknown as 海 山 二 (Hǎi Shān èr, English: theSecond Star of Sea and Mountain),[33] referring to Seaand Mountain, an asterism that Eta Carinae forms withs Carinae, λ Centauri and λ Muscae.[34]

In 2010, astronomers Duane Hamacher and David Frewfrom Macquarie University in Sydney showed that theBoorong Aboriginal people of northwestern Victoria,Australia, witnessed the outburst of Eta Carinae in the1840s and incorporated it into their oral traditions asCollowgulloric War, the wife of War (Canopus, theCrow — pronounced "Waah").[35] This is the onlydefinitive indigenous record of Eta Carinae's outburstidentified in the literature to date.

Notes[1] "SIMBAD query result: V* eta Car – Variable Star" (http:/ / simbad. u-strasbg. fr/ simbad/ sim-id?Ident=Eta+ Carinae). Centre de Données

astronomiques de Strasbourg. . Retrieved 2008-04-25.—some of the data is located under "Measurements".[2] "GCVS Query=Eta+Car" (http:/ / www. sai. msu. su/ gcvs/ cgi-bin/ search. cgi?search=Eta+ Car). General Catalogue of Variable Stars @

Sternberg Astronomical Institute, Moscow, Russia. . Retrieved 2010-11-24.[3] Fernández Lajús, Eduardo (Dec 19, 2011). "Optical monitoring of Eta Carinae" (http:/ / etacar. fcaglp. unlp. edu. ar/ ). Universidad Nacional

de La Plata. . Retrieved 2011-12-26.[4] "The Binarity of η Carinae Revealed from Photoionization Modeling of the Spectral Variability of the Weigelt Blobs B and D" (http:/ /

iopscience. iop. org/ 0004-637X/ 624/ 2/ 973/ fulltext/ 60929. text. html). The Astrophysical Journal. . Retrieved 2012-05-23.—some of thedata is located under "Measurements".

[5] Kashi, A.; Soker, N. (2009). "Possible implications of mass accretion in Eta Carinae". New Astronomy 14: 11.doi:10.1016/j.newast.2008.04.003.

[6] Gull, T. R.; Damineli, A. (2010). "JD13 – Eta Carinae in the Context of the Most Massive Stars". Proceedings of the InternationalAstronomical Union 5: 373. doi:10.1017/S1743921310009890.

[7][7] The mass is so high that there is no clearly defined boundary between the star and the surrounding nebula. The effective temperature andradius correspond to a position where τ(ross) is around unity.

[8] Gull, T. R.; Vieira, G.; Bruhweiler, F.; Nielsen, K. E.; Verner, E.; Danks, A. (2005). "The Absorption Spectrum of High‐Density StellarEjecta in the Line of Sight to η Carinae". The Astrophysical Journal 620: 442. doi:10.1086/426885.

[9] "VIZIER Details for Eta Carinae in Gould's Uranomatria Argentina" (http:/ / vizier. u-strasbg. fr/ viz-bin/ VizieR-5?-out. add=. & -source=V/135/ catalog& recno=1428). Centre de Données astronomiques de Strasbourg. . Retrieved 2011-02-14.

[10] Wagman 2003, pp. 84–85[11] "Historical light curve" (http:/ / etacar. fcaglp. unlp. edu. ar/ plots/ historic. jpg). . Retrieved 2011-12-26.[12] Courtland, Rachel (7 August 2008). "'Supernova imposter' begins to dim unexpectedly" (http:/ / space. newscientist. com/ article/

dn14488-supernova-impostor-begins-to-dim-unexpectedly. html?feedId=online-news_rss20). NewScientist news service. . Retrieved 30December 2012.

[13] Rest, A.; J. L. Prieto, N. R. Walborn, N. Smith, F. B. Bianco, R. Chornock, D. L. Welch, D. A. Howell, M. E. Huber, R. J. Foley, W. Fong,B. Sinnott, H. E. Bond, R. C. Smith, I. Toledo, D. Minniti, K. Mandel (2012-02-16). "Light echoes reveal an unexpectedly cool η Carinaeduring its nineteenth-century Great Eruption" (http:/ / dx. doi. org/ 10. 1038/ nature10775). Nature 482 (7385): 375–378. arXiv:1112.2210.Bibcode 2012Natur.482..375R. doi:10.1038/nature10775. ISSN 0028-0836. PMID 22337057. . Retrieved 2012-02-17.

[14] Neal-Jones, Nancy; Steigerwald, Bill (1 November 2005). "NASA Satellite Detects Massive Star Partner" (http:/ / www. nasa. gov/ home/hqnews/ 2005/ nov/ HQ_05353_massive_star. html). NASA Goddard Space Flight Center. . Retrieved 30 December 2012.

[15] Frommert, Hartmut & Kronberg, Christine (February 2, 1998). "Peculiar star Eta Carinae, in Carina" (http:/ / messier. seds. org/ xtra/ ngc/etacar. html). Students for the Exploration and Development of Space (SEDS). . Retrieved 2012-02-20.

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[16] Stockdale, Christopher J.; Rupen, Michael P.; Cowan, John J.; Chu, You-Hua; Jones, Steven S. (2001). "The fading radio emission from SN1961v: evidence for a Type II peculiar supernova?". The Astronomical Journal 122 (1): 283. arXiv:astro-ph/0104235.Bibcode 2001AJ....122..283S. doi:10.1086/321136.

[17] Naeye, Robert (4 April 2007). "Supernova Impostor Goes Supernova" (http:/ / www. nasa. gov/ centers/ goddard/ news/ topstory/ 2007/supernova_imposter. html). NASA Goddard Space Flight Center. . Retrieved 30 December 2012.

[18] Shiga, D. (2007). "Star's odd double explosion hints at antimatter trigger" (http:/ / space. newscientist. com/ article. ns?id=dn11563&feedId=online-news_rss20). New Scientist 2598 (2626): 18. doi:10.1016/S0262-4079(07)62628-1. .

[19] see various articles in R.M. Humphreys & K.Z. Stanek (eds.) (2005). "The Fate of the Most Massive Stars". ASP Conference 332.Astronomical Society of the Pacific.

[20] "Chandra Takes X-ray Image of Repeat Offender" (http:/ / science. nasa. gov/ newhome/ headlines/ ast08oct99_1. htm). Science News.NASA. 8 October 1999. .

[21] Hamaguchi, K.; Corcoran, M. F.; Gull, T.; Ishibashi, K.; Pittard, J. M.; Hillier, D. J.; Damineli, A.; Davidson, K. et al. (2007). "X‐RaySpectral Variation of η Carinae through the 2003 X‐Ray Minimum". The Astrophysical Journal 663: 522. doi:10.1086/518101.

[22] Falceta-Gonçalves, D.; Jatenco-Pereira, V.; Abraham, Z. (2005). "Wind-wind collision in the η Carinae binary system: a shell-like eventnear periastron". MNRAS 357 (3): 895. arXiv:astro-ph/0404363. Bibcode 2005MNRAS.357..895F. doi:10.1111/j.1365-2966.2005.08682.x.

[23] Damineli, A. (1996). "The 5.52 Year Cycle of Eta Carinae". ApJ 460 (1): L49. Bibcode 1996ApJ...460L..49D. doi:10.1086/309961.[24] White, Stephen. "Radio outburst of Eta Carinae" (http:/ / www. astro. umd. edu/ ~white/ images/ eta_time_full. html). Department of

Astronomy, University of Maryland College Park. . Retrieved 30 December 2012.[25] Corcoran, Michael (10 December 2008). "RXTE X-ray lightcurve" (http:/ / asd. gsfc. nasa. gov/ Michael. Corcoran/ eta_car/

etacar_rxte_lightcurve/ index. html). Goddard Space Flight Center. . Retrieved 30 December 2012.[26] Kashi, A.; Soker, N. (2007). "Modelling the Radio Light Curve of Eta Carinae". Monthly Notices of the Royal Astronomical Society 378 (4):

1609. arXiv:astro-ph/0702389. Bibcode 2007astro.ph..2389K. doi:10.1111/j.1365-2966.2007.11908.x.[27] Smith, Nathan; Owocki, Stanley P. (2006). "On the Role of Continuum-driven Eruptions in the Evolution of Very Massive Stars". The

Astrophysical Journal 645 (1): L45. arXiv:astro-ph/0606174. Bibcode 2006ApJ...645L..45S. doi:10.1086/506523.[28] Sanders, Robert (4 April 2007). "Massive star burps, then explodes" (http:/ / www. berkeley. edu/ news/ media/ releases/ 2007/ 04/

04_supernova. shtml) (Press release). UC Berkeley News. .Naeye, Robert; Sanders, Robert (4 April 2007). "Massive star burps, then explodes" (http:/ / www. chandra. harvard. edu/ press/ 07_releases/press_040407. html) (Press release). Chandra X-ray Observatory. Bibcode 2007cxo..pres....5.. .

[29] http:/ / www. sciencemag. org/ content/ 184/ 4141/ 1079. abstract[30] "Star dies in monstrous explosion" (http:/ / news. bbc. co. uk/ 2/ hi/ science/ nature/ 6633609. stm). BBC News. 8 May 2007. . Retrieved 30

December 2012.[31] Arnon Dar; A. De Rujula (2002). "The threat to life from Eta Carinae and gamma ray bursts". 24. Astrophysics and Gamma Ray Physics in

Space. pp. 513–523. arXiv:astro-ph/0110162. Bibcode 2001astro.ph.10162D.[32] Kostjuk, N. (2002). "table3.dat" (ftp:/ / cdsarc. u-strasbg. fr/ pub/ cats/ IV/ 27A/ table3. dat). Institute of Astronomy of Russian Academy of

Sciences. . Retrieved 30 December 2012.[33] 陳 輝 樺, ed. (28 July 2006). "AEEA 天 文 教 育 資 訊 網" (http:/ / aeea. nmns. edu. tw/ 2006/ 0607/ ap060728. html). . Retrieved 30

December 2012. (Chinese)[34] 陳 久 金 (2005). 中 國 星 座 神. 台 灣 書 房 出 版 有 限 公 司. ISBN 978-986-7332-25-7. (Chinese)[35] Hamacher, D. W.; Frew, D. J. (2010). "An Aboriginal Australian Record of the Great Eruption of Eta Carinae". Journal of Astronomical

History and Heritage 13 (3): 220–234. arXiv:1010.4610. Bibcode 2010arXiv1010.4610H.

References

Cited text• Wagman, Morton (2003). Lost Stars: Lost, Missing and Troublesome Stars from the Catalogues of Johannes

Bayer, Nicholas Louis de Lacaille, John Flamsteed, and Sundry Others. Blacksburg, VA: The McDonald &Woodward Publishing Company. ISBN 978-0-939923-78-6.

