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    Heavenly bodies

    Astronomy ( Astronomy, study of the universe and the celestial bodies, gas, and dust within it. Astronomyincludes observations and theories about the solar system, the stars, the galaxies, and the general

    structure of space. Astronomy also includes cosmology, the study of the universe and its past andfuture. People who study astronomy are called astronomers, and they use a wide variety of methods

    to perform their research. These methods usually involve ideas of physics, so most astronomers are

    also astrophysicists, and the terms astronomer and astrophysicist are basically identical. Some areas

    of astronomy also use techniques of chemistry, geography, geology and biology.

    Encarta Reference Library 2005.

    Cosmology(g n vR vMwZ K we vb )Cosmology field of study that brings together the natural sciences, particularly astronomy andphysics, in a joint effort to understand the physical universe as a unified whole.Cosmology, study of

    the universe as a whole, including its distant past and its future. Cosmologists study the universe

    observationallyby looking at the universeand theoreticallyby using physical laws andtheories to predict how the universe should behave. Cosmology is a branch of astronomy, but the

    observational and theoretical techniques used by cosmologists involve a wide range of other

    sciences, such as physics and chemistry. Cosmology is distinguished from cosmogony, which used

    to mean the study of the origin of the universe but now usually refers only to the study of the originof the solar system.

    White dwarf star( White dwarf stars, so called because of the white colour of the first few that were discovered, are

    characterized by a low luminosity, a mass on the order of that of the Sun, and a radius comparable

    to that of the Earth. Because of their large mass and small dimensions, such stars are dense andcompact objects with average densities approaching 1,000,000 times that of water.White Dwarf, old

    star that has exhausted its available nuclear fuel and collapsed, yet continues to radiate light from

    thermal energy (heat energy) trapped in it during its collapse. This is the final luminous phase in the

    evolution of low- to medium-mass stars.White dwarf stars are common throughout the earthsgalaxy, the Milky Way. The first few stages in the evolution of a white dwarf are the same as for

    other stars. A cloud of interstellar hydrogen gas and dust particles condenses under the mutuallyattractive force of gravitation until the temperature at the center of the cloud is high enough to causethe fusion of hydrogen atoms to form helium . Hydrogen fusion releases electromagnetic radiation,

    which produces an outward pressure. When the outward radiation pressure and the inward

    gravitational force reach equilibrium, the star stabilizes as a main-sequence starthe longest phasein the life of any star .

    Galaxy(b g j x)Galaxy, a massive ensemble of hundreds of millions of stars, all gravitationally interacting, andorbiting about a common center. Nature has provided an immensely varied array of galaxies,

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    ranging from faint, diffuse dwarf objects to brilliant, spiral-shaped giants. Virtually all galaxies

    appear to havebeen formed soon after the universe began, and they pervade space, even into thedepths of the farthest reaches penetrated by powerful modern telescopes. Galaxies usually exist in

    clusters, some of which in turn are grouped into larger clusters measuring hundreds of millions of

    light-years across. (A light-year is the distance traversed by light in one year, traveling at a velocityof 300,000 kilometres per second, or 650,000,000 miles per hour.) These so-called superclusters are

    separated by nearly empty voids, causing the gross structure of the universe to look somewhat like anetwork of sheets and chains of galaxies.Galaxies differ from one another in shape, with variations resulting from the way in which the

    systems were formed. Depending on the initial conditions in the pregalactic gas some

    15,000,000,000 years ago, galaxies formed either as slowly turning, smoothly structured, round

    systems of stars and gas or as rapidly rotating pinwheels of such entities. Other differences betweengalaxies have been observed and are thought to reflect evolutionary changes. Some galaxies are rife

    with activity: they are the sites of star formation with its attendant glowing gas and clouds of dust

    and molecular complexes. Others, by contrast, arequiescent, having long ago ceased to form new

    stars. Perhaps the most conspicuous evolutionary changes in galaxies occur in their nuclei, whereevidence suggests that in many cases supermassive objectsprobably black holesformed when

    the galaxies were young. Such phenomena occurred several billion years ago and are now observedas brilliant objects called quasars.The existence of galaxies was not recognized until the early 20th century. Since then, however,

    galaxies have become one of the focal points of astronomical investigation. The notable

    developments and achievements in the study of galaxies are surveyed here. Included in thediscussion are the external galaxies (i.e., those lying outside the Milky Way Galaxy, the local

    galaxy to which the Sun and Earth belong), their distribution in clusters and superclusters, and the

    evolution of galaxies and quasars.

    Nebula( wb n vwi K v)Nebula, in astronomy, a localized conglomerate of the gaseous and finely divided dust particles that

    are spread throughout interstellar space. Before the invention of the telescope, the term nebula

    (Latin, cloud) was applied to all celestial objects of a diffuse appearance. As a result, many

    objects now known to be star clusters or galaxies were called nebulas.Nebulas exist within other galaxies as well as in our own Milky Way galaxy. They are classified as

    planetary nebulas, supernova remnants, and diffuse nebulas, including reflecting, emission, and

    dark nebulas. Small, very bright nebulas known as Herbig-Haro objects are found in dense

    interstellar clouds, and are probably the products of gas jets expelled by new stars in the process offormation.

    Star( Z vi v)Star is a massive shining self-luminous celestial sphere of hot gas that shines by radiation derivedfrom its internal energy sources.. Of all the stars in the universe, our Sun is the nearest to Earth and

    the most extensively studied. The stars visible to the naked eye all belong to the Milky Way Galaxy,

    the massive ensemble of stars that contains our solar system .

    Many stars occur in pairs, multiple systems, and clusters. The members of such stellar groups arephysically relatedthrough common origin and are bound by mutual gravitational attraction.

    Somewhat related to star clusters are stellar associations, which consist of loosegroups of physically

    similar stars that have insufficient mass as a group to remain together as an organization.

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    The Ten Nearest Stars

    name visual apparent magnitude distance visual luminosity

    and spectrum in light-

    years A* B*

    A* B*

    Alpha Centauri** 0.1 G2 V 1.5 K5 V 4.3 1.30 0.36000

    Barnard's star*** 9.5 M5 V 5.9 0.00044

    Wolf 359 13.5 M6e 7.6 0.00002

    Lalande 21185*** 7.5 M2 V 8.1 0.0052

    Sirius**** -1.5 A1 V 7.2 dA5 8.6 23 0.00800

    Luyten 726-8 12.5 M6e V 13.0 M6e 8.9 0.00006 0.00004

    Ross 154 10.6 M5e V 9.4 0.0004

    Ross 248 12.2 M6e V 10.3 0.00011

    Epsilon Eridani 3.7 K2 V 10.7 0.30

    *A and B are brighter and fainter components, respectively, of star. **The two

    components of Alpha Centauri have masses of 1.0 and 0.9 solar mass, respectively.

    They are separated by 23.1 astronomical units and revolve around one anotherwith a period 79.9 years. A third component of the system is an 11th magnitude M5

    dwarf. ***These stars have unseen, planetlike companions whose presence is

    revealed by gravitational attraction on the visible star. ****The two components of

    Sirius are separated by 19.9 astronomical units, 50.1 years. The bright component

    has 2.2 solar masses; the faint component is a white dwarf of 0.9 solar mass.

    The Ten Brightest Stars

    name visual magnitude* and spectrum distance in visual luminositylight-years**

    A*** B*** A*** B***

    Sirius -1.50 A1 V +7.20 dA5 8.6 23.0 0.0080Canopus -0.73 F0 Ib 98.0 1,450.0

    Alpha Centauri +0.10 G2 V +1.50 K5 V 4.3 1.3 0.3600

    Vega +0.04 A0 V 26.0 52.0

    Arcturus 0.00 K2 III 36.0 110.0Capella +0.05 G III +10.20 M1 V 45.0 160.0

    Rigel +0.08 B8 Ia +6.60 B9 600.0 25,000.0 70.0000

    Procyon +0.34 F5 IV +10.80 d 11.4 7.6 0.0005Betelgeuse +0.41 M2 I (var.) 600.0 21,000.0

    Achernar +0.47 B5 IV-V 65.0 210.0

    *Negative magnitudes are brightest, and one magnitude difference corresponds to a

    difference in brightness of 2.5 times; e.g., a star of magnitude -1 is 10 times

    brighter than one of magnitude +1.5. **One light-year equals about 9.46(10^12) kilometres;

    ^ indicates exponentiation. ***A and B are brighter and fainter components, respectively, of star.

    Cassiopeia

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    Cassiopeia , northern constellation, near the celestial pole. It is distinguished by a group of five

    stars, of second to fourth magnitude, in the form of a rough letter W. The brightest supernova onrecord appeared in the constellation in 1572 and was observed by the Danish astronomer Tycho

    Brahe. Brighter than the planet Venus, for about 16 months Cassiopeia was visible to the naked eye

    even at noon. It is named for the mythological Ethiopian queen Cassiopeia, the mother ofAndromeda.

