AstronomyTopic 06:
Galaxies
Stephen’s Quintet
Galaxies
Galaxy: A group of millions or billions of stars, gas, and dust held together by gravity
Our sun and all visible stars are members of our galaxy, the Milky Way
Our Galaxy
The Milky Way is visible as a band of light crossing the sky during summer and winter
Represents the concentrated light of the billions of stars in our galaxy
At night, we witness the view from within our galaxy
Milky Way Galaxy
A disk of stars that contains 100-200 billion stars
100,000 light years in diameter, about 3000 LY thick
The center of the Milky Way (nuclear bulge), swells to about 10,000 LY thick
Milky Way Diagram
http://zebu.uoregon.edu/~imamura/123/images/mw-schematic.jpg
Milky Way
Infrared Milky Way (COBE)
http://www.astro.psu.edu/users/niel/astro1/slideshows/class21/slides-21.html
The view from within, in IR light
Sun and Galaxy
Sun located about 2/3 distance away from the center (30,000 LY)
The entire Milky Way rotates around its center
Sun orbits the center of the galaxy at 563,000 mph
One galactic revolution takes 220 million years
Sun’s Revolution
http://www.envirotruth.org/images/graphics/suns_path.jpg
Milky Way, a Spiral Galaxy
Spiral galaxy, most are concentrated in a central nucleus
Spiral arms of stars wind outward from the nucleus
M101
Milky Way Map
http://members.nova.org/~sol/chview/milkyway.jpg
Star Clusters: Open Clusters
A few dozen to thousands of stars loosely bound together by gravity
Found mainly in the galaxy’s disk and spiral arms
More than 1000 have been discovered in the MW
Young stars that recently formed from nebulosity
Jewel Box open cluster (NGC 4755)
Star Clusters: Globular Clusters
Huge, concentrated balls of thousands to millions of stars
Found in galactic halo, a spherical region centered on the nucleus
Contain the oldest known stars
About 150 globular clusters