External links• NASA image released Feb. 24, 2012 (http:/ / www. flickr. com/ photos/ gsfc/ 6780345900/ in/ photostream)• HST Treasury Project and General Information on Eta Carinae (http:/ / etacar. umn. edu/ )• Eta Carinae profile (http:/ / www. solstation. com/ x-objects/ eta-car. htm)• Is there a "clock" in Eta Carinae? – Brazilian research about the star (http:/ / www. etacarinae. iag. usp. br/ )• Broad Band Optical Monitoring (http:/ / etacar. fcaglp. unlp. edu. ar/ )

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• X-ray Monitoring by RXTE (http:/ / asd. gsfc. nasa. gov/ Michael. Corcoran/ eta_car/ etacar_rxte_lightcurve/index. html)

• ESO press release (http:/ / www. eso. org/ outreach/ press-rel/ pr-2003/ pr-31-03. html) about the possibility of asupernova in 10 to 20 millennia

• The 2003 Observing Campaign (http:/ / eud. gsfc. nasa. gov/ Michael. Corcoran/ eta_car/ 2003. 5/ index. html)• Davidson, Kris et al. (1999). "An Unusual Brightening Of Eta Carinae". The Astronomical Journal (The

Astronomical Journal) 118 (4): 1777. Bibcode 1999AJ....118.1777D. doi:10.1086/301063.• Nathan, Smith (1998). "The Behemoth Eta Carinae: A Repeat Offender" (http:/ / www. astrosociety. org/ pubs/

mercury/ 9804/ eta. html). Astronomical Society of the Pacific. Retrieved 2006-08-13.• Eta Carinae at SIMBAD (http:/ / simbad. u-strasbg. fr/ sim-id. pl?protocol=html& Ident=Eta+ Carinae)

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Comet HaleBopp 55

Comet Hale–Bopp

C/1995 O1 (Hale–Bopp)

Discovery

Discovered by Alan Hale / Thomas Bopp

Discovery date July 23, 1995

Alternativedesignations

The Great Comet of1997,C/1995 O1

Orbital characteristics A [1]

Epoch 2450460.5

Aphelion 370.8 AU[2]

Perihelion 0.914 AU[2]

Semi-major axis 186 AU

Eccentricity 0.995086

Orbital period 2520[3]–2533[2] yr(Barycentric 2391 yr)[4]

Inclination 89.4°

Last perihelion April 1, 1997[2]

Next perihelion ~4385[5]

Comet Hale–Bopp (formally designated C/1995 O1) was perhaps the most widely observed comet of the 20thcentury and one of the brightest seen for many decades. It was visible to the naked eye for a record 18 months, twiceas long as the previous record holder, the Great Comet of 1811.Hale–Bopp was discovered on July 23, 1995, at a great distance from the Sun, raising expectations that the cometwould brighten considerably by the time it passed close to Earth. Although predicting the brightness of comets withany degree of accuracy is very difficult, Hale–Bopp met or exceeded most predictions when it passed perihelion onApril 1, 1997. The comet was dubbed the Great Comet of 1997.

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DiscoveryThe comet was discovered on July 23, 1995 by two independent observers, Alan Hale and Thomas Bopp, both in theUnited States.[6]

Hale had spent many hundreds of hours searching for comets without success, and was tracking known comets fromhis driveway in New Mexico when he chanced upon Hale–Bopp just after midnight. The comet had an apparentmagnitude of 10.5 and lay near the globular cluster M70 in the constellation of Sagittarius.[7] Hale first establishedthat there was no other deep-sky object near M70, and then consulted a directory of known comets, finding that nonewere known to be in this area of the sky. Once he had established that the object was moving relative to thebackground stars, he emailed the Central Bureau for Astronomical Telegrams, the clearing house for astronomicaldiscoveries.[8]

Bopp did not own a telescope. He was out with friends near Stanfield, Arizona observing star clusters and galaxieswhen he chanced across the comet while at the eyepiece of his friend's telescope. He realized he might have spottedsomething new when, like Hale, he checked his star maps to determine if any other deep-sky objects were known tobe near M70, and found that there were none. He alerted the Central Bureau for Astronomical Telegrams through aWestern Union telegram. Brian Marsden, who had run the bureau since 1968, laughed, "Nobody sends telegramsanymore. I mean, by the time that telegram got here, Alan Hale had already e-mailed us three times with updatedcoordinates."[9]

The following morning, it was confirmed that this was a new comet, and it was given the designation C/1995 O1.The discovery was announced in International Astronomical Union circular 6187.[7][10]

The comet may have been observed by ancient Egyptians during the reign of pharaoh Pepi I (2332–2283 BC). InPepi's pyramid in Saqqara is a text referring to an "nhh-star" as a companion of the pharaoh in the heavens, where"nhh" is the hieroglyph for long hair.[11]

Early observationHale–Bopp's orbital position was calculated as 7.2 astronomical units (AU) from the Sun, placing it between Jupiterand Saturn and by far the greatest distance from Earth at which a comet had been discovered by amateurs.[12][13]

Most comets at this distance are extremely faint, and show no discernible activity, but Hale–Bopp already had anobservable coma.[7] An image taken at the Anglo-Australian Telescope in 1993 was found to show thethen-unnoticed comet some 13 AU from the Sun,[14] a distance at which most comets are essentially unobservable.(Halley's Comet was more than 100 times fainter at the same distance from the Sun.)[15] Analysis indicated later thatits comet nucleus was 60±20 kilometres in diameter, approximately six times the size of Halley.[2][16]

Its great distance and surprising activity indicated that comet Hale–Bopp might become very bright indeed when itreached perihelion in 1997. However, comet scientists were wary – comets can be extremely unpredictable, andmany have large outbursts at great distance only to diminish in brightness later. Comet Kohoutek in 1973 had beentouted as a 'comet of the century' and turned out to be unspectacular.[8]

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Perihelion

The comet became a spectacular sight inearly 1997.

Hale–Bopp became visible to the naked eye in May 1996, and although itsrate of brightening slowed considerably during the latter half of that year,[17]

scientists were still cautiously optimistic that it would become very bright. Itwas too closely aligned with the Sun to be observable during December 1996,but when it reappeared in January 1997 it was already bright enough to beseen by anyone who looked for it, even from large cities with light-pollutedskies.[18]

The Internet was a growing phenomenon at the time, and numerous websitesthat tracked the comet's progress and provided daily images from around theworld became extremely popular. The Internet played a large role inencouraging the unprecedented public interest in comet Hale–Bopp.[19]

As the comet approached the Sun, it continued to brighten, shining at2nd magnitude in February, and showing a growing pair of tails, the blue gastail pointing straight away from the Sun and the yellowish dust tail curvingaway along its orbit. On March 9, a solar eclipse in China, Mongolia andeastern Siberia allowed observers there to see the comet in the daytime.[20] Hale–Bopp had its closest approach toEarth on March 22, 1997 at a distance of 1.315 AU.[21]

As it passed perihelion on April 1, 1997 the comet developed into a spectacular sight. It shone brighter than any starin the sky except Sirius, and its dust tail stretched 40–45 degrees across the sky.[22][23] The comet was visible wellbefore the sky got fully dark each night, and while many great comets are very close to the Sun as they passperihelion, comet Hale–Bopp was visible all night to northern hemisphere observers.[24]

After perihelionAfter its perihelion passage, the comet moved into the southern celestial hemisphere, and its show was over as far asmost of the northern hemisphere was concerned. The comet was much less impressive to southern hemisphereobservers than it had been in the northern hemisphere, but southerners were able to see the comet gradually fadefrom view during the second half of 1997. The last naked-eye observations were reported in December 1997, whichmeant that the comet had remained visible without aid for 569 days, or about 18 and a half months.[17] The previousrecord had been set by the Great Comet of 1811, which was visible to the naked eye for about 9 months.[17]

The comet continued to fade as it receded, but is still being tracked by astronomers. In October 2007, 10 years afterthe perihelion and at distance of 25.7 AU from Sun, the comet was still active as indicated by the detection of theCO-driven coma.[25] Herschel Space Observatory images taken in 2010 suggest comet Hale–Bopp is covered in afresh frost layer.[26] Hale–Bopp was again detected in December 2010 when it was 30.7AU from the Sun,[26] andagain on 2012 Aug 7 when it was 33.2AU from the Sun.[27] Astronomers expect that the comet will remainobservable with large telescopes until perhaps 2020, by which time it will be nearing 30th magnitude.[28] By thistime it will become very difficult to distinguish the comet from the large numbers of distant galaxies of similarbrightness.

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Orbital changes

Hale–Bopp at perihelion on April 1, 1997

The comet likely made its last perihelion4,200 years ago.[29] Its orbit is almostperpendicular to the plane of the ecliptic,which ensures that close approaches toplanets are rare. However, in April 1996 thecomet passed within 0.77 AU of Jupiter,close enough for its orbit to be affected bythe planet's gravity.[29] The comet's orbitwas shortened considerably to a period ofroughly 2,533 years,[2] and it will nextreturn to the inner Solar System around theyear 4385.[5] Its greatest distance from theSun (aphelion) will be about 370 AU,[2] reduced from about 525 AU.[30][31]

Over many orbits, the cumulative effect of gravitational perturbations on comets with high orbital inclinations andsmall perihelion distances is generally to reduce the perihelion distance to very small values. Hale–Bopp has about a15% chance of eventually becoming a sungrazing comet through this process.[32]

It has been calculated that the previous visit by Hale–Bopp occurred in July 2215 BC.[30] The comet may havepresented a similar sight to people then, as the estimated closest approach to Earth was 1.4 AU, but no records of ithave survived. Hale–Bopp may have had a near collision with Jupiter in early June 2215 BC, which probably causeda dramatic change in its orbit, and 2215 BC may have been its first passage through the inner Solar System.[30]

The estimated probability of impacting Earth in future passages through the inner Solar System is remote, about 2.5x 10−9 per orbit.[33] However, given that the comet nucleus is around 60 km in diameter,[2] the consequences of suchan impact would be apocalyptic. A calculation given by Weissman[33] conservatively estimates the diameter at35 km; an estimated density of 0.6 g/cm3 then gives a cometary mass of 1.3 x 1019 g. An impact velocity of52.5 km/s yields an impact energy of 1.9 x 1032 ergs, or 4.4 x 109 megatons, about 44 times the estimated energy ofthe K-T impact event.