    Big Dipper( mwl g j ) Big Dipper, common name applied to a conspicuous constellation in the northern celestial hemisphere, nearthe North Pole. It was known to the ancient Greeks as the Bear and the Wagon and to the Romans as Ursa

    Major (the Great Bear) and Septentriones (Seven Plowing Oxen). The seven brightest stars of the

    constellation form the easily identified outline of a giant dipper. In Europe, the pattern is known as the Plow,Charles's (Charlemagne's) Wain, and the Wagon; among the Hindus, it represents the seven rishis, or holy

    ancient sages.

    Of the seven stars constituting the Big Dipper, six are of the second magnitude and one is of the third

    magnitude. Two of the second-magnitude stars, alpha ()and beta () Ursa Major, which form the outer edge

    of the bowl, point directly to the North Star, or Polaris, and hence are called the Pointers. At the bend of thehandle of the Big Dipper is the readily visible double star known as Mizar, or zeta Ursa Major. Mizar, the

    first visual double star discovered, consists of two components having magnitudes of 2.4 and 4, respectively.

    The brighter component was itself found in spectroscopic studies (1889) to be a double star; subsequently, in

    1908, it was discovered that the other component also is a spectroscopic double.

    Big Dipper

    Planet( Mn )Planet, any major celestial body that orbits a star and does not emit visible light of its own butinstead shines by reflected light. Smaller bodies that also orbit a star and are not satellites of a planet

    are called asteroids or planetoids. In the solar system, there are nine planets: Mercury, Venus, Earth,

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    Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto. Planets that orbit stars other than the Sun are

    collectively called extrasolar planets. Some extrasolar planets are nearly large enough to becomestars themselves. Such borderline planets are called brown dwarfs.

    PleiadesPleiades , in astronomy, loose cluster of 400 to 500 stars, about 415 light-years from the solar

    system in the direction of the constellation Taurus. The stars are about 1 light-year apart, on the

    average, and photographs show them to be surrounded by a nebulosity that shines by their reflected

    light. The cluster was named by the ancient Greeks after the Seven Sisters of mythology.Observers have claimed to be able to see with the naked eye as many as 12 of the stars in the

    cluster.

    Sirius( j y zK )Sirius (Greek Seirios,scorching), also Dog Star, brightest star in the sky, situated in theconstellation Canis Major. The star was highly venerated by the ancient Egyptians, who regarded it

    as a token of the rising of the Nile and of a subsequent good harvest. Many Egyptian temples wereconstructed in such a way that the light of Sirius reached the inner chambers. The hottest part of thesummer coincides with the heliacal rising of Sirius, and thus acquired the name dog days.

    The brilliance of Sirius is in large part a consequence of its relative nearness to the earth. The

    distance of the star from the earth is 8.7 light-years, or 51 trillion mi, and it is therefore one of theclosest stars. It can be seen from every part of the earth. The mass of the star is 2.4 times that of the

    sun, and its surface temperature is higher than that of the sun. Irregularities in the motion of Sirius

    led the German astronomer Friedrich Bessel to believe that the star was accompanied by a hitherto

    unseen companion star. The companion was detected for the first time 18 years later in 1862 by theAmerican astronomer Alvan Clark; it was later shown to be a white dwarf star.

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    Canis Major and Canis Minor Canis Major and Canis Minor (greater dog and lesser dog in Latin,), two constellations of stars,the former lying southeast and the latter east of Orion, and separated by the Milky Way. According

    to ancient mythology, these constellations represent dogs trotting at the heels of the Greek hunter

    Orion. Canis Major contains Sirius (also called the Dog Star), the brightest star in the heavens, andCanis Minor contains Procyon, far less bright than Sirius but still a star of the first magnitude.

    Midsummer, when Sirius rises at dawn, was associated by the ancients with the Dog Star, and this

    period is still known as the dog days or canicular days.

    Orionin astronomy, major constellation lying at about 5 hours 30 minutes right ascension (the coordinate

    on the celestial sphere analogous to longitude on the Earth) and zero declination (at the celestialequator), named for the Greek mythological hunter. Orion is one of the most conspicuous

    constellations and contains many bright stars. One of these, Betelgeuse (Alpha Orionis), a variable

    star, is easily distinguished by its reddish colour. The total brightness of Rigel, in the hunter's leg,when measured over all visible light, is greater than that of Betelgeuse. The third brightest star in

    the constellation is Bellatrix. Orion's girdle, or beltconsisting of three bright starslies nearly on

    the celestial equator. His sword, south of the belt, contains the great Orion Nebula, visible to theunaided eye, an emission nebula containing hundreds of young stars. Faint extensions of this nebula

    fill almost the whole constellation.

    Black Hole cosmic body of extremely intense gravity from which nothing, not even light, can escape. A black

    hole can be formed by the death of a massive star. When such a starhas exhausted its internalthermonuclear fuels at the end of its life, it becomes unstable and gravitationally collapses inward

    upon itself. The crushing weight of constituent matter falling in from all sides compresses the dyingstar to a point of zero volume and infinite density called the singularity. Details of the structure of a

    black hole are calculated from Albert Einstein's general theory of relativity. The singularity

    constitutes the centre of a black hole and is hidden by the object's surface, the event horizon.

    Black Hole an extremely dense celestial body that has been theorized to exist in the universe. Thegravitational field of a black hole is so strong that, if the body is large enough, nothing, including

    electromagnetic radiation, can escape from its vicinity. The body is surrounded by a spherical

    boundary, called a horizon, through which light can enter but not escape; it therefore appears totally

    black.The black-hole concept was developed by the German astronomer Karl Schwarzschild in 1916 on

    the basis of physicist Albert Einsteins general theory of relativity. The radius of the horizon of aSchwarzschild black hole depends only on the mass of the body, being 2.95 km times the mass ofthe body in solar units (the mass of the body divided by the mass of the Sun). If a body is

    electrically charged or rotating, Schwarzschilds results are modified. An ergosphere formsoutside the horizon, within which matter is forced to rotate with the black hole; in principle, energy

    can be emitted from the ergosphere.

    In 1994 astronomers used the Hubble Space Telescope (HST) to uncover the first convincing

    evidence that a black hole exists. They detected an accretion disk (disk of hot, gaseous material)circling the center of the galaxy M87 with an acceleration that indicated the presence of an object

    2.5 to 3.5 billion times the mass of the Sun. By 2000, astronomers had detected supermassive black

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    holes in the centers of dozens of galaxies and had found that the masses of the black holes were

    correlated with the masses of the parent galaxies. More massive galaxies tend to have more massiveblack holes at their centers. Learning more about galactic black holes will help astronomers learn

    about the evolution of galaxies and the relationship between galaxies, black holes, and quasars.

    The English physicist Stephen Hawking has suggested that many black holes may have formed inthe early universe. If this were so, many of these black holes could be too far from other matter to

    form detectable accretion disks, and they could even compose a significant fraction of the total massof the universe. For black holes of sufficiently small mass it is possible for only one member of anelectron-positron pair near the horizon to fall into the black hole, the other escaping . The resulting

    radiation carries off energy, in a sense evaporating the black hole. Any primordial black holes

    weighing less than a few thousand million metric tons would have already evaporated, but heavier

    ones may remain.The American astronomer Kip Thorne of California Institute of Technology in Pasadena,

    California, has evaluated the chance that black holes can collapse to form "wormholes," connections

    between otherwise distant parts of the universe. He concludes that an unknown form of "exotic

    matter" would be necessary for such wormholes to survive.

    According to Hawking's theory, numerous tiny primordial black holes, possibly with a mass equalto that of an asteroid or less, might have been created during the big bang, a state of extremely hightemperatures and density in which the universe is thought to have originated roughly 10 billion

    years ago. These so-called mini black holes, unlike the more massive variety, lose mass over time

    and disappear. Subatomic particles such as protons and their antiparticles (i.e., antiprotons) may becreated very near a mini black hole. If a proton and an antiproton escape its gravitational attraction,

    they annihilate each other andin so doing generate energyenergy that they in effect drain from the

    black hole. If this process is repeated again and again, the black hole evaporates, having lost all of

    its energy and thereby its mass, since these are equivalent.

    Black DwarfBlack Dwarf, burnt-out core of an old star that no longer emits light, generally believed to follow

    the white dwarf stage as the final stage in the evolution of small to medium mass stars.A white dwarf cools as it emits radiation, and so over time its color shifts from blue to white to

    yellow to red and finally, after billions of years, it no longer shines in the visible portion of the

    spectrum, at which point it appears black. Astronomers call these small, dense, and cold crystalline

    cores black dwarfs. Such an object could only be detected with existing technology if it were part ofa binary star system. In a binary system, two stars orbit a common center of mass, so the

    gravitational effect the black dwarf had on its neighboring visible star would be detectable. Thus

    far, no black dwarf stars have been conclusively detected.

    supernovaplural supernovas, or supernovae, any of a class of violently exploding stars whose luminosity aftereruption suddenly increases millions or even billions oftimes its normal level.