Scientific resultsComet Hale–Bopp was observed intensively by astronomers during its perihelion passage, and several importantadvances in cometary science resulted from these observations. The dust production rate of the comet was very high(up to 2.0×106 kg/s),[34] which may have made the inner coma optically thick.[35] Based on the properties of the dustgrains—high temperature, high albedo and strong 10 μm silicate emission feature—the astronomers concluded thedust grains are smaller than observed in any other comet.[36]

Hale–Bopp showed the highest ever linear polarization detected for any comet. Such polarization is the result ofsolar radiation getting scattered by the dust particles in the coma of the comet and depends on the nature of thegrains. It further confirms that the dust grains in the coma of comet Hale–Bopp were smaller than inferred in anyother comet.[37]

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Sodium tail

Comet Hale–Bopp's neutral sodium tail(the straight tail extending up to the left

from the nucleus)[38]

One of the most remarkable discoveries was that the comet had a third type oftail. In addition to the well-known gas and dust tails, Hale–Bopp alsoexhibited a faint sodium tail, only visible with powerful instruments withdedicated filters. Sodium emission had been previously observed in othercomets, but had not been shown to come from a tail. Hale–Bopp's sodium tailconsisted of neutral atoms (not ions), and extended to some 50 millionkilometres in length.[38]

The source of the sodium appeared to be the inner coma, although notnecessarily the nucleus. There are several possible mechanisms for generatinga source of sodium atoms, including collisions between dust grainssurrounding the nucleus, and 'sputtering' of sodium from dust grains byultraviolet light. It is not yet established which mechanism is primarilyresponsible for creating Hale–Bopp's sodium tail, and the narrow[38] anddiffuse[39] components of the tail may have different origins.[40]

While the comet's dust tail roughly followed the path of the comet's orbit and the gas tail pointed almost directlyaway from the Sun, the sodium tail appeared to lie between the two. This implies that the sodium atoms are drivenaway from the comet's head by radiation pressure.[38]

Deuterium abundanceThe abundance of deuterium in comet Hale–Bopp in the form of heavy water was found to be about twice that ofEarth's oceans. If Hale–Bopp's deuterium abundance is typical of all comets, this implies that although cometaryimpacts are thought to be the source of a significant amount of the water on Earth, they cannot be the only source.[41]

Deuterium was also detected in many other hydrogen compounds in the comet. The ratio of deuterium to normalhydrogen was found to vary from compound to compound, which astronomers believe suggests that cometary iceswere formed in interstellar clouds, rather than in the solar nebula. Theoretical modelling of ice formation ininterstellar clouds suggests that comet Hale–Bopp formed at temperatures of around 25–45 Kelvin.[41]

OrganicsSpectroscopic observations of Hale–Bopp revealed the presence of many organic chemicals, several of which hadnever been detected in comets before. These complex molecules may exist within the cometary nucleus, or might besynthesised by reactions in the comet.[42]

Detection of argonHale–Bopp was the first comet where the noble gas argon was detected.[43] Noble gases are chemically inert andhighly volatile, and since different noble elements have different sublimation temperatures, they can be used forprobing the temperature histories of the cometary ices. Krypton has a sublimation temperature of 16–20 K and wasfound to be depleted more than 25 times relative to the solar abundance,[44] while argon with its higher sublimationtemperature was enriched relative to the solar abundance.[43] Together these observations indicate that the interior ofHale–Bopp has always been colder than 35–40 K, but has at some point been warmer than 20 K. Unless the solarnebula was much colder and richer in argon than generally believed, this suggests that the comet formed beyondNeptune in the Kuiper belt region and then migrated outward to the Oort cloud.[43]

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Rotation

Comet Hale–Bopp over Zabriskie Point,Death Valley, USA

Comet Hale–Bopp's activity and outgassing were not spread uniformly overits nucleus, but instead came from several specific jets. Observations of thematerial streaming away from these jets[45] allowed astronomers to measurethe rotation period of the comet, which was found to be about 11 hours 46minutes.[46]

Binary nucleus question

In 1997 a paper was published that hypothesised the existence of a binarynucleus to fully explain the observed pattern of comet Hale–Bopp's dustemission observed in October 1995. The paper was based on theoreticalanalysis, and did not claim an observational detection of the proposed satellitenucleus, but estimated that it would have a diameter of about 30 km, with themain nucleus being about 70 km across, and would orbit in about three daysat a distance of about 180 km.[47] This analysis was confirmed byobservations in 1996 using Wide-Field Planetary Camera 2 of the HubbleSpace Telescope which had taken images of the comet that revealed thesatellite.[48]

Although observations using adaptive optics in late 1997 and early 1998 showed a double peak in the brightness ofthe nucleus,[49] controversy still exists over whether such observations can only be explained by a binary nucleus.[16]

The discovery of the satellite was not confirmed by other observations.[50][51] Also, while comets have beenobserved to break up before,[52] no case has previously been found of a stable binary nucleus. Given the very smallmass of this comet, the orbit of the binary nucleus would be easily disrupted by the gravity of the Sun and planets.

UFO claims

Leader of the doomsday cult, Heaven'sGate, who organized a mass suicide inhopes of spiritually riding along with a

UFO he thought followed the Hale-Boppcomet.

In November 1996 amateur astronomer Chuck Shramek of Houston, Texastook a CCD image of the comet, which showed a fuzzy, slightly elongatedobject nearby. When his computer sky-viewing program did not identify thestar, Shramek called the Art Bell radio program Coast to Coast AM toannounce that he had discovered a "Saturn-like object" following Hale–Bopp.UFO enthusiasts, such as remote viewing proponent Courtney Brown, soonconcluded that there was an alien spacecraft following the comet.[53]

Several astronomers, including Alan Hale,[54] claimed the object was simplyan 8.5-magnitude star, SAO141894, which did not appear on Shramek'scomputer program because the user preferences were set incorrectly.[55]

Later, Art Bell even claimed to have obtained an image of the object from ananonymous astrophysicist who was about to confirm its discovery. However,astronomers Olivier Hainaut and David J. Tholen of the University of Hawaii stated that the alleged photo was analtered copy of one of their own comet images.[56]

A few months later, in March 1997, the cult Heaven's Gate committed mass suicide with the intention of teleportingto a spaceship they believed was flying behind the comet.[57]

Nancy Lieder, a self-proclaimed contactee who claims to receive messages from aliens through an implant in her brain, stated that Hale–Bopp was a fiction designed to distract the population from the coming arrival of "Nibiru" or "Planet X", a giant planet whose close passage would disrupt the Earth's rotation, causing global cataclysm.[58]

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Although Lieder's original date for the apocalypse, May 2003, has now passed, the imminent arrival of Nibiru is stillpredicted by various conspiracy websites, most of whom tie it to the 2012 phenomenon.[59] This also passed.

Legacy

Comet Hale–Bopp, at a distance ofnearly 2 billion kilometres from the Sun.

Credit: ESO

Its lengthy period of visibility and extensive coverage in the media meant thatHale–Bopp was probably the most-observed comet in history, making a fargreater impact on the general public than the return of Halley's Comet in1986, and certainly seen by a greater number of people than witnessed any ofHalley's previous appearances. For instance, 69% of Americans had seenHale–Bopp by April 9, 1997.[60]

Hale-Bopp was a record-breaking comet—the farthest comet from the Sundiscovered by amateurs,[21] with the largest well-measured cometary nucleusknown after 95P/Chiron,[16] and it was visible to the naked eye for twice aslong as the previous record-holder.[17] It was also brighter than magnitude 0for eight weeks, longer than any other recorded comet.[21]

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Computing and Minor Planet Sections. . Retrieved 2009-12-17.[4] Horizons output. "Barycentric Osculating Orbital Elements for Comet C/1995 O1 (Hale-Bopp)" (http:/ / ssd. jpl. nasa. gov/ horizons.

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[5] "Solex 10 estimate for Next Perihelion of C/1995 O1 (Hale-Bopp)" (http:/ / home. surewest. net/ kheider/ astro/ Hale-Bopp4385. txt). .Retrieved 2009-12-18.

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[7] Hale, A.; Bopp, T.; Stevens, J. (July 23, 1995). "IAU Circular No. 6187" (http:/ / www. cbat. eps. harvard. edu/ iauc/ 06100/ 06187.html#Item1). IAU. . Retrieved 2011-07-05.

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[54] Hale, Alan (1997). "Hale-Bopp Comet Madness" (http:/ / www. csicop. org/ si/ show/ hale-bopp_comet_madness/ ). Skeptical Inquirer 21(2): 25–28. .

[55] Burnham, Robert; Levy, David H. (2000). Great Comets. Cambridge University Press. pp. 191. ISBN 978-0-521-64600-0.[56] Tholen, David J. (January 15, 1997). "Fraudulent use of a IfA/UH picture [[[sic (http:/ / web. archive. org/ web/ 20070930225155/ http:/ /

www. sc. eso. org/ ~ohainaut/ Hale_Bopp/ hb_ufo_tholen. html)]]"]. European Organisation for Astronomical Research in the SouthernHemisphere. Archived from the original (http:/ / www. sc. eso. org/ ~ohainaut/ Hale_Bopp/ hb_ufo_tholen. html) on September 30, 2007. .Retrieved 2008-10-14.

[57] Robinson, Wendy Gale. "Heaven's Gate: The End" (http:/ / jcmc. indiana. edu/ vol3/ issue3/ robinson. html). Journal of Computer-MediatedCommunication 3 (3). .

[58] George Johnson (1997-03-28). "Comets Breed Fear, Fascination and Web Sites" (http:/ / www. nytimes. com/ 1997/ 03/ 28/ us/comets-breed-fear-fascination-and-web-sites. html?sec=health& spon=& pagewanted=all). The New York Times. . Retrieved 2009-09-27.

[59] David Morrison. "The Myth of Nibiru and the End of the World in 2012" (http:/ / www. csicop. org/ si/ 2008-05/ morrison. html). SkepicalEnquirer. . Retrieved 2009-04-28.

[60] Aguirre, Edwin L. (July 1997). "The Great Comet of 1997" (http:/ / pqasb. pqarchiver. com/ skyandtelescope/ access/ 886291661.html?dids=886291661:886291661& FMT=CITE& FMTS=CITE:PAGE& date=Jul+ 1997& author=Edwin+ L+ Aguirre& desc=The+ Great+Comet+ of+ 1997). Sky and Telescope. .

External links• Cometography.com: Comet Hale-Bopp (http:/ / www. cometography. com/ lcomets/ 1995o1. html)• NASA Hale-Bopp page (http:/ / www. jpl. nasa. gov/ comet)• Hale-Bopp (http:/ / ssd. jpl. nasa. gov/ sbdb. cgi?sstr=Hale-Bopp#content) at the JPL Small-Body Database

• Shadow and Substance.com: Static orbital diagram (http:/ / www. shadowandsubstance. com/ Past graphics/Comet Hale-Bopp orbit. htm)

• Comet Nucleus Animation (http:/ / one. revver. com/ watch/ 90657/ flv/ affiliate/ 14715)

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Alvarez hypothesis 64

Alvarez hypothesisThe Alvarez hypothesis posits that the mass extinction of the dinosaurs and many other living things was caused bythe impact of a large asteroid on the Earth sixty-five million years ago, called the Cretaceous–Paleogene extinctionevent. Evidence indicates that the asteroid fell in the Yucatán Peninsula, at Chicxulub, Mexico. The hypothesis isnamed after the father-and-son team of scientists Luis and Walter Alvarez, who first suggested it in 1980. In March2010 an international panel of scientists endorsed the asteroid hypothesis, specifically the Chicxulub impact, as beingthe cause of the extinction. A team of 41 scientists reviewed 20 years of scientific literature and in so doing alsoruled out other theories such as massive volcanism. They had determined that a 10–15 km (6–9 mi) space rockhurtled into earth at Chicxulub. The rock's size could be approximately the entire size of Martian moon Deimos(mean radius 6.2 km); the collision would have released the same energy as 100 teratonnes of TNT (420 ZJ), over abillion times the energy of the atomic bombs dropped on Hiroshima and Nagasaki.[1]

HistoryIn 1980, a team of researchers led by Nobel prize-winning physicist Luis Alvarez, his son geologist Walter Alvarezand chemists Frank Asaro and Helen Michels discovered that sedimentary layers found all over the world at theCretaceous–Paleogene boundary (Cretaceous–Tertiary boundary or K–T boundary) contain a concentration ofiridium hundreds of times greater than normal. Iridium is extremely rare in the Earth's crust because it is very dense,and therefore most of it sank into the Earth's core while the earth was still molten. The Alvarez team suggested thatan asteroid struck the earth at the time of the Cretaceous–Paleogene boundary.[2] There were other earlierspeculations on the possibility of an impact event, but no evidence had been uncovered at that time.[3]

EvidenceThe evidence for the Alvarez impact hypothesis is supported by chondritic meteorites and asteroids which contain amuch higher iridium concentration than the Earth's crust. The isotopic ratio of iridium in asteroids is similar to thatof the Cretaceous–Paleogene boundary layer but significantly different from the ratio in the Earth's crust. Chromiumisotopic anomalies found in Cretaceous–Paleogene boundary sediments are similar to that of an asteroid or a cometcomposed of carbonaceous chondrites. Shocked quartz granules, glass spherules and tektites, indicative of an impactevent, are common in the Cretaceous–Paleogene boundary, especially in deposits from around the Caribbean. All ofthese constituents are embedded in a layer of clay, which the Alvarez team interpreted as the debris spread all overthe world by the impact.[2] The location of the impact was unknown when the Alvarez team developed theirhypothesis, but later scientists discovered the Chicxulub Crater in the Yucatán Peninsula, now considered the likelyimpact site.

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Alvarez hypothesis 65

Badlands near Drumheller, Alberta where erosion has exposed the K–Pg boundary.

Using estimates of the total amount ofiridium in the K–T layer, and assuming thatthe asteroid contained the normal percentageof iridium found in chondrites, the Alvarezteam went on to calculate the size of theasteroid. The answer was about 10kilometers (6 mi) in diameter, about the sizeof Manhattan.[2] Such a large impact wouldhave had approximately the energy of 1 x108 megatons, i.e. about 2 million times asgreat as the most powerful thermonuclearbomb ever tested.

Impact

The most easily observable consequence ofsuch an impact would be a vast dust cloud which would block sunlight and prevent photosynthesis for a few years.This would account for the extinction of plants and phytoplankton and of all organisms dependent on them(including predatory animals as well as herbivores). But small creatures whose food chains were based on detrituswould have a reasonable chance of survival. It is estimated that sulfuric acid aerosols were injected into thestratosphere, leading to a 10–20% reduction of solar transmission normal for that period. It would have taken at leastten years for those aerosols to dissipate.[4]

Global firestorms may have resulted as incendiary fragments from the blast fell back to Earth. Analyses of fluidinclusions in ancient amber suggest that the oxygen content of the atmosphere was very high (30–35%) during thelate Cretaceous. This high O2 level would have supported intense combustion. The level of atmospheric O2plummeted in the early Tertiary Period. If widespread fires occurred, they would have increased the CO2 content ofthe atmosphere and caused a temporary greenhouse effect once the dust cloud settled, and this would haveexterminated the most vulnerable survivors of the "long winter".The impact may also have produced acid rain, depending on what type of rock the asteroid struck. However, recentresearch suggests this effect was relatively minor. Chemical buffers would have limited the changes, and the survivalof animals vulnerable to acid rain effects (such as frogs) indicate this was not a major contributor to extinction.[5]

Impact hypotheses can only explain very rapid extinctions, since the dust clouds and possible sulphuric aerosolswould wash out of the atmosphere in a fairly short time — possibly under ten years.Although further studies of the K–T layer consistently show the excess of iridium, the idea that the dinosaurs wereexterminated by an asteroid remained a matter of controversy among geologists and paleontologists for more than adecade.

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References[1] Schulte, P.; Alegret, L.; Arenillas, I.; Arz, J. A.; Barton, P. J.; Bown, P. R.; Bralower, T. J.; Christeson, G. L. et al. (5 March 2010). "The

Chicxulub Asteroid Impact and Mass Extinction at the Cretaceous- Paleogene Boundary". Science 327 (5970): 1214–1218.Bibcode 2010Sci...327.1214S. doi:10.1126/science.1177265. PMID 20203042.

[2] Alvarez, LW, Alvarez, W, Asaro, F, and Michel, HV (1980). "Extraterrestrial cause for the Cretaceous–Tertiary extinction". Science 208(4448): 1095–1108. Bibcode 1980Sci...208.1095A. doi:10.1126/science.208.4448.1095. PMID 17783054.

[3] De Laubenfels, MW (1956). "Dinosaur Extinctions: One More Hypothesis" (http:/ / www. norwebster. com/ astrohit/ ) (subscriptionrequired). Journal of Paleontology 30 (1): 207–218. . Retrieved 2007-05-22.

[4] Ocampo, A, Vajda, V & Buffetaut, E (2006). Unravelling the Cretaceous–Paleogene (KT) Turnover, Evidence from Flora, Fauna andGeology in Biological Processes Associated with Impact Events (Cockell, C, Gilmour, I & Koeberl, C, editors) (http:/ / www. springerlink.com/ content/ vw75014157p2p278/ ). SpringerLink. pp. 197–219. ISBN 978-3-540-25735-6. . Retrieved 2007-06-17.

[5] Kring, DA (2003). "Environmental consequences of impact cratering events as a function of ambient conditions on Earth". Astrobiology 3(1): 133–152. Bibcode 2003AsBio...3..133K. doi:10.1089/153110703321632471. PMID 12809133.

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Article Sources and Contributors 67

Article Sources and ContributorsCentaurus A  Source: http://en.wikipedia.org/w/index.php?oldid=531972161  Contributors: AhMedRMaaty, Albester, Aranel, Auke Slotegraaf, Blankfaze, Braincricket, Bryan Derksen, Caco devidro, Chris the speller, Clh288, CommonsDelinker, Conscious, Count Iblis, Danim, Davidhorman, Dr. Submillimeter, Earthandmoon, Eleuther, Elphion, Fotaun, Friendlystar, Gallina3795,Giraffedata, Gnixon, Hcobb, Headbomb, HorsePunchKid, Hurricane Devon, JamesHoadley, Jauhienij, Jleous, Jmencisom, John of Reading, Joseph Dwayne, Jyril, Kheider, Kozuch, LarsLindberg Christensen, Luiscalcada, Martarius, MortimerCat, Nick, Old wombat, OlivierHainaut, Originalwana, Peter Ellis, Pine, RJHall, Reyk, Rjwilmsi, Rl, Roberto Mura, Shawn81, SimonP,Stepa, Svanimpe, Sverdrup, Taggard, Telescopi, Uber nemo, Uriel8, WilliamKF, WolfmanSF, Worldtraveller, Wwheaton, Zzzzzzzzzzz, 29 anonymous edits

Andromeda Galaxy  Source: http://en.wikipedia.org/w/index.php?oldid=534427376  Contributors: 2D, 4pq1injbok, 68Kustom, 84user, A2Kafir, Aaaxlp, Aeon1006, Afudge, Ageekgal, Ahpook,Aisphording, Aitias, Ajames2212, Ajax-and-Achilles, Ajuk, Akathetruth111, Alfio, Alnokta, Ancheta Wis, AndHow90, Andre Engels, Andrea105, AndyZ, Angelpbj, Angr, Animum, AnonymousDissident, Antandrus, Antonsusi, Arakunem, ArdWar, Ardric47, Arpingstone, Art LaPella, AshLin, Ashmoo, AsperaAT, AstroMalasorte, AstroNomer, Astrobiologist, Astronautics, Attilios,Audriusa, Avenue, Avillia, AxelBoldt, BOMBINI, BPositive, Bachcell, Baldhur, Bencherlite, Bender235, Bigturtle, Blow, Bobblewik, Br'er Rabbit, Brainbark, BrianBird, Brianhurley, Brodika,Bronger, Bryan Derksen, Cadiomals, Cakechild, Canon.vs.nikon, Capricorn42, Casliber, Catgut, Celuici, Chahl, Chaos syndrome, Charleshull6, Chelseamarie322, ChicXulub, Chr.K.,ChrisGualtieri, Circeus, Cireshoe, ClaudioSantos, Clh288, Clockwork Droid, Cmapm, Cometstyles, Comp Ninja, Cp111, Cremepuff222, Curps, Cyde, Cyrius, DaMatriX, Dak1ng995, DanielCenteno, Darkskynet, Darkspace72, Darren curtis, Daverocks, David J Johnson, DavidK93, Dawnseeker2000, DeadEyeArrow, Deflective, Dendlai, DerHexer, Dino, Discospinster, DisplacedBrit, Djameel, Doradus, Dorftrottel, Downwards, Dr. Submillimeter, DrFO.Jr.Tn, Dudley Miles, Duk, Duodecimal, Dwimsey, Dzhim, Editor2020, El C, Elvarg, Emilio floris, Epbr123,ErgoSum88, Eric Kvaalen, Etacar11, Eteq, EvenGreenerFish, Evercat, Excirial, Fabianneyer, Falkonry, Farseer, Fhqwhgads14172, Fjörgynn, FlavrSavr, Fluttershy is so cute!, Fotaun, Fredsmith2,Funandtrvl, Furrykef, Garrett0305, Garrisonroo, Gauss, Glane23, Gogo Dodo, Gracenotes, Graham87, Greaseball the locomotive, GregorB, GrouchyDan, Gtrmp, Haham hanuka, Halfblue,Hamiltonstone, Hamster3, Hans Dunkelberg, Harp, Hashar, Headbomb, Hekerui, Hellbus, Henry Flower, Herbee, Hoagssculptor, Horselover Frost, Hurricane Devon, IVAN3MAN, Iam onandromeda, Icairns, Icemuon, InfiniteThinking, Ing.CP, J.delanoy, Jacqueslacouth, Jagged 85, James McBride, James086, JamesHoadley, Jauhienij, Jdlambert, Jeffmedkeff, Jhertel, Jj1236,Jmencisom, JoJan, Joedeshon, Joeyspqr, John D. Croft, JohnOwens, Jon513, Jonathunder, JorisvS, Joseph Dwayne, Joseph Solis in Australia, Jph, Jusdafax, Jyril, K30, KGyST, KSmrq,Katydidit, Kembangraps, Keraunos, Kevin Saff, Kgrad, Khalid Mahmood, Kheider, Killdevil, Kloy1334, Knightshield, Kostisl, Kriss12, KuduIO, Kukamunga13, Kwamikagami, Landroo, LarryV, Lars Lindberg Christensen, LeaveSleaves, Lee, Leo C Stein, Lightmouse, Ling.Nut, Little Mountain 5, Lopo, Lotje, M00npirate, MER-C, Magioladitis, Mako098765, Mark Arsten, MarkFoskey, Markocms, Marshallsumter, Massimo Macconi, Mateuszica, Mathwhiz90601, MattHaffner, Matthew, Maurice Carbonaro, Maxim, Maxim Razin, Meewam, Megaman en m, Meltonkt,Mensch, Mephistophelian, Metalhead94, Mgiganteus1, Mhking, Michael C Price, Michael9422, Michaelbusch, Mike Payne, Mike Peel, Mike s, Mkativerata, Moland Spring, Montrealais,Moon&Nature, Mortense, Moverton, Mschlindwein, Mu301, Myrvin, N328KF, NFAN3, Nandesuka, Nareek, Nate Silva, NatureA16, NeilN, Nerroth, Netoholic, NewEnglandYankee, Nickptar,Nine9s, Nivedh, Noctibus, Nono64, Octane, Okthef, OperaJoeGreen, Orgio89, Originalwana, Outriggr, Oxymoron83, PBiton, POOPY321, Pagw, Paine Ellsworth, Parmis17, PeteXor, Peter B.,Peterkennett, Petermarcus, Petersam, Phelpsjr, Piperh, Planetary, Plasticup, Pmcray, Pol098, Poolkris, Possum, Proxima Centauri, Prvc, Pstank92, Puzl bustr, Qui1che, RJHall, RL579, Racerx11,Rajamatage, RandomCritic, Ranger One, RaptorHunter, Raskolnikov The Penguin, Razorflame, RedHillian, Reddi, Retiono Virginian, Rezin8, RickV88, Ricnun, Rjwilmsi, Rnt20, Robert K S,Roberto Mura, Roentgenium111, Ronald.davis, RossF18, Rotem Dan, Rothorpe, RoyBoy, RoyalPurple1337, Rrburke, Rtkat3, Rune.welsh, Rwnn, SCEhardt, SMP, Sadalsuud, Salsa Shark,Sammy61157, Sarfa, Saros136, Scetoaux, SchuminWeb, Scope creep, Scott3, Scwlong, Sfan00 IMG, Shadowjams, Sheitan, Shyam, Slade95816, Smartech, Sneezor, Snowmanradio, Spacepotato,Spoirier, SqueakBox, Ssd, Stack, Ste4k, Stevepeterson, Stickee, Stupid2, Sumanthvepa, Superborsuk, Supercoop, Sverdrup, Swliv, THEN WHO WAS PHONE?, Tango, Telescopi, Tgeairn, TheBearded One, The Flying Spaghetti Monster, The Geography Elite, The High Fin Sperm Whale, The Illusive Man, The Rambling Man, The sunder king, TheDJ, TheTito, Theodork, Thingg,Thoughtcriminall, Thricecube, Thumperward, Thuresson, Tide rolls, Tigerdg, Tim Starling, Timwi, Titodutta, Titus III, Tman555, Tobby72, Tokeupdude, Tom.Reding, Tomruen, Tonisee,Torirene, TotoBaggins, Trinitrix, Troy 07, TutterMouse, Twang, Tyrol5, U-95, Uber nemo, Unyoyega, Utcursch, UtherSRG, Vedran8080, Vega2, Verdi1, VirtualDave, Vpereziii, Vsst, VystrixNexoth, Vyznev Xnebara, Vzb83, Wer900, Whouk, Wikieditor06, WilliamKF, Wjfox2005, Wlegro, WolfmanSF, Woohookitty, Worldtraveller, Wsvlqc, Wurdnurd, XJamRastafire, Xiaomao123,Zain Ebrahim111, Zaslav, ZeroEgo, Zhouqin, Zyxw, Zzzzzzzzzzz, と あ る 白 い 猫, 514 anonymous edits