    The term supernova is derived from nova (Latin: new), the name for another type ofexplodingstar. Supernovas resemble novas in several respects. Both are characterized by a

    tremendous, rapid brightening lasting for a few weeks, followed by a slow

    dimming.Spectroscopically, they show blue-shifted emission lines, which imply that hot gases are

    blown outward. But a supernova explosion, unlike a nova outburst, is a cataclysmic event for a star,

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    one that essentially ends its active (i.e., energy-generating) lifetime. When a star goes supernova,

    considerable amounts of its matter, equaling the material of several Suns, may be blasted into spacewith such a burst of energy as to enable the explodingstar to outshine its entire home galaxy.

    Historically, only seven supernovas are known to have been recorded before the early 17th century,with the most famous occurring in AD 1054. It was seen in one of the horns of the constellation

    Taurus. The remnants of this explosion are visible today as the Crab Nebula, which is composed ofglowing ejecta of gases flying outward in an irregular fashion and a rapidly spinning, pulsatingneutron star, called a pulsar, in the centre. The supernova of 1054 was recorded by Chinese and

    Korean observers; it also may have been seen by southwestern AmericanIndians, as suggested by

    certain rock paintings discovered in Arizona and New Mexico. It was bright enough to be seen

    during the day, and its great luminosity lasted for weeks. Other prominent supernovas are known tohave been observed from Earth in 185, 393, 1006, 1181, 1572, and 1604.

    The closest and most easily observed of the hundreds of supernovas that have been recorded since

    1604 was first sighted on the morning of Feb. 24, 1987, by the Canadian astronomer Ian K. Sheltonwhile working at the Las Campanas Observatory in Chile. Designated SN 1987A, this formerly

    extremely faint object attained a magnitude of 4.5 within just a few hours, thus becoming visible tothe unaided eye. The newly appearing supernova was located in the Large Magellanic Cloud at adistance of about50,000 parsecs. It immediately became the subject of intense observation by

    astronomers throughout the Southern Hemisphere and has been observed by the Hubble Space

    Telescope. SN 1987A's brightness peaked in May with a magnitude of about 3 and slowly declinedin the following months.

    Supernovas may be divided into two broad classes, Type I and Type II, according to the way in

    which they detonate. Type I supernovas may be up to three times brighter than Type II; they alsodiffer from Type II supernovas in that their spectra contain no hydrogen lines and they expand

    about twice as rapidly.

    The supernova detonation occurs when material falls in from the outer layers of the star and then

    rebounds off the core, which has stopped collapsing and suddenly presents a hard surface to the

    infalling gases. The shock wave generated by this collision propagates outward and blows off the

    star's outer gaseous layers. The amount of material blasted outward depends on the star's originalmass.

    In some cases, the core collapse may be too great to produce a supernova and the imploding star is

    compressed into an even smaller and denser body than a neutron starnamely, a black hole.Infalling material disappears into the black hole, the gravitational field of which is so intense that

    not even light can escape. The entire star is not taken in by the black hole, since much of the falling

    envelope of the star either rebounds from thetemporary formation of a spinning neutron core ormisses passing through the very centre of the core and is spun off instead.

    Supernova explosions release not only tremendous amounts of radio radiation and X-radiation butalso cosmic rays and many of the heavier elements that make up the components of the solar

    system, including the Earth, into the interstellar medium. Spectral analyses show that abundances of

    the heavier elementsare greater than normal, indicating that these elements do indeed form during

    the course of the explosion. The shell of remnants continues to expand until, at a very advancedstage, it dissolves into the interstellar medium. Compare nova. See also black hole.

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    SupergiantSupergiant , extremely large, luminous star that can be seen from vast distances across space.

    Supergiants are stars that have evolved through several stages, converting the nuclear fuels in their

    cores to successively heavier elements at each stage. They often explode as supernovas when thenuclear fuels in their cores are exhausted.

    Supergiants form in the same way that ordinary stars forma cloud of hydrogen gas and interstellardust compresses under the gravitational attraction of its matter for itself until the temperature at thecenter of the cloud is hot enough to fuse hydrogen to form helium. The central region where

    hydrogen fusion occurs is called the core. After hydrogen fusion occurs in a new star,

    electromagnetic radiation is released from the core. The radiation creates an outward pressure that

    balances the gravitational force, and the cloud eventually stabilizes as a main-sequence stara starin the first and longest phase of its luminous existence.

    The primary difference between a star destined to become a supergiant and a more typical star is its

    mass. Astronomers estimate that a star must be at least six to ten times more massive than the

    earths sun in order to have a core massive enough to make the star a supergiant. The additionalmass leads to stronger gravitational forces that create the temperature and pressure conditions in the

    core needed to induce the fusion of heavy elements late in the life of the star. The additional massalso causes higher core temperatures in the stars early phases and results in more rapid nuclearreactions than smaller stars. Astrophysicists estimate that the rate of fusion in a star is roughly

    proportional to the fourth power of its mass. Thus, a star with a mass of ten times the earths sun

    consumes hydrogen about 104

    (10,000) times faster than the sun. Such a star will consume the

    hydrogen in its core within a few million years, whereas a star of the suns mass will last a thousandtimes longer.

    After the hydrogen in a stars core has been consumed, the outward radiation pressure that

    supported the star dissipates and the star collapses. The outer layers of the star compress enough tocause fusion of the hydrogen in the layer next to the core. This creates a hydrogen-burning shell that

    causes the outer layers of the star to expand while the inner core of the star continues to contract.

    The outer layers cool and turn red as they expand, and the star becomes a red giant.If the mass of a red giant stars core exceeds a critical limit, which is approximately 0.7 to 1.0 times

    the entire mass of the earths sun, the core will compress until its temperature reaches 100 millionChot enough to induce the fusion of helium atoms to form carbon. The radiation released by

    helium fusion causes the red giant to swell to 500 times the size of the sun or larger and become a

    red supergiant. Because the mass of a stars core is only about 10 percent of its total mass, only starswith a mass of six to ten suns or more can become supergiants.

    Antares is a red supergiant star in the constellation Scorpio. It is so large that if it were placed at the

    center of Earths solar system, it would engulf the orbits of Mercury, Venus, Earth, and Mars.Betelgeuse in the constellation Orion is another well-known and easily identifiable red supergiant

    star. The maximum light output of a red supergiant corresponds to an absolute magnitude of about

    9the equivalent light output of 600,000 suns. Knowing the maximum light output of these brightstars allows astronomers to use them as standard candles to judge distances to far parts of theMilky Way or even to other galaxies where the stars can be seen.

    When the nuclear reactions in the core of a supergiant come to an end, the core collapses a finaltime. Smaller supergiants have cores that are less than a critical value known as the Chandrasekhar

    limit, which is about 1.4 times the mass of the sun . The core material of such stars will collapse

    into a state known as a degenerate electron state and the core will become a white dwarf star.

    Stars with cores of mass between the Chandrasekhar limit and about three times the entire mass ofthe sun will eventually become iron. The degenerate electron state of a white dwarf does not have

    the mechanical strength to support such a massive dense body, and so the core collapses further

    until the electrons and atomic nuclei in the core are squeezed together to form neutrons. When this

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    happens, the core is squeezed into a spherical shape only 20 km (12 mi) in diameter to form a

    neutron star. If the mass of the supergiants core is more than three times the entire mass of the sun,the neutron star condenses even furtherliterally disappearing from the visible universe as it

    becomes a black hole. In both cases, the outer envelope of the star is blown away to form a

    planetary nebula by a supernova explosion that accompanies the collapse of the core. Theseplanetary nebulas contain hydrogen as well as heavy elementssuch as carbon, oxygen, nitrogen,

    and ironthat were synthesized in the core of the star during its earlier phases. Thus, supergiantsare a major source of the heavier elements found in the universe. In fact, astronomers generally

    agree that the earth and all of the earths living organisms are made of matter that was blasted into

    space by a supernova resulting from the collapse of a supergiant star more than 5 billion years ago

    Neutron StarNeutron Star, rapidly spinning, extremely dense astronomical object. Neutron stars are composed

    primarily of neutrons, minute, neutrally charged particles that exist in the nuclei of atoms. Aneutron star is created when the core of a supergiant stara massive star that has evolved so that it

    burns heavy elements instead of hydrogenhas converted all of the material in its core to iron. Atthis stage, no further nuclear reactions can take place to liberate energy, and so the core collapsesunder the mutual gravitational attraction of its own matter .

    Neutron stars were first predicted by Indian physicist Subrahmanyan Chandrasekhar and others in

    the 1930s. These theorists predicted that when a massive supergiant star exhausts the nuclear fuel inits core, the core will collapse and condense under gravitational forces. If the mass of the core

    exceeds about 1.4 times the full mass of the suna value known as the Chandrasekhar limitthe

    core will collapse with such force that the positively-charged protons and negatively-charged

    electrons of the core will be crushed together to form electrically neutral neutrons. The resultingtheoretical body was called a neutron star.

    The rotational speed of a collapsing supergiant core increases for the same reason that the rotational

    speeds of spinning ice skaters increase when they pull in their armsthe conservation of angularmomentum. As material that was more distant from the center of the star moves in closer, its

    rotational speed must increase to compensate and conserve angular momentum. A star that

    originally required days or months to revolve once on its axis would suddenly accelerate to spin

    several hundred revolutions per second. Only the tremendous forces generated by gravitation andnuclear interactions keep it from flying apart.

    At the time Chandrasekhar predicted the existence of neutron stars, calculations indicated that the

    stars would be relatively cold and small and therefore too dim to observe through an optical

    telescope. In 1967, however, astronomers realized that the magnetic field of a neutron star wouldalso be extremely condensed and would rotate at the same rapid rate as the neutron star. The intense

    magnetic field at the neutron stars surfaceperhaps a trillion times more intense than the magnetic

    field of the earthwould cause electrons moving near its magnetic poles to radiate energy in theform of radio waves, creating a signal that would sweep across space with each revolution of the

    star. An observer positioned within the sweep of radio-frequency radiation would observe a radio

    signal that pulsates at the same frequency as the rotation of the star. For this reason, neutron starswere also named pulsars because of the hypothetical radio frequency pulsations they were presumed

    to emit. In early 1968, only a few months after the existence of pulsars was predicted, two

    astronomers at the Mullard Radio Astronomy Observatory at the University of Cambridge observed

    the first such pulsating radio frequency source (see Radio Astronomy). Other pulsars were soondiscovered, and other evidence indicating that the observed pulsars are neutron stars rapidly

    followed.

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    One of the most compelling pieces of evidence that pulsars are neutron stars was the discovery of a

    pulsar at the center of the Crab Nebula. Astrophysicists studying neutron stars felt that the collapse

    of a supergiant stars core would be accompanied by a violen t explosion that would blow away the

    outer envelope of the star. The explosive event was named a supernova. Many suspected that the

    Crab Nebula was the remains of an explosion that Chinese astronomers had recorded 900 yearsearlier, since it was in the same region of the sky. Discovery of a pulsar at the exact center of the

    Crab Nebula confirmed the theoretical connection between supernovas, neutron stars, and pulsars.The sequence of events following the formation of a neutron star appears to be as follows: The outer

    envelope of a collapsing star, which comprises 80 percent or more of the stars total mass, alsocollapses when nuclear fusion in the core ceases, but it does not become part of the neutron star.

    When the outer envelope falls through the intense gravitational field of the neutron star to the

    neutron stars surface, the material of the outer envelope gains an enormous amount of energy.When this highly energetic material hits the surface of the neutron star, massive thermonuclear

    reactions are ignited all over the surface of the neutron star simultaneously, blowing the outer

    envelope into a vast spherical bubble of gas and debris that surrounds the new neutron star.

    The total mass of a star immediately before it becomes a neutron star is probably about five timesthe final mass of the neutron star. Thus, most of the material of the star is blasted into space in the

    supernova explosion that follows the formation of the neutron star. Stars also lose mass in manyother ways as they progress from one phase to the next. A star that eventually becomes a neutronstar must first go through several intermediate phases, from a young star probably all the way to the

    formation of iron in its core. Thus, the minimum mass of a star whose core will eventually become

    a neutron star could be as much as 20 times the mass of the sun or more.The gravitational and nuclear forces that hold a neutron star together combine to create the most

    dense, exotic material in the visible universe. Calculations predict that a neutron star is perhaps only

    10 to 20 km in diameter, yet it retains all of the mass of the stars coreat least 1.4 times the full

    mass of the sun. The density of such material could be as high as 1015

    grams per cubic centimeter(1,000,000,000,000,000 gm/cc). A teaspoonful of this material would weigh ten billion tons on the

    surface of the earth.

    Physicists estimate that a neutron star probably has an atmosphere a few centimeters (about 1 inch)thick. Beneath the atmosphere is a surface crust about 1 km (about 0.6 mi) thick, which is made of

    iron 10,000 times more dense and stiff than any iron found on the earth. Despite the great stiffness

    of the surface material, the tremendous gravitational forces of neutron stars limit the height of

    mountains on their surfaces to only few centimeters (about 1 inch) in height.Beneath the superdense iron crust is a superfluid sea of neutronsa strange, liquidlike substance

    that is even more dense than the iron crust, yet has no resistance to movement (see Superfluidity).

    At the center of a neutron star is a core of exotic nuclear particles found under no other conditions

    in the known universe. The rapid rotation of neutron stars causes their equators to bulge, and theytake on the shape of a flattened ball.

    When a neutron star is a member of a close binary star system, the intense gravitational field of the

    neutron star can distort the outer layers of the companion star and pull material from the companionstar onto the neutron star. This material accelerates under the influenc e of the neutron stars gravityto enormous speeds, then crashes into the surface in thermonuclear explosions that release intense

    beams of X rays and gamma rays.X-Ray Astronomy; Gamma-Ray Astronomy.If the mass of a collapsing stars core is less than the Chandrasekhar limit, it cannot generate

    enough gravitational force to cause the fusion of electrons and protons to form neutrons. The

    collapse of such a star will instead stop at a less extreme statethe white dwarf stage. If a white

    dwarf is part of an interacting binary star system, it may eventually accumulate enough mass toexceed the Chandrasekhar limit, at which point it will condense into a neutron star.

    White dwarfs and neutron stars share a unique property: As they accumulate matter, they actually

    grow smaller, not larger. This shrinking occurs because the additional mass increases the

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    gravitational pull of the stars material for itself, which squeezes the matter even tighter. If the mass

    of the collapsing core is greater than about three times the full mass of the sun, the gravitationalforce will exceed the strength of the material, and the core will collapse until it disappears from the

    visible universe altogether. This extreme state of gravitational collapse is known as a black hole.

    Astronomers speculate that neutron stars in interacting binary star systems can become black holesby accumulating mass in the same way that white dwarf stars in interacting binary star systems

    become more massive.

    Little DipperLittle Dipper, constellation of the northern sky, situated close to the Big Dipper. Known to the

    Romans as Ursa Minor, or Little Bear, the Little Dipper may be found on winter evenings to the left

    of and above the Big Dipper, with its handle pointed upward. Polaris, or Alpha ( ) Ursae Minoris,

    commonly known as the North Star, or polestar, marks the end of the handle of the Little Dipper;presently the polestar is situated slightly less than 1 from the North Pole. The North Star is a

    second-magnitude star, the brightest in the constellation.

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    Heavenly bodies )

    Astronomy ( Astronomy, study of the universe and the celestial bodies, gas, and dust within it. Astronomyincludes observations and theories about the solar system, the stars, the galaxies, and the general

    structure of space. Astronomy also includes cosmology, the study of the universe and its past andfuture. People who study astronomy are called astronomers, and they use a wide variety of methodsto perform their research. These methods usually involve ideas of physics, so most astronomers are

    also astrophysicists, and the terms astronomer and astrophysicist are basically identical. Some areas

    of astronomy also use techniques of chemistry, geography, geology and biology.

    Cosmology(g n vR vMwZ K we vb )Cosmology field of study that brings together the natural sciences, particularly astronomy and

    physics, in a joint effort to understand the physical universe as a unified whole.Cosmology, study of

    the universe as a whole, including its distant past and its future. Cosmologists study the universeobservationallyby looking at the universeand theoreticallyby using physical laws and

    theories to predict how the universe should behave. Cosmology is a branch of astronomy, but theobservational and theoretical techniques used by cosmologists involve a wide range of other

    sciences, such as physics and chemistry. Cosmology is distinguished from cosmogony, which usedto mean the study of the origin of the universe but now usually refers only to the study of the origin

    of the solar system.

    White dwarf star( White dwarf stars, so called because of the white colour of the first few that were discovered, are

    characterized by a low luminosity, a mass on the order of that of the Sun, and a radius comparable

    to that of the Earth. Because of their large mass and small dimensions, such stars are dense andcompact objects with average densities approaching 1,000,000 times that of water.White Dwarf, old

    star that has exhausted its available nuclear fuel and collapsed, yet continues to radiate light from

    thermal energy (heat energy) trapped in it during its collapse. This is the final luminous phase in theevolution of low- to medium-mass stars.White dwarf stars are common throughout the earthsgalaxy, the Milky Way. The first few stages in the evolution of a white dwarf are the same as for

    other stars. A cloud of interstellar hydrogen gas and dust particles condenses under the mutually

    attractive force of gravitation until the temperature at the center of the cloud is high enough to causethe fusion of hydrogen atoms to form helium . Hydrogen fusion releases electromagnetic radiation,

    which produces an outward pressure. When the outward radiation pressure and the inward

    gravitational force reach equilibrium, the star stabilizes as a main-sequence starthe longest phase

    in the life of any star .