Pleiades  Source: http://en.wikipedia.org/w/index.php?oldid=532416767  Contributors: (, (jarbarf), A.M.962, A2Kafir, Acceleracer, AdelaMae, Aeusoes1, Agorf, Aherunar, Ahoerstemeier,Akshaysrinivasan, Alexander Meleg, Alfio, Allistar.m, Andres, Anomalocaris, Antandrus, Anttler, Arch dude, Archird, ArdWar, Aristitleism, ArkinAardvark, Arpingstone, Arvind Vyas, Asciic,AstroMalasorte, Authenticmaya, Bacteria, Beland, Benbest, BillFlis, Bitil Guilderstrone, Bkonrad, BollyDave, Bryan Derksen, CALR, CJLL Wright, CPBOOTH, Calvinkrishy, Chaojoker,Chapmlg, Chenx064, Chinasaur, Chris Kyrzyk, Chrisbolt, Christian75, Chuunen Baka, Ckatz, Clh288, Clokverkorange, Clumsily, CommonsDelinker, Completesentence, Connecting the dots,Credulity, Cuaxdon, Curps, D, D3h, DNewhall, Dakotdk, Danny, Daveisall, Dbachmann, Deeptrivia, Denni, Deor, Deutschgirl, Dhanasekaran muthu, DiamondDave, DickisonR, Dlohcierekim,DokX, [email protected], DopefishJustin, Dourios, Download, Dr. Submillimeter, Długosz, Eall Ân Ûle, Ebizur, Eep², El C, El Roih, Eleuther, Ema Zee, Emphyrio, Emufarmers, EoGuy,Epastore, Error, Extra999, FaerieInGrey, FeanorStar7, Feezo, Florian Blaschke, Freelancer lsf, Friendlystar, Fæ, GLaDOS, Gail, Galoubet, Gazurtoid, Geboy, Gene Nygaard, Ghostgamer,Giftlite, Gilgamesh, Giorgio Baroni, Glenn L, Grandia01, Gurch, H2g2bob, Hadal, Hairy Dude, Hamiltonstone, Hans Dunkelberg, Headbomb, Henrodon, Hgilbert, Hunnjazal, Hurricane Devon,Hyacinth, Ilikeverin, Ilvon, Insomniacpuppy, Instinct, Irishguy, Iseeaboar, Itub, Itzcuauhlti, Ixfd64, J.delanoy, JPD, Jake Nelson, JamminJonah, Jauhienij, Jaw7, Jayron32, Jc-S0CO, Jeronimo,Jfortier, Jim77742, Jjorgensen, Jmrowland, Jo3sampl, John Price, John Spraggs, Joseph Dwayne, Joule36e5, Julia Rossi, K.shayanthan, Kaffi, Kahuroa, Kalsermar, Kanġi Oĥanko, Katydidit,Kbahey, Keraunos, Ketiltrout, Kevin Saff, Kheider, Khukri, Kinkreet, Kipala, Klausness, Knucmo2, Knyets, Koavf, Kusma, Kwamikagami, La goutte de pluie, Lampsalot, Lcmortensen, Leia,Lemmy315, Livajo, Livinglight, Lkitrossky, Llajwa, Logos5557, Lovibond, Lusanaherandraton, Mahmudmasri, Makana Chai, Maldodan, Marasama, Markjoseph125, Mathonius, MauriceCarbonaro, Mav, Max rspct, Maxim, Midgrid, Mike Dallwitz, Misfit815, MisfitToys, MistyMorn, MrFish, Muruganastro, Mussnoon, Mzajac, N328KF, Nadyes, Nareek, NaySay, Nazar, NcSchu,Nehrams2020, Newone, Nicboyde, Nik42, Nine9s, Niteowlneils, Njardarlogar, Noclevername, Nordisk varg, Ntsimp, Olessi, Olivier, Ortolan88, Paknur, Palica, Pauli133, PetaRZ, Pharaoh of theWizards, Pickledfootwear, PrivateWiddle, Processr, Prodego, Prvc, Pudeo, Puffin10, PuzzletChung, Pvloc90, Qbr12, QualityDr, Quinton12123, Qwfp, R'n'B, RG2, RHaworth, RJHall, RQG,RadicalOne, RadioFan, Regushee, Remdabest, Renato Caniatti, RexNL, Rjwilmsi, Rlupsa, Roberto Mura, Roth ritter, Rubicon, Ryan Norton, Sadalsuud, Samuelsen, Sannse, Satyriasis,Savemaxim, Sax Russell, Scog, Section103, Selket, Serendipodous, Serpent's Choice, Shadegan(goru), Sherefyounan, Sigo, Smaines, Smiller385, Snowolf, Some thing, Sonicology, Sonjaaa,Spacepotato, Spiko-carpediem, Spiritflute, Sry85, Stargazer 7000, Stephenbedingfield, Stevebritgimp, Stevemarlett, Sublime5891, Surajt88, Sushi, Sverdrup, Tarquin, Teddybearcan, Telescopi,Temp07, Tesi1700, Thatguynamedgreg, The Raven's Apprentice, Theda, TheresJamInTheHills, Thuyen, Tobias Conradi, Tom Peters, Tomruen, Tpbradbury, Trengarasu, TriniMuñoz, Trinitrix,Trusilver, Tucci528, Twas Now, Uicyend, UnitedStatesian, Uriel8, UserDoe, UtherSRG, Vedran8080, Velt76, Venture4, VladV, Vzb83, Wahabijaz, Wallslide, Wayward, Wer900, Wesino,Wiglaf, Wikianon, WilyD, Wizardist, Woohookitty, Worldtraveller, Wtmitchell, Xdenizen, Yogesh Khandke, Zereshk, Zerokitsune, Zonination, ZorkFox, Zundark, Zyxoas, Zyxw, عباد ديرانية ,باسم,சஞ்சீவி சிவகுமார், 428 anonymous edits