    Galaxy(b g j x)Galaxy, a massive ensemble of hundreds of millions of stars, all gravitationally interacting, and

    orbiting about a common center. Nature has provided an immensely varied array of galaxies,

    ranging from faint, diffuse dwarf objects to brilliant, spiral-shaped giants. Virtually all galaxiesappear to havebeen formed soon after the universe began, and they pervade space, even into the

    depths of the farthest reaches penetrated by powerful modern telescopes. Galaxies usually exist in

    clusters, some of which in turn are grouped into larger clusters measuring hundreds of millions oflight-years across. (A light-year is the distance traversed by light in one year, traveling at a velocity

    of 300,000 kilometres per second, or 650,000,000 miles per hour.) These so-called superclusters are

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    separated by nearly empty voids, causing the gross structure of the universe to look somewhat like a

    network of sheets and chains of galaxies.The existence of galaxies was not recognized until the early 20th century. Since then, however,

    galaxies have become one of the focal points of astronomical investigation. The notable

    developments and achievements in the study of galaxies are surveyed here. Included in thediscussion are the external galaxies (i.e., those lying outside the Milky Way Galaxy, the local

    galaxy to which the Sun and Earth belong), their distribution in clusters and superclusters, and theevolution of galaxies and quasars.

    Nebula( wb n vwi K v)Nebula, in astronomy, a localized conglomerate of the gaseous and finely divided dust particles that

    are spread throughout interstellar space. Before the invention of the telescope, the term nebula

    (Latin, cloud) was applied to all celestial objects of a diffuse appearance. As a result, manyobjects now known to be star clusters or galaxies were called nebulas.

    Nebulas exist within other galaxies as well as in our own Milky Way galaxy. They are classified as

    planetary nebulas, supernova remnants, and diffuse nebulas, including reflecting, emission, anddark nebulas. Small, very bright nebulas known as Herbig-Haro objects are found in dense

    interstellar clouds, and are probably the products of gas jets expelled by new stars in the process of

    formation.

    Star( Z vi v)Star is a massive shining self-luminous celestial sphere of hot gas that shines by radiation derivedfrom its internal energy sources.. Of all the stars in the universe, our Sun is the nearest to Earth and

    the most extensively studied. The stars visible to the naked eye all belong to the Milky Way Galaxy,

    the massive ensemble of stars that contains our solar system .Many stars occur in pairs, multiple systems, and clusters. The members of such stellar groups are

    physically relatedthrough common origin and are bound by mutual gravitational attraction.

    Somewhat related to star clusters are stellar associations, which consist of loosegroups of physicallysimilar stars that have insufficient mass as a group to remain together as an organization.

    Nearest Starsname visual apparent magnitude distance visual luminosity

    and spectrum in light-

    years A* B*

    A* B*

    Alpha Centauri** 0.1 G2 V 1.5 K5 V 4.3 1.30 0.36000

    Barnard's star*** 9.5 M5 V 5.9 0.00044

    Wolf 359 13.5 M6e 7.6 0.00002

    Lalande 21185*** 7.5 M2 V 8.1 0.0052

    Sirius**** -1.5 A1 V 7.2 dA5 8.6 23 0.00800

    Brightest Starsname visual magnitude* and spectrum distance in visual luminosity

    light-years**A*** B*** A*** B***

    Sirius -1.50 A1 V +7.20 dA5 8.6 23.0 0.0080

    Canopus -0.73 F0 Ib 98.0 1,450.0

    Alpha Centauri +0.10 G2 V +1.50 K5 V 4.3 1.3 0.3600Vega +0.04 A0 V 26.0 52.0

    Arcturus 0.00 K2 III 36.0 110.0

    Capella +0.05 G III +10.20 M1 V 45.0 160.0

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    CassiopeiaCassiopeia , northern constellation, near the celestial pole. It is distinguished by a group of five

    stars, of second to fourth magnitude, in the form of a rough letter W. The brightest supernova on

    record appeared in the constellation in 1572 and was observed by the Danish astronomer TychoBrahe. Brighter than the planet Venus, for about 16 months Cassiopeia was visible to the naked eye

    even at noon. It is named for the mythological Ethiopian queen Cassiopeia, the mother of

    Andromeda.

    Big Dipper( mwl g j ) Big Dipper, common name applied to a conspicuous constellation in the northern celestial hemisphere, nearthe North Pole. It was known to the ancient Greeks as the Bear and the Wagon and to the Romans as Ursa

    Major (the Great Bear) and Septentriones (Seven Plowing Oxen). The seven brightest stars of the

    constellation form the easily identified outline of a giant dipper. In Europe, the pattern is known as the Plow,

    Charles's (Charlemagne's) Wain, and the Wagon; among the Hindus, it represents the seven rishis, or holy

    ancient sages.

    Of the seven stars constituting the Big Dipper, six are of the second magnitude and one is of the third

    magnitude. Two of the second-magnitude stars, alpha ()and beta () Ursa Major, which form the outer edge

    of the bowl, point directly to the North Star, or Polaris, and hence are called the Pointers. At the bend of thehandle of the Big Dipper is the readily visible double star known as Mizar, or zeta Ursa Major. Mizar, the

    first visual double star discovered, consists of two components having magnitudes of 2.4 and 4, respectively.

    The brighter component was itself found in spectroscopic studies (1889) to be a double star; subsequently, in

    1908, it was discovered that the other component also is a spectroscopic double.

    Big Dipper

    Planet( Mn )Planet, any major celestial body that orbits a star and does not emit visible light of its own but

    instead shines by reflected light. Smaller bodies that also orbit a star and are not satellites of a planetare called asteroids or planetoids. In the solar system, there are nine planets: Mercury, Venus, Earth,

    Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto. Planets that orbit stars other than the Sun are

    collectively called extrasolar planets. Some extrasolar planets are nearly large enough to become

    stars themselves. Such borderline planets are called brown dwarfs.

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    PleiadesPleiades , in astronomy, loose cluster of 400 to 500 stars, about 415 light-years from the solar

    system in the direction of the constellation Taurus. The stars are about 1 light-year apart, on theaverage, and photographs show them to be surrounded by a nebulosity that shines by their reflected

    light. The cluster was named by the ancient Greeks after the Seven Sisters of mythology.Observers have claimed to be able to see with the naked eye as many as 12 of the stars in the

    cluster.

    Sirius( j y zK )Sirius (Greek Seirios,scorching), also Dog Star, brightest star in the sky, situated in theconstellation Canis Major. The star was highly venerated by the ancient Egyptians, who regarded it

    as a token of the rising of the Nile and of a subsequent good harvest. Many Egyptian temples wereconstructed in such a way that the light of Sirius reached the inner chambers. The hottest part of the

    summer coincides with the heliacal rising of Sirius, and thus acquired the name dog days.

    The brilliance of Sirius is in large part a consequence of its relative nearness to the earth. The

    distance of the star from the earth is 8.7 light-years, or 51 trillion mi, and it is therefore one of theclosest stars. It can be seen from every part of the earth. The mass of the star is 2.4 times that of the

    sun, and its surface temperature is higher than that of the sun. Irregularities in the motion of Sirius

    led the German astronomer Friedrich Bessel to believe that the star was accompanied by a hithertounseen companion star. The companion was detected for the first time 18 years later in 1862 by theAmerican astronomer Alvan Clark; it was later shown to be a white dwarf star.

    Canis Major and Canis Minor Canis Major and Canis Minor (greater dog and lesser dog in Latin,), two constellations of stars,

    the former lying southeast and the latter east of Orion, and separated by the Milky Way. Accordingto ancient mythology, these constellations represent dogs trotting at the heels of the Greek hunter

    Orion. Canis Major contains Sirius (also called the Dog Star), the brightest star in the heavens, andCanis Minor contains Procyon, far less bright than Sirius but still a star of the first magnitude.

    Midsummer, when Sirius rises at dawn, was associated by the ancients with the Dog Star, and this

    period is still known as the dog days or canicular days.

    Orionin astronomy, major constellation lying at about 5 hours 30 minutes right ascension (the coordinate

    on the celestial sphere analogous to longitude on the Earth) and zero declination (at the celestial

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    equator), named for the Greek mythological hunter. Orion is one of the most conspicuous

    constellations and contains many bright stars. One of these, Betelgeuse (Alpha Orionis), a variablestar, is easily distinguished by its reddish colour. The total brightness of Rigel, in the hunter's leg,

    when measured over all visible light, is greater than that of Betelgeuse. The third brightest star in

    the constellation is Bellatrix. Orion's girdle, or beltconsisting of three bright starslies nearly onthe celestial equator. His sword, south of the belt, contains the great Orion Nebula, visible to the

    unaided eye, an emission nebula containing hundreds of young stars. Faint extensions of this nebulafill almost the whole constellation.

    Black Hole cosmic body of extremely intense gravity from which nothing, not even light, can escape. A blackhole can be formed by the death of a massive star. When such a starhas exhausted its internal

    thermonuclear fuels at the end of its life, it becomes unstable and gravitationally collapses inward

    upon itself. The crushing weight of constituent matter falling in from all sides compresses the dying

    star to a point of zero volume and infinite density called the singularity. Details of the structure of ablack hole are calculated from Albert Einstein's general theory of relativity. The singularity

    constitutes the centre of a black hole and is hidden by the object's surface, the event horizon.

    Black Hole an extremely dense celestial body that has been theorized to exist in the universe. The

    gravitational field of a black hole is so strong that, if the body is large enough, nothing, including

    electromagnetic radiation, can escape from its vicinity. The body is surrounded by a sphericalboundary, called a horizon, through which light can enter but not escape; it therefore appears totally

    black.

    Black DwarfBlack Dwarf, burnt-out core of an old star that no longer emits light, generally believed to followthe white dwarf stage as the final stage in the evolution of small to medium mass stars.

    A white dwarf cools as it emits radiation, and so over time its color shifts from blue to white toyellow to red and finally, after billions of years, it no longer shines in the visible portion of thespectrum, at which point it appears black. Astronomers call these small, dense, and cold crystalline

    cores black dwarfs. Such an object could only be detected with existing technology if it were part ofa binary star system. In a binary system, two stars orbit a common center of mass, so the

    gravitational effect the black dwarf had on its neighboring visible star would be detectable. Thus

    far, no black dwarf stars have been conclusively detected.

    Supernovaplural supernovas, or supernovae, any of a class of violently exploding stars whose luminosity after

    eruption suddenly increases millions or even billions oftimes its normal level.

    The term supernova is derived from nova (Latin: new), the name for another type of

    explodingstar. Supernovas resemble novas in several respects. Both are characterized by atremendous, rapid brightening lasting for a few weeks, followed by a slow

    dimming.Spectroscopically, they show blue-shifted emission lines, which imply that hot gases are

    blown outward. But a supernova explosion, unlike a nova outburst, is a cataclysmic event for a star,one that essentially ends its active (i.e., energy-generating) lifetime. When a star goes supernova,considerable amounts of its matter, equaling the material of several Suns, may be blasted into space

    with such a burst of energy as to enable the explodingstar to outshine its entire home galaxy.

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    SupergiantSupergiant , extremely large, luminous star that can be seen from vast distances across space.

    Supergiants are stars that have evolved through several stages, converting the nuclear fuels in their

    cores to successively heavier elements at each stage. They often explode as supernovas when thenuclear fuels in their cores are exhausted.

    Supergiants form in the same way that ordinary stars forma cloud of hydrogen gas and interstellardust compresses under the gravitational attraction of its matter for itself until the temperature at thecenter of the cloud is hot enough to fuse hydrogen to form helium. The central region where

    hydrogen fusion occurs is called the core. After hydrogen fusion occurs in a new star,

    electromagnetic radiation is released from the core. The radiation creates an outward pressure that

    balances the gravitational force, and the cloud eventually stabilizes as a main-sequence stara starin the first and longest phase of its luminous existence.

    Neutron StarNeutron Star, rapidly spinning, extremely dense astronomical object. Neutron stars are composed

    primarily of neutrons, minute, neutrally charged particles that exist in the nuclei of atoms. Aneutron star is created when the core of a supergiant stara massive star that has evolved so that itburns heavy elements instead of hydrogenhas converted all of the material in its core to iron. At

    this stage, no further nuclear reactions can take place to liberate energy, and so the core collapses

    under the mutual gravitational attraction of its own matter .Neutron stars were first predicted by Indian physicist Subrahmanyan Chandrasekhar and others in

    the 1930s. These theorists predicted that when a massive supergiant star exhausts the nuclear fuel in

    its core, the core will collapse and condense under gravitational forces. If the mass of the core

    exceeds about 1.4 times the full mass of the suna value known as the Chandrasekhar limitthecore will collapse with such force that the positively-charged protons and negatively-charged

    electrons of the core will be crushed together to form electrically neutral neutrons. The resulting

    theoretical body was called a neutron star.

    Little DipperLittle Dipper, constellation of the northern sky, situated close to the Big Dipper. Known to the

    Romans as Ursa Minor, or Little Bear, the Little Dipper may be found on winter evenings to the leftof and above the Big Dipper, with its handle pointed upward. Polaris, or Alpha ( ) Ursae Minoris,commonly known as the North Star, or polestar, marks the end of the handle of the Little Dipper;

    presently the polestar is situated slightly less than 1 from the North Pole. The North Star is a

    second-magnitude star, the brightest in the constellation.

    Solar System

    Solar system is assemblage consisting of the Sunan average star in the Milky Way Galaxyandthose bodies orbiting around it: 9 major planets, at least 60 planetary satellites, countless asteroids

    and comets, and the vast interplanetary medium. Four of the major planets have ring systems, and

    seven have one or more satellites. The several thousand minor planets, or asteroids, are

    predominantly in orbits between Mars and Jupiter, while most of the several billion comets travelaround the Sun in a spherical shell approximately 50,000 times farther out than the Earth. The

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    interplanetary mediuman exceedingly tenuous plasma (ionized gas) laced with concentrations of

    dustextends outward from the Sun to great distances.

    Observations of the motions of the Sun, the Moon, and the visibleplanets by early investigators gave

    rise to the science of astronomy. These objects are still studied today in an attempt to understand

    their origin and evolution, which can aid in determining whether there may be other similar systems

    among the millions of stars in the galaxy.

    General considerations:

    Containing more than 99 percent of the mass of the solar system, theSun lies at the centre of thesystem; all the planets (and the asteroids) move around it in elliptical orbits in the same direction as

    the Sun rotates. Looking down on the system from a vantage point above the North Pole of the

    Earth, an observer would find that all the orbital motions are in a counterclockwise direction. The

    shape of an ellipse is defined in terms of its eccentricity, e. For a circle, e = 0; for a parabola, e =1.0. Venus and Neptune have the most circular orbits, with eccentricities of 0.007 and 0.009,

    respectively. Another attribute of a planet's orbit is inclination, which is the angle that it makes with

    the plane of the Earth's orbit. The closest and most distant planets have the greatest inclinations:

    Mercury's orbit is inclined at 7 and Pluto's at 17.

    The earliest of such theories were certainly much less constrained. A scientific approach to the

    origin of the solar system became possible only after the publication of Isaac Newton's laws of

    motion and gravitation in 1687. Even after this breakthrough, many years elapsed while scientistsstruggled with applications of Newton's laws to explain the apparent motions of planets, satellites,

    comets, and asteroids. Meanwhile, the first semblance of a modern theory for solar system origin

    was proposed by the German philosopher Immanuel Kant in 1755. Kant's central idea was that the

    system began as a cloud of dispersed particles. He assumed that the mutual gravitational attractionsof the particles caused them to start moving and colliding, at which point chemical forces kept them

    bonded together. As some of these aggregates became larger than others, they grew still more

    rapidly, ultimately forming the planets. Because Kant was not highly versed in either physics ormathematics, he did not recognize the intrinsic limitations of his primitive approach. His model

    does not account for planets moving around the Sun in the same direction and in the same plane, as

    they are observed to do, nor does it explain the revolution of planetary satellites.

    A significant step forward was made by Pierre-Simon Laplace of France some 40 years later.

    Laplace was a brilliant mathematician who was particularly successful in the field of celestial

    mechanics. After publishing a monumental treatise on this subject, Laplace wrote a popular book on

    astronomy, with anappendix in which he made some suggestions about the origin of the solarsystem. It is this relatively minor work for which he is best remembered. Laplace's model begins

    with the Sun already formed and its atmosphere extending beyond the distance at which the farthest

    planet would be created. Knowing nothing about the source of energy in stars, Laplace assumed thatthe rotating Sun would start to cool as it radiated away its heat. In response to this cooling, as the

    pressure exerted by its gases declined, the Sun contracted. Owing to the law of conservation of

    angular momentum, the decrease in size would have to be accompanied by an increase in the Sun'srotational velocity. Centrifugal acceleration pushed the material in the atmosphere outward, while

    the gravitational attraction pulled it toward the central mass; when these forces just balanced, a ring

    of material was left behind. This process would have continued through the formation of several

    concentric rings, each of which subsequently coalesced to form a planet. The satellites are thoughtto have originated from similar rings produced by the forming planets.