Orion (constellation)  Source: http://en.wikipedia.org/w/index.php?oldid=533810815  Contributors: -Ril-, 28bytes, 350z33, @pple, A. Parrot, Aaseeger, Abc518, Academic Challenger, Accurizer, Addshore, Adrian.benko, Ag1246, Ahoerstemeier, Aitias, Alai, Alansohn, Alcazar84, Alex earlier account, Algebraist, All Is One, All Worlds, Allens, Alphachapmtl, Altaïr, Amateurastro, Andonic, Andromeda321, Andy M. Wang, Andycjp, Andyman14, Angela, Animum, Ankur, Antandrus, ArcAngel, Archanamiya, Arsia Mons, Arthena, Arunsingh16, Asarelah, AstroHurricane001, AstroMalasorte, Avb, B. Wolterding, B.d.mills, B00P, Baccyak4H, Bark, BarretB, BartBenjamin, Bay Flam, Bazonka, Bellerophon5685, Belovedfreak, Bencherlite, Bender235, Beno1000, BereanDAD, Berig, Bertrand Bellet, Big Bird, BjKa, Blaxthos, Bobianite, Bongwarrior, BorgQueen, Brain40, Brent.austin, Bronger, Bruce Marlin, Bruinfan12, Bryan Derksen, Buck O'Nollege, Burner0718, Bwfletcher, C-nap13, C.Logan, CArules123, CIreland, CWY2190, CactusWriter, Cal-linux, CalRis25, Calvin 1998, Cam, Canderson7, Casliber, Ceyockey, Chaojoker, Chevy111, ChicXulub, Cholmes75, Chris Roy, ChrisGualtieri, Chrisdolan, Chriss.2, Citylover, Clandestine1138, Closedmouth, ClovisPt, Coffee, Colonies Chris, Cometstyles, Cool3, Coriander, Corvus cornix, Cosmium, Cras26, Cremepuff222, Crism, CrniBombarder!!!, Crypticfirefly, Curps, Dabbler, Dabrow, Danilchick, Darcyj, Darth Panda, Dbachmann, Deadlyops, Deagle AP, Decltype, Deconstructhis, Deflective, Devatipan, Dicklyon, Dimator, Dirkbb, Doceddi, Donarreiskoffer, Doppelgangland, Dorinh, Dotancohen, Doulos Christos, Download, Dpv, DrZarkov, Dragonbones, Dravick, Dschwen, Durova, Dusti, Earthsky, East718, EchetusXe, EconomicsGuy, EdGl, Editor2020, Egil, EkwanIMSA, El C, ElizWard, Emact, Equendil, Eroica, Errolhunt, Euchiasmus, EugeneZelenko, Excirial, FDuffy, FKmailliW, Fbifriday, Fieldday-sunday, Flex, Flibjib8, Flyguy649, Fosnez, Fram, Friendlystar, Fyyer, Fæ, GPHemsley, Gamemaker51, Gauss, George.dickeson, Giffdev, Giftlite, Gizzakk, Gomada, Good Olfactory, Goustien, Greenguy1090, GregAsche, Gscshoyru, Ha4iu5hj, HaeB, Hairy Dude, Hakluyt bean, Halfelven Ranger, Hanif 90, Hans Dunkelberg, Haveacigaro, Hede2000, HelloAnnyong, Hey jude, don't let me down, Hmrox, HollyI, Howcheng, Hrishikes, Hu12, HueSatLum, Hyperdeath, Ian.thomson, Icairns, Ijnicholas, Ikalpo, Ikkyu2, Ilvon, Immunize, Infrogmation, Into The Fray, Iron Dragon91, J.delanoy, JNW, JSpung, JYolkowski, JaGa, Jackflet44, James Bartosik, Jamiejojesus, Janneok, Jayjg, Jeandré du Toit, Jeff G., Jengod, Jim1138, Jimdevlin, Jimjom44, Jncraton, Joanjoc, John Hyams, John K, John254, John5246, Jor, Joyous!, Jujutacular, Jyril, Kalsermar, Kariteh, KarlM, Keilana, Ketiltrout, Khalid Mahmood, Khalil633, Kimjae20, King of Hearts, Kintetsubuffalo, KirtZJ, Kitch, Kjlewis, Knightzy99, KnowledgeOfSelf, KoshVorlon, Krellis, Kristaga, Ks0stm, Kukini, Kurek6, Kwamikagami, LAX, LFaraone, Lamb99, Landroo, Lar, LedgendGamer, Legion fi, Leonardorejorge, Leonfreak17, Leoni2, Lestatdelc, Liekkis, Lights, Lol0074, Looxix, Lorenzarius, Lowzeewee, LrdChaos, Lumos3, MER-C, Madhero88, Magiclite, Magnus Manske, Man vyi, Mani1, Marek69, Marnen, Martarius, MartinHarper, Materialscientist, Mattgirling, Maximaximax, Michaelbusch, Mike Peel, Mlt29994, Montrealais, Moomoomoo, Mouser, Mygerardromance, MylesCallum, Mysid, Mzajac, NW's Public Sock, Nadyes, NawlinWiki, NeilFraser, Nick Ottery, Nick Wilson, Nik42, Niteowlneils, Nivix, Noctibus, Nongendered, Nono64, NorwegianBlue, O.Koslowski, Officiallyover, Ohnoitsjamie, OlEnglish, OldakQuill, Omg you suck balls, Onorem, Organic cookie, Ospalh, Oxymoron83, Palica, ParisianBlade, Patrick, Paul Drye, PaulGarner, Pb30, Persian Poet Gal, Petecarney, Petersam, Phaldo, Pharaoh of the Wizards, Pharos, Philly jawn, Pie4all88, Pizza Puzzle, PlanetStar, Plynn9, Pmanderson, Poolkris, Poopiedooo, Possum, Prodego, Puchiko, Punisher2404, Qoholeth, Quadell, R, RJDdrum1, RJHall, Radagast83, Random astronomer, RandomCritic, Rapscallion, Raven in Orbit, Rayd8, Red Bulls Fan, Relay11, Reywas92, Rhopkins8, Rich Farmbrough, Richontaban, Riffsyphon1024, RingtailedFox, Rise Against713, Risramlal, Roadsidemultimedia 2, Rob Hooft, Roberta F., Robsavoie, Rory096, Rothorpe, Rrburke, Ruben31, Rursus, Rushbugled13, Rāmā, S.Chepurin, SamuelTheGhost, Sankalpdravid, Sarz, SassyLilNugget, Satellizer, SchfiftyThree, ScottDavis, Sectori, Shalom Yechiel, Shawn81, ShelfSkewed, Shortiewaswo, Shortna, Silversatellite, Skatebiker, Skeptic2, Skirrid, Skizzik, SkyMachine, Slakr, Smartech, Snowolf, Sole Soul, Sonicology, SpaceFlight89, Spacepotato, Spitfire19, Sreyantha chary, Starlover2005, Starstriker7, StaticGull, Stele, Stephenb, Stephenchou0722, Steve03Mills, Stevemarlett, Stevertigo, Stevey7788, Summer Song, Sverdrup, TShilo12, Taam, TakingUpSpace,

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Tamás, Tanketai, Tdvance, Telescopi, Tellyaddict, Tenguopr, TestimonyofJesus, The Cunctator, The Seventh Taylor, The Thing That Should Not Be, The sunder king, TheTito, Thebestofall007,Theelf29, Thingg, Thor Dockweiler, Thortveitite, Thu, Tide rolls, Til Eulenspiegel, Till Credner, ToastKing1000, Tomruen, Travelbird, TriniMuñoz, Trystan, Turk oğlan, Twang, Twas Now, TyrAnasazi, UNIT A4B1, USMA, Ultor Solis, Urania's Muse, UtherSRG, Vacuum, Venture4, Verdi1, Versus22, Vgent, Vrenator, Vssun, Vuong Ngan Ha, Wassini, Wavelength, Wereon,Wes.faires, West.andrew.g, Whale plane, Whpq, Wiglaf, Wiki alf, Wiki13, Wikibob, Wikipeedio, Wikipelli, Wikislemur, Willard, WilliamKF, Wizardist, Wkboonec, Wmpittendreigh, Woody,Woohookitty, Worldtraveller, Wrnchhead76, Wwoods, Xavierxolando, Xosé, Xtifr, Yzkoc, Zanaq, Zaphod Beeblebrox, Zoicon5, Zollerriia, Zondrah89, ZtCS, 1025 anonymous edits

Orion Nebula  Source: http://en.wikipedia.org/w/index.php?oldid=533530378  Contributors: 25or6to4, 32X, Abdullah Murad, Ahoerstemeier, Alex Arnold, Alexius08, Alfio, Alsandro,Andromeda321, AndyJ231, AndyZ, Ardric47, Arianewiki1, Arnomane, Arpingstone, AstroMalasorte, Astropithicus, Awardgive, Awolf002, B.d.mills, Bastique, Bay Flam, Bender235,Bjankuloski06en, Bob Burkhardt, Bobblewik, Bozlich, Bryan Derksen, CAkira, Caltas, Can't sleep, clown will eat me, Chaos syndrome, ChicXulub, Chippym22, Chris the speller, CielProfond,Clh288, Cometstyles, Conscious, Ctrl build, D6, DannyZ, Davepape, Deeptrivia, Deryck Chan, Dobie80, Docu, Dr. Submillimeter, DubaiTerminator, Dustin winski, Earth358, Edgar181, Emijrp,Epbr123, Erik, Etacar11, Eumolpo, Ewlyahoocom, Exir Kamalabadi, FKmailliW, Fairychild, Falcorian, Ferrettini, Forever Dusk, Fotaun, Fountains of Bryn Mawr, Friendlystar, Fëaluinix, Gauss,George100, Gikü, GoneAwayNowAndRetired, Gscshoyru, Gzkn, H, Hajatvrc, Headbomb, Heiny, Hewholooks, Hmoul, Howcheng, ILovePlankton, Iwillharasu, J.delanoy, JamesHoadley,Jan1nad, Jargon777, Jascal, Jauhienij, Jeff Saxton, Jerzy, Jleous, Jmencisom, John Belushi, Jon Harald Søby, JorisvS, Joseph Dwayne, Jyril, KGyST, Kadellar, Katieh5584, Katydidit, Ke6jjj,Keflavich, Kellyprice, Kendrick7, Kevin Saff, Khalid Mahmood, Khazar2, Knowledge Seeker, Koavf, Kobrabones, Korg, Lars Lindberg Christensen, Ligulem, Lilac Soul, Looxix, Luiscalcada,Lzz, M1ss1ontomars2k4, Mackensen, Marco Castellani, Melesse, Mgiganteus1, Mickal555, Mike Peel, Mild Bill Hiccup, MisfitToys, Mishuletz, Mitth'raw'nuruodo, Mordemur, MuDavid,Newone, Ngc7023, Nine9s, Nivix, Nono64, Officiallyover, Ohconfucius, OperaJoeGreen, Originalwana, Palica, Pandora, Patrick, Paul Drye, Peppe83, PetaRZ, Petersam, PhGustaf, Pingku,Pioannid, Polaris999, Puddington, R'n'B, RJHall, Rentier, Rich Farmbrough, Rickjamez, RingtailedFox, Rjwilmsi, Rnt20, Roberto Mura, Rock4arolla, Rrburke, Rrv1988, Rshida, Rsrikanth05,Samsara, Sannse, ScottyBoy900Q, Shadowjams, Shizhao, Skeptic2, Slakr, Spacepotato, Spanky71, Ssd, Sverdrup, TJRC, Tarotcards, Tarquin, Telescopi, TestPilot, The Thing That Should NotBe, TheAlphaWolf, Tide rolls, Tim Starling, Tripledot, TrygveFlathen, Twinsday, Urbanclearway, Urhixidur, Vedran8080, Vgent, Wafulz, Wassini, Wikiborg4711, Wikisteff, WilliamKF,WolfmanSF, Worldtraveller, XJamRastafire, YGingras, 150 anonymous edits

Eta Carinae  Source: http://en.wikipedia.org/w/index.php?oldid=531014612  Contributors: 121snake121, 1996vishak, 1dragon, 7903xc, Achoo5000, Aherunar, Alfio, Anagnorisis, AndreEngels, AndreaPersephone, Andrew the coolest, Anville, Aranel, ArdWar, Arsia Mons, Ashill, AstroHurricane001, Attilios, B.d.mills, B00P, Babbage, Bazonka, Bender235, BillC, Binrdow,BjKa, Bkonrad, Bob rulz, Bowlhover, Bryan Derksen, Burkhard.Plache, Butsuri, Canterel, Captain Seafort, Casliber, Caspianm, Chiaro, ChileVerde, CommonsDelinker, Corcoran, Curps, Cyrius,David Newton, Deadlock, Delldot, Diamonddavej, Digulla, Doradus, Dorftrottel, Dougmc, Dr.Gonzo, Eassin, El C, Elwood00, Etacar11, Fosnez, Fournax, Francesco Malafarina, Furrykef,Gadykozma, Gaz, Gdr, Geboy, Georgewilliamherbert, GhostTrain, Gilliam, Gmaxwell, Golbez, Graham87, GregorB, GrimmigerHagen, Guitarist6987876, HarryAlffa, Headbomb, Hmmm,IVAN3MAN, IW.HG, IanOsgood, Ikemeister1551, Ilvon, Imzogelmo, InternetMeme, It Is Me Here, Ixfd64, JBsupreme, Jafro, Jamie C, Jaysweet, Jbeans, JeLuF, Jennavecia, Jessemerriman,Jezza333, Jkl, Jmacwiki, JodyB, Josh215, JoshuaZ, Julesd, Jusdafax, Jyril, Kanthoney, Kazvorpal, Keflavich, Kellyprice, Ken Arromdee, Ketiltrout, Kheider, King Hildebrand, Kometsuga,Kwamikagami, Lars Lindberg Christensen, LiamE, Lithopsian, Livajo, Lomn, Luna Santin, MER-C, Markjoseph125, Marshallsumter, Martarius, Mateuszica, Maury Markowitz, Mazca,Metebelis, Michael Frind, Modest Genius, Moonraker12, Moverton, Namangwari, Neitherday, Newone, Njardarlogar, Nonagonal Spider, Noosphere, NuclearWarfare, Obiwan042, Orioane,PFSLAKES1, Paine Ellsworth, Palica, Patrick1982, Pcd72, PedroPVZ, Percommode, Pi.1415926535, PlanetStar, Plasticup, Prvc, Q43, RJHall, RandomCritic, Rattlesnake, Rbryson74, Rccoms,Rich Farmbrough, Rickington, Ricky58267, RingtailedFox, Rjwilmsi, Rnt20, RodC, Rothorpe, Rursus, Ryan suchocki, S. Korotkiy, Saravask, ScAvenger, Siddiqui, Skatebiker, Skeletor 0, Skorede, SkyLined, Skylark42, Slightsmile, Smalljim, Sonicology, Spacepotato, Spikebrennan, StringTheory11, Sumanch, Sverdrup, SwordSmurf, Tarlneustaedter, Tgm8, TheWhistleGang, Tirerim,Todd Lowery, Tom, Tom Lougheed, Tompw, TopGUN71691, Tracer9999, Trevor.tombe, Trovatore, Troyt, Tycho Magnetic Anomaly-1, Unagiflum, UncleBubba, Vicki Rosenzweig, Viriditas,WLRoss, Wdfarmer, Wer900, Wetman, WilliamKF, WolfmanSF, Worldtraveller, XJamRastafire, Zundark, 孟 柏 民, 228 anonymous edits

Comet Hale–Bopp  Source: http://en.wikipedia.org/w/index.php?oldid=532692164  Contributors: 0, A305w, AZphotogallery, Addshore, Adrian.benko, Ahoerstemeier, Ahpook, Alchemy187,Alsandro, Amirber, Angusmclellan, Apcgurutech, Apteva, Arakunem, Arcturus, Ascánder, Ataleh, Atkinson 291, Autarch, Avenue, Avicennasis, B, B.d.mills, Badmachine, Basilicofresco,Bender235, Bentogoa, Blizzard1, BobShair, Bobblewik, Bobo192, Bodzasfanta, Bpeps, Brighterorange, British Commando, CDN99, CTF83!, CWenger, Caltas, Carmichael, Cfailde,Charlesdrakew, Chesnok, Cirt, Cracked acorns, Cramyourspam, Crash Underride, Crashdoom, CrniBombarder!!!, Curps, Cyanoa Crylate, DVdm, Dailycare, Dalillama, Dana boomer, DanteAlighieri, Darth Panda, DaveHer, Deltabeignet, Demf, DerHexer, Deviator13, Dicklyon, Discospinster, Dolovis, Dominus, Donarreiskoffer, Donfbreed, DrKiernan, Drydom, Duoduoduo,Dyslexic agnostic, EarthPerson, Ed Poor, Edward321, Eisnel, Eleschinski2000, Enirac Sum, Erikeltic, Ethan Mitchell, Euryalus, Evil saltine, Ewlyahoocom, Feitclub, Fjörgynn, Fly by Night,Francesco Betti Sorbelli, Fukumoto, GB fan, Gail, Gaius Cornelius, Gene Nygaard, Geni, Gerda Arendt, Gilliam, Gimmetrow, GirasoleDE, Giuliopp, Glyph27notfound, Gnoitall, Gogo Dodo,Gpietsch, Grant Gussie, GregorB, H, HJensen, Headbomb, Hetar, Hike395, Hydrargyrum, Icairns, J.delanoy, JPX7, Jcd113, Jenser, Jhapeman, JorisvS, Joshua Scott, Joshuapaquin, Jyp, Jyril,KLooove, Kanags, Kbaliyan, Ke6jjj, Ken g6, Ketiltrout, Kheider, KnightRider, Kolbasz, Kurykh, Kwamikagami, Lantrix, Levelistchampion, Lfh, Logan, Loloyedwazup, Lupin, MER-C, Macy,Magioladitis, Malvolio80, Marek69, Markus Poessel, Marskell, Martschink, Matt Crypto, Mattbuck, Maurog, Mike Peel, Mike s, MinutiaeMan, MisfitToys, Mkfairdpm, Modest Genius,Moonriddengirl, Mvgossman, Mystykmyk, Nadiasupergirl, Nertzy, Nickshanks, Noctibus, NuclearWarfare, Nv8200p, P. S. Burton, Pacaro, Pachtova, Pagrashtak, Paxsimius, Penwhale,Peruvianllama, Phil Boswell, Phoenix58, Piano non troppo, Pierre2012, Piledhigheranddeeper, Pinkadelica, ProfGiles, PseudoOne, Pthag, Q85, Quadell, Qui1che, Random astronomer, Raul654,ResidentAnthropologist, RetiredUser2, Revolución, Rich Farmbrough, Rjwilmsi, Rmhermen, RobertG, Rock4arolla, Rothorpe, RunninRiot, Ruodyssey, Ruslik0, Sakar, Salilmathur, Samuel Sol,SandyGeorgia, Satori, ScAvenger, SchuminWeb, Scwlong, Serendipodous, Sh f80, Shanes, Shirulashem, Sidgrollix, Siim, Skizzik, Slawekb, Some jerk on the Internet, Stars1408, Steveprutz,Stone, Sun Creator, Susvolans, Szabomgyula, Tablizer, Tacos tacos tacos15, Taxman, Tbc2, Teresia, The Haunted Angel, The Olive Branch, The Thing That Should Not Be, The way, the truth,and the light, Theresa knott, Thincat, Thingg, Threedots dead, Thunderbird2, Tim Chambers, Tobias Hoevekamp, Tommy2010, Tomruen, Tothebarricades.tk, Tracer9999, Treisijs, Triphop,Unclenuclear, Vzb83, Vũ Ngọc Phan, Wafulz, Warlordgeneral1216, Wayne Hardman, Wayward, Whosasking, WilliamKF, WojPob, WolfmanSF, Woohookitty, Worldtraveller, Wrotesolid,Xndr, Xtifr, Xuehxolotl, Yamamoto Ichiro, Yekrats, Yuckfoo, Zubenelgenubi, Zundark, Zxcvbnm, Ævar Arnfjörð Bjarmason, 361 anonymous edits

Alvarez hypothesis  Source: http://en.wikipedia.org/w/index.php?oldid=527210832  Contributors: Alexzabbey, Anyeverybody, Balloonguy, Brinlong, Download, Dr. Blofeld, Eastlaw,Emerson7, EncycloPetey, Finlay McWalter, Fletcher, Ikluft, Joseph Solis in Australia, Kbdank71, Keyboardica, Learner59, Melaen, Mikker, NawlinWiki, Nwbeeson, Ouzo, Parsa, Paul H.,Peregrine Fisher, Rjwilmsi, Shanes, Smith609, Stephenb, WolfmanSF, 18 anonymous edits