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    Laplace's model led naturally to the observed result of planets revolving around the Sun in the same

    plane and in the same direction as the Sun rotates. Because the theory of Laplace incorporatedKant's idea of planets coalescing from dispersed material, these two approaches for planet formation

    are often combined in a single model called the Kant-Laplace nebular hypothesis. This model for

    solar system formation was widely accepted for about 100 years. During this period, the apparentregularity of motions in the solar system was contradicted by the discovery of asteroids with highly

    eccentric orbits and satellites with retrograde orbits. Another problem with the nebular hypothesiswas the fact that, while the Sun contains 99.9 percent of the mass of the solar system, the planets(principally the outer planets) carry more than 99 percent of the system's angular momentum. To

    conform to this theory, either the Sun would have to be rotating more rapidly or the planets would

    have to be revolving around it more slowly.

    In the early decades of the 20th century, several scientists independently decided that these

    deficiencies of the nebular hypothesis were so great that it was no longer tenable. The AmericansThomas Chrowder Chamberlin and Forest Ray Moulton, along with Sir James Jeans and Sir

    Harold Jeffreys, bothof Britain, independently developed variations on the idea that the planets were

    formed catastrophicallyi.e., by the close encounter of the Sun with another star. The basis of this

    model was that, when the two bodies passed at close range, material would be drawn out from oneor both stars, and this material would later coalesce to form planets. A somewhat discouraging

    aspect of this theory was the implication that the formation of solar systems must be extremely rare,

    because sufficiently close encounters between stars occur very seldom, and thus very few wouldhave taken place during the lifetime of the galaxy.

    The next significant development occurred during the middle of the 20th century, as scientists

    became more aware of the processes by which stars themselves must form and acquired a more

    mature understanding of the behaviour of gases under astrophysical conditions. This perspectiveled to the realization that hot gases stripped from a stellar atmosphere would simply dissipate in

    space; they would not condense to form planets. Hence the basic idea of solar system formation

    through stellar encounters was physically impossible. Furthermore, the growth in knowledge about

    the interstellar mediumthe gas and dust distributed in the space separating the starsindicatedthat large clouds of such matter exist and that stars form in these clouds. Planets must somehow be

    created in the process that forms the stars themselves. This awareness prompted scientists toreconsider certain basic processes that resembled some of theearlier notions of Kant and Laplace.

    Modern ideas:

    The current approach is to treat the origin of the solar system as part of the general problem of star

    formation. A steadily increasing amount of observational data is available to constrain models forthis process. This information ranges from observations of star-forming regions in giant interstellar

    clouds to subtle clues revealed in the existing chemical composition of the objects present in the

    solar system.

    The current paradigm for solar system origin suggests that its formation began with the collapse of

    part of an interstellar cloud of gas and dust, with an initial mass only 10 to 20 percent larger than

    the present mass of the Sun. This collapse could be initiatedby random fluctuations of densitywithin the cloud, one or more of which might result in the accumulation of enough material to cause

    the cloud to collapse. The initial cloud would have to be roughly spherical in shape. Because it is

    revolving around the centre of the galaxy, the outer edge of the cloud (i.e., the one farthest from thegalactic centre) moves more slowly than the inner part. Hence the collapse of the cloud would cause

    it to rotate, and in order to conserve angular momentum the speed ofrotation would increase as the

    cloud contracts. As the contraction continues, the cloud flattens, as it is easier for matterto follow

    the attraction of gravity along the axis of rotation than perpendicular to it, where the opposing

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    centrifugal force is greatest. Thus a flattened disk is formed around a central condensation (as in the

    model of Laplace). This configuration, commonly referred to as the solar nebula, would resemblethe shape of a typical spiral galaxy on an enormously reduced scale. As gas and dust are pulled in

    toward the central condensation, their potential energy is converted to kinetic energy and the

    temperature of the material rises. Ultimately the temperature becomes great enough in the interior ofthe condensation for nuclear reactions to begin, thereby giving birth to a star.

    Comet Comet (Latin stella cometa, hairy star) is relatively small, icy celestial body revolving around theSun. When a comet nears the Sun, some of the ice in the comet turns into gas. The gas and loose

    dust freed from the ice create a long, luminous tail that streams behind the comet.

    Natural Satellite Natural Satellite is a celestial body that orbits a larger celestial body. The larger body is referred to

    as the satellites primary. Natural satellites that orbit planets are often called moons.

    The best-known natural satellite is Earth's Moon. The Moon is unusually large relative to the size of

    its primary (Earth); in fact, it is significantly larger than the planet Pluto. The Moons surface, likethe surfaces of most of the natural satellites in the solar system, is heavily cratered and geologically

    inactive.

    Asteroid( Mnvby)Asteroid is one of the many small or minor rocky planetoids that are members of the solar systemand that move in elliptical orbits primarily between the orbits of Mars and Jupiter.

    Asteroid 243 Ida Asteroids are chunks of rock and metal too small to be considered planets. They

    orbit the Sun and are situated primarily between the orbits of Mars and Jupiter. Asteroid 433 Eros InFebruary 2000, the Near-Earth Asteroid Rendezvous (NEAR) Shoemaker spacecraft reached the

    asteroid 433 Eros and began transmitting images back to scientists on Earth. This image, taken on

    February 14, is detailed enough to show the boulders and craters that dot Eross surface.Asteroid

    Collision with Earth Many scientists believe that a large asteroid or comet struck Earth about 65

    million years ago, changing the Earths climate enough to kill off the dinosaurs.

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    Meteorite( D v)Meteorite, meteor that reaches the surface of Earth or of another planet before it is entirely

    consumed. Meteorites found on Earth are classified into types, depending on their composition:irons, those composed chiefly of iron, a small percentage of nickel, and traces of other metals such

    as cobalt; stones, stony meteors consisting of silicates; and stony irons, containing varying

    proportions of both iron and stone.

    Characteristic Mercury Venus Earth Mars Jupiter Saturn Uranus Neptune Pluto

    Equatorial radius (Earth radii) 0.3825 0.9488 1 0.5325 11.21 9.449 4.007 3.883 0.1874

    Equatorial inclination (degrees) 0.01 2.64 23.5 25.2 3.13 26.7 82.2 28.3 57.4

    Mass (Earth masses) 0.055 0.815 1 0.107 318 95.2 14.5 17.1 0.002

    Average density (g/cm3) 5.4 5.2 5.5 3.9 1.3 0.69 1.3 1.6 1.8

    Rotational period (days) 58.6 -240 1 1.03 0.414 0.444 -0.718 0.671 -6.4

    Orbital period (years) 0.2408 0.6152 1 1.881 11.86 29.46 84.01 164.8 247.9

    Average distance from the Sun (AUs) 0.3871 0.7233 1 1.524 5.203 9.59 19.10 30 39.30

    Orbital eccentricity (ratio) 0.206 0.00674 0.0167 0.0935 0.0489 0.0576 0.0497 0.00995 0.248

    Orbital inclination (degrees) 7 3.39 0.0003 1.85 1.30 2.49 0.772 1.77 17.2

    Moons (number) 0 0 1 2 39 32 27 8 1

    Planet's radius expressed as a multiple of Earth's radius (6,378 km)

    Planet's mass expressed as a multiple of Earth's mass (5.971024 kg)

    Mercury

    Mercury is one of the planets in the solar system. Mercury orbits closest to the Sun of all the

    planets, at an average distance of approximately 58 million km . The planets diameter is 4,879 km,and its volume and mass are about one-eighteenth that of Earth. Mercurys mean density is

    approximately equal to that of Earth and is higher than that of any of the other planets. The force of

    gravity on the planet's surface is about one-third of that on Earth's surface or about twice the surfacegravity on the Moon.

    Gaspra, an asteroid of the main belt, in a composite o

    two images taken by the Galileo spacecraft during its

    flyby on October 29, 1991. Pocked with numerous sm

    craters, Gaspra measures about 20 km in its longest

    dimension. Its irregular shape and groovelike linearmarkings suggest that it was once part of a larger bo

    that experienced one or more shattering collisions.

    Colours in the composite image have been enhanced

    computer to highlight subtle variations in reflectivity

    and other surface characteristics.