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Image Sources, Licenses and ContributorsImage:ESO Centaurus A LABOCA.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:ESO_Centaurus_A_LABOCA.jpg  License: Creative Commons Attribution 3.0  Contributors:ESO/WFI (Optical); MPIfR/ESO/APEX/A.Weiss et al. (Submillimetre); NASA/CXC/CfA/R.Kraft et al. (X-ray)Image:Radio galaxy Centaurus A by ALMA.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Radio_galaxy_Centaurus_A_by_ALMA.jpg  License: Creative Commons Attribution3.0  Contributors: JmencisomImage:NGC 5128 MPG ESO 2.2-metre.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:NGC_5128_MPG_ESO_2.2-metre.jpg  License: Creative Commons Attribution 3.0 Contributors: Jmencisom, 1 anonymous editsImage:Firestorm of Star Birth in Galaxy Centaurus A.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Firestorm_of_Star_Birth_in_Galaxy_Centaurus_A.jpg  License: PublicDomain  Contributors: User:Dipankan001Image:CentaurusA2.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:CentaurusA2.jpg  License: Public domain  Contributors: Original uploader was Hurricane Devon at en.wikipediaImage:CenA-nearIR.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:CenA-nearIR.jpg  License: Creative Commons Attribution 3.0  Contributors: ESO/Y. BeletskyImage:CentaurusA3.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:CentaurusA3.jpg  License: Public Domain  Contributors: NASA/JPL-Caltech/SST J. Keene (SSC/Caltech)Original uploader was Hurricane Devon at en.wikipediaImage:NGC_5128.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:NGC_5128.jpg  License: Public Domain  Contributors: Boivie, Eleferen, Jarekt, KGyST, Mo-Slimy, Nordelch,Ruslik0, Twincinema, 1 anonymous editsImage:Centaurus A.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Centaurus_A.jpg  License: unknown  Contributors: ESOFile:Centaurus A jets.ogv  Source: http://en.wikipedia.org/w/index.php?title=File:Centaurus_A_jets.ogv  License: Public Domain  Contributors: NASA/Goddard Space Flight CenterFile:Centauros a-spc.png  Source: http://en.wikipedia.org/w/index.php?title=File:Centauros_a-spc.png  License: GNU Free Documentation License  Contributors: Martin HardcastleFile:Andromeda Galaxy (with h-alpha).jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Andromeda_Galaxy_(with_h-alpha).jpg  License: Creative Commons Attribution 2.0 Contributors: Adam EvansFile:Pic iroberts1.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Pic_iroberts1.jpg  License: Public Domain  Contributors: Isaac Roberts (d. 1904)File:Andromeda constellation map (1).png  Source: http://en.wikipedia.org/w/index.php?title=File:Andromeda_constellation_map_(1).png  License: GNU Free Documentation License Contributors: Original uploader was Bronger at en.wikipedia. Later version(s) were uploaded by Rursus at en.wikipedia.File:Stars in the Andromeda Galaxy's disc.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Stars_in_the_Andromeda_Galaxy's_disc.jpg  License: unknown  Contributors:ComputerHotline, Jmencisom, Ruslik0File:WISE- Andromeda.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:WISE-_Andromeda.jpg  License: Public Domain  Contributors: NASA/JPL-Caltech/UCLAFile:Andromeda galaxy.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Andromeda_galaxy.jpg  License: Public Domain  Contributors: NASA/JPL/California Institute ofTechnologyFile:Local Group.svg  Source: http://en.wikipedia.org/w/index.php?title=File:Local_Group.svg  License: Creative Commons Attribution-Sharealike 2.5  Contributors: Richard PowellFile:Infraredandromeda.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Infraredandromeda.jpg  License: Public Domain  Contributors: en:TheodorkFile:Andromeda galaxy Ssc2005-20a1.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Andromeda_galaxy_Ssc2005-20a1.jpg  License: Public Domain  Contributors:User:Ceranthor, User:Noodle snacks, User:SuperborsukFile:A Swift Tour of M31.OGG  Source: http://en.wikipedia.org/w/index.php?title=File:A_Swift_Tour_of_M31.OGG  License: Public Domain  Contributors: NASAFile:1993-18-a-web.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:1993-18-a-web.jpg  License: Public domain  Contributors: Tod R. Lauer, NASAFile:M31 Core in X-rays.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:M31_Core_in_X-rays.jpg  License: Public Domain  Contributors: S. Murray, M. Garcia, et al., Authors &editors: Robert Nemiroff (MTU) & Jerry Bonnell (USRA) NASA Technical Rep.: Jay Norris.File:Andromeda active core.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Andromeda_active_core.jpg  License: Public domain  Contributors: NASA, ESA and A. Schaller (forSTScI)File:Pleiades large.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Pleiades_large.jpg  License: Public Domain  Contributors: NASA, ESA, AURA/Caltech, Palomar ObservatoryThe science team consists of: D. Soderblom and E. Nelan (STScI), F. Benedict and B. Arthur (U. Texas), and B. Jones (Lick Obs.)File:Nebra Scheibe.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Nebra_Scheibe.jpg  License: GNU Free Documentation License  Contributors: Dbachmann, Itu, Lionel Allorge,Martin H., Mmcannis, Rainer Zenz, Roomba, WstFile:Pleiades Spitzer big.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Pleiades_Spitzer_big.jpg  License: Public Domain  Contributors: John Stauffer (Spitzer Science Center,Caltech)credits: Credit: NASA/JPL-Caltech/J. Stauffer (SSC/Caltech)File:Pleiades-comet-Machholz.jpeg  Source: http://en.wikipedia.org/w/index.php?title=File:Pleiades-comet-Machholz.jpeg  License: Attribution  Contributors: Rochus HessFile:X-ray image of the Pleiades.gif  Source: http://en.wikipedia.org/w/index.php?title=File:X-ray_image_of_the_Pleiades.gif  License: Public Domain  Contributors: Original uploaderWorldtraveller at en.wikipediaFile:Reflection nebula IC 349 near Merope.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Reflection_nebula_IC_349_near_Merope.jpg  License: Public Domain  Contributors:Original uploader was Worldtraveller at en.wikipedia Later versions were uploaded by Wikibob at en.wikipedia.File:M45map.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:M45map.jpg  License: Public Domain  Contributors: Bulwersator, QuibikFile:Pleiades-Taurus-Stellarium.png  Source: http://en.wikipedia.org/w/index.php?title=File:Pleiades-Taurus-Stellarium.png  License: Creative Commons Attribution-ShareAlike 1.0 Generic Contributors: StellariumFile:Orion IAU.svg  Source: http://en.wikipedia.org/w/index.php?title=File:Orion_IAU.svg  License: Creative Commons Attribution 3.0  Contributors: Antonsusi, Hogg 22, Kxx, 1 anonymouseditsFile:Orion 3008 huge.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Orion_3008_huge.jpg  License: GNU Free Documentation License  Contributors: MouserFile:Orionurania.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Orionurania.jpg  License: Public Domain  Contributors: Original uploader was Urania's Muse at en.wikipediaFile:Orion-guide dark.svg  Source: http://en.wikipedia.org/w/index.php?title=File:Orion-guide_dark.svg  License: GNU Free Documentation License  Contributors: User:Mysid.Image:OrionCC.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:OrionCC.jpg  License: Creative Commons Attribution-Sharealike 3.0  Contributors: User:Till CrednerFile:Orion constelation PP3 map PL.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Orion_constelation_PP3_map_PL.jpg  License: GNU Free Documentation License Contributors: Original uploader was Blueshade at pl.wikipediaImage:Cintura di Orione binocolo.png  Source: http://en.wikipedia.org/w/index.php?title=File:Cintura_di_Orione_binocolo.png  License: Creative Commons Attribution-Sharealike 3.0 Contributors: Roberto MuraImage:Orion Belt.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Orion_Belt.jpg  License: Public Domain  Contributors: Davide De Martin (http://www.skyfactory.org); Credit:Digitized Sky Survey, ESA/ESO/NASA FITS LiberatorFile:Sig07-006.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Sig07-006.jpg  License: Public Domain  Contributors: ASA/JPL-Caltech/D. Barrado y Navascués (LAEFF-INTA)File:Aratea 58v.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Aratea_58v.jpg  License: Public Domain  Contributors: Goodness Shamrock, WarburgImage:Orion Nebula - Hubble 2006 mosaic 18000.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Orion_Nebula_-_Hubble_2006_mosaic_18000.jpg  License: Public domain Contributors: NASA, ESA, M. Robberto (Space Telescope Science Institute/ESA) and the Hubble Space Telescope Orion Treasury Project TeamFile:Hubble Snaps View of the Orion Nebula.ogv  Source: http://en.wikipedia.org/w/index.php?title=File:Hubble_Snaps_View_of_the_Orion_Nebula.ogv  License: Public Domain Contributors: NASAFile:Orion composite1.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Orion_composite1.jpg  License: Public Domain  Contributors: Original uploader was Skatebiker aten.wikipediaImage:M42m.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:M42m.jpg  License: Public Domain  Contributors: Charles MessierFile:Henry Drape Orion nebula 1880 inverted.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Henry_Drape_Orion_nebula_1880_inverted.jpg  License: Public Domain Contributors: Henry Draper

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File:Orion-Nebula A A Common.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Orion-Nebula_A_A_Common.jpg  License: Public Domain  Contributors: Andrew AinslieCommon (1841-1903)Image:Trapezium cluster optical and infrared comparison.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Trapezium_cluster_optical_and_infrared_comparison.jpg  License:Public domain  Contributors: Dcljr, Kauczuk, Martin H., Wouterhagens, 3 anonymous editsImage:M42proplyds.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:M42proplyds.jpg  License: Public domain  Contributors: C.R. O'Dell/Rice University; NASAImage:Star Formation Fireworks in Orion.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Star_Formation_Fireworks_in_Orion.jpg  License: Public domain  Contributors:ESA/Hubble & NASAImage:HH47 animation.gif  Source: http://en.wikipedia.org/w/index.php?title=File:HH47_animation.gif  License: Public Domain  Contributors: Patrick HartiganImage:Ripples.png  Source: http://en.wikipedia.org/w/index.php?title=File:Ripples.png  License: Creative Commons Attribution-Sharealike 3.0  Contributors: ESO/J. Emerson/VISTA.Acknowledgment: Cambridge Astronomical Survey UnitImage:Orion.nebula.arp.750pix.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Orion.nebula.arp.750pix.jpg  License: Public Domain  Contributors: Credit: NASA, C.R. O'Dell andS.K. Wong (Rice University)Image:Orion Nebula WFI.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Orion_Nebula_WFI.jpg  License: Creative Commons Attribution 3.0  Contributors: ESO and IgorChekalinImage:OrionHunterWilson.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:OrionHunterWilson.jpg  License: Creative Commons Attribution-Sharealike 3.0  Contributors:HewholooksImage:Orion Nebulae.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Orion_Nebulae.jpg  License: Creative Commons Attribution-Sharealike 3.0  Contributors: Ioannidis PanosImage:Sig07-006.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Sig07-006.jpg  License: Public Domain  Contributors: ASA/JPL-Caltech/D. Barrado y Navascués (LAEFF-INTA)Image:M42 - The Orion Nebula.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:M42_-_The_Orion_Nebula.jpg  License: unknown  Contributors: ESOImage:ESO-M42-Phot-03a-01.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:ESO-M42-Phot-03a-01.jpg  License: Creative Commons Attribution 3.0  Contributors: ESOImage:VISTA infrared Orion Nebula.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:VISTA_infrared_Orion_Nebula.jpg  License: Creative Commons Attribution 3.0 Contributors: ESO/J. Emerson/VISTA. Acknowledgment: Cambridge Astronomical Survey UnitImage:Spitzer's Orion.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Spitzer's_Orion.jpg  License: Public Domain  Contributors: NASA, JPL-Caltech, J. Stauffer (SSC/Caltech)Image:Orion Nebulas biggest stars.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Orion_Nebulas_biggest_stars.jpg  License: Public domain  Contributors: NASA, ESA, M.Robberto ( Space Telescope Science Institute/ESA) and the Hubble Space Telescope Orion Treasury Project TeamFile:Orion Rainbow.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Orion_Rainbow.jpg  License: Public Domain  Contributors: Image credit: NASA/ESA/JPL-Caltech/IRAMFile:EtaCarinae.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:EtaCarinae.jpg  License: Public domain  Contributors: Nathan Smith (University of California, Berkeley), and NASAFile:ECARmulticolor4.tnl.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:ECARmulticolor4.tnl.jpg  License: Public Domain  Contributors: Dr. John M. Horack , Director ofScience Communications, Editor: Dave Dooling Curator: Linda Porter NASA Official: M. Frank RoseFile:Eta Carinae Nebula 1.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Eta_Carinae_Nebula_1.jpg  License: Public domain  Contributors: Hubble image: NASA, ESA, N. Smith(University of California, Berkeley), and The Hubble Heritage Team (STScI/AURA) CTIO data: N. Smith (University of California, Berkeley) and NOAO/AURA/NSFFile:Gamma ray burst.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Gamma_ray_burst.jpg  License: Attribution  Contributors: Nicolle Rager Fuller of the NSFFile:ESO-Eta Carinae-phot-17a-08-normal.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:ESO-Eta_Carinae-phot-17a-08-normal.jpg  License: unknown  Contributors: ESOFile:Comet Hale-Bopp 1995O1.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Comet_Hale-Bopp_1995O1.jpg  License: Creative Commons Attribution-Sharealike 3.0 Contributors: E. Kolmhofer, H. Raab; Johannes-Kepler-Observatory, Linz, Austria (http://www.sternwarte.at)File:Comet-Hale-Bopp-29-03-1997 hires adj.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Comet-Hale-Bopp-29-03-1997_hires_adj.jpg  License: Creative CommonsAttribution-Sharealike 2.0  Contributors: Philipp SalzgeberFile:Hale-Bopp orbit.svg  Source: http://en.wikipedia.org/w/index.php?title=File:Hale-Bopp_orbit.svg  License: Creative Commons Attribution-Sharealike 3.0  Contributors: Randomastronomer (talk). Original uploader was Random astronomer at en.wikipediaFile:Hale-Bopp sodium tail.gif  Source: http://en.wikipedia.org/w/index.php?title=File:Hale-Bopp_sodium_tail.gif  License: unknown  Contributors: Original uploader was Worldtraveller aten.wikipediaFile:Comet Hale-Bopp Death Valley.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Comet_Hale-Bopp_Death_Valley.jpg  License: GNU Free Documentation License Contributors: w:User:Mkfairdpm from English WikipediaFile:Marshall Applewhite.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Marshall_Applewhite.jpg  License: unknown  Contributors: A video produced by Heaven's Gate (religiousgroup)File:ESO-Comet Hale-Bopp-Phot-07a-01-hires.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:ESO-Comet_Hale-Bopp-Phot-07a-01-hires.jpg  License: Creative CommonsAttribution 3.0  Contributors: ESOImage:KT boundary 054.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:KT_boundary_054.jpg  License: Public Domain  Contributors: Original uploader was Glenlarson aten.wikipedia

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