    NASA/JPL/Caltech

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    VenusVenus is one of the planets in the solar system, the second in distance from the Sun. Except for the

    Sun and the Moon, Venus is the brightest object in the sky. The planet is called the morning star

    when it appears in the east at sunrise, and the evening star when it is in the west at sunset. In ancienttimes the evening star was called Hesperus and the morning star Phosphorus or Lucifer. Because of

    the distances of the orbits of Venus and Earth from the Sun, Venus is never visible more than threehours before sunrise or three hours after sunset.When viewed through a telescope, the planet exhibits phases like the Moon. Maximum brilliance (a

    stellar magnitude of -4.4, 15 times as bright as the brightest star) is seen in the crescent phase when

    Venus is closer to Earth. Venuss full phase appears smaller and dimmer because it occurs when the

    planet is on the far side of the Sun from Earth. The phases and positions of Venus in the sky repeatevery 1.6 years . Transits of Venus (when the planet moves across the face of the Sun as seen from

    Earth) are rare, occurring in pairs at intervals of a little more than a century. The next two transits

    will be in 2004 and 2012.

    Earth

    Earth is one of nine planets in the solar system, the only planet known to harbor life, and the

    home of human beings. From space Earth resembles a big blue marble with swirling white cloudsfloating above blue oceans. About 71 percent of Earths surface is covered by water, which is

    essential to life. The rest is land, mostly in the form of continents that rise above the oceans.

    Earth An oxygen-rich and protective atmosphere, moderate temperatures, abundant water, and a

    varied chemical composition enable Earth to support life, the only planet known to harbor life. The

    planet is composed of rock and metal, which are present in molten form beneath its surface. Earthssurface is surrounded by a layer of gases known as the atmosphere, which extends upward from the

    surface, slowly thinning out into space. Below the surface is a hot interior of rocky material and two

    core layers composed of the metals nickel and iron in solid and liquid form.

    Mars

    Mars is one of the planets in the solar system, it is the fourth planet from the Sun and orbits the Sun

    at an average distance of about 228 million km . Mars is named for the Roman god of war and is

    sometimes called the red planet because it appears fiery red in Earths night sky.

    Mars is a relatively small planet, with about half the diameter of Earth and about one-tenth Earths

    mass. The force of gravity on the surface of Mars is about one-third of that on Earth. Mars has twice

    the diameter and twice the surface gravity of Earths Moon. The surface area of Mars is almostexactly the same as the surface area of the dry land on Earth. Mars is believed to be about the same

    age as Earth, having formed from the same spinning, condensing cloud of gas and dust that formed

    the Sun and the other planets about 4.6 billion years ago.

    JupiterJupiter is fifth planet from the Sun and the largest planet in the solar system. The fourth brightest

    object in Earths sky, after the Sun, the Moon, and Venus, Jupiter is more than three times brighterthan Sirius, the brightest star. Due to its prominence in the sky, the Romans named the planet for

    their chief god, Jupiter.Jupiter orbits the Sun at an average distance of 780 million km , which is about five times the

    distance from Earth to the Sun. Jupiters year, or the time it takes to complete an orbit about the

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    Sun, is 11.9 Earth years, and its day, or the time it takes to rotate on its axis, is about 9.9 hours, less

    than half an Earth day.Unlike the rocky inner planets of the solar system (Mercury, Venus, Earth, and Mars), Jupiter is a

    ball of dense gas and has no solid surface. Jupiter may have a core composed of rock-forming

    minerals like those trapped in comet ices, but the core makes up less than 5 percent of the planets

    mass. The force of gravity at the level of the highest clouds in Jupiters atmosphere is about 2.5

    times the force of gravity at Earths surface..Jupiter, encircled by at least 61 satellites and a series of thin rings, is similar to a miniature solar

    system. For this reason, Jupiter is of great interest to planetary scientists and others who are

    concerned with the formation of planetary systems. Sixteen of Jupiter's moons are discussed in this

    section; the remaining 45 are relatively recent discoveries and have not yet been extensivelystudied.

    Eleven of these newly discovered moons have been named: Themisto, Iocaste, Harpalyke,

    Praxidike, Taygete, Chalden, Kalyke, Callirrhoe, Megaclite, Isonoe, and Erinome. The rest are

    referred to by numbers that reflect the year and order in which they were discovered.

    SaturnSaturn , sixth planet in order of distance from the Sun, and the second largest in the solar system.Saturn's most distinctive feature is its ring system, which was first seen in 1610 by Italian scientist

    Galileo, using one of the first telescopes. He did not understand that the rings were separate from

    the body of the planet, so he described them as handles (ansae). The Dutch astronomer ChristiaanHuygens was the first to describe the rings correctly. In 1655, desiring further time to verify his

    explanation without losing his claim to priority, Huygens wrote a series of letters in code, which

    when properly arranged formed a Latin sentence that read in translation, It is girdled by a thin flat

    ring, nowhere touching, inclined to the ecliptic. The rings are named in order of their discovery,and from the planet outward they are known as the D, C, B, A, F, G, and E rings. These rings are

    now known to comprise more than 100,000 individual ringlets, each of which circles the planet.

    As seen from Earth, Saturn appears as a yellowish objectone of the brightest in the night sky.Observed through a telescope, the A and B rings are easily visible, whereas only under optimal

    conditions can the D and E rings be seen. Sensitive Earth-based telescopes have detected nine

    satellites, and in the haze of Saturn's gaseous envelope, pale belts and zones parallel to the equator

    can be distinguished.Saturn has 18 confirmed moons and as many as 14 proposed new, unconfirmed moons. In the past

    many proposed new moons have turned out to be just dense spots in Saturn's rings, but the Cassini

    spacecraft should be able to definitively catalog Saturn's moons. The diameters of Saturn's satellites

    range from 20 to 5,150 km

    Uranus

    Uranus is a major planet in the solar system, seventh planet from the Sun. Uranus revolves outsidethe orbit of Saturn and inside the orbit of Neptune . The average distance from Uranus to the Sun is

    2.87 billion km. Uranus has an inner rocky core that is surrounded by a vast ocean of water mixed

    with rocky material. From the core, this ocean extends upward until it meets an atmosphere ofhydrogen, helium, and methane. Uranus has 11 known rings and 27 confirmed moons. The mass of

    Uranus is 14.5 times greater than the mass of Earth, and its volume is 67 times greater than that of

    Earth. The force of gravity at the surface of Uranus is 1.17 times the force of gravity on Earth.

    Because of its great size and mass, scientists classify Uranus as one of the giant or Jovian (likeJupiter) planetsalong with Jupiter, Saturn, and Neptune.

    Uranus was the first planet that people discovered by using a telescope. Sir William Herschel, a

    German-born British musician and astronomer, discovered the planet in 1781.

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    Neptune

    Neptune is major planet in the solar system, eighth planet from the Sun and fourth largest indiameter. Neptune maintains an almost constant distance, about 4.5 billion km, from the Sun.

    Neptune revolves outside the orbit of Uranus and for most of its orbit moves inside the elliptical

    path of the outermost planet Pluto . Every 248 years, Plutos elliptical orbit brings the planet inside

    Neptunes nearly circular orbit for about 20 years, temporarily making Neptune the farthest planet

    from the Sun. The last time Plutos orbit brought it inside Neptunes orbit was in 1979. In 1999Plutos orbit carried it back outside Neptunes orbit.

    Pluto

    Pluto is ninth planet from the Sun and outermost known planet of the solar system. Pluto revolves

    about the Sun once in 247.7 Earth years at an average distance of 5.91 billion km . The planetsorbit is so eccentric that at certain points along its path Pluto is slightly closer to the Sun than is

    Neptune. Pluto is about 2,360 km in diameter, about two-thirds the size of Earth's moon.

    Discovered in 1930, Pluto is the most recent planet in the solar system to be detected.Pluto is far away from Earth, and no spacecraft has yet been sent to the planet. All the information

    astronomers have on Pluto comes from observation through large telescopes. Pluto was discoveredas the result of a telescopic search inaugurated in 1905 by American astronomer Percival Lowell,who postulated the existence of a distant planet beyond Neptune as the cause of slight irregularities

    in the orbits of Uranus and Neptune. Continued after Lowells death by members of the Lowell

    Observatory staff, the search ended successfully in 1930, when American astronomer ClydeWilliam Tombaugh found Pluto.

    The planets of the solar system, in a montage of images scaled to show the approximate sizes of the

    bodies relative to one another. Outward from the Sun, which is represented to scale by the yellow

    segment at the extreme left, are the four rocky terrestrial planetsMercury, Venus, Earth, and Marsthe

    four hydrogen-rich giant planetsJupiter, Saturn, Uranus, and Neptuneand the icy, comparatively tiny

    Pluto.

    NASA/Lunar and Planetary Laboratory

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    Supernova 1987A

    The Hubble Space Telescope took this photo of the aftermath of the 1987A supernova in 1994, seven years

    after the light from the exploding star first reached Earth. The supernova occurred in the Large Magellanic

    Cloud, a satellite galaxy of the Milky Way. Scientists do not yet agree on the mechanism that created the

    rings surrounding the remnants of the star.

    NASA

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    Microsoft Encarta Reference Library 2005. 1993-2004 Microsoft Corporation. Allrights reserved.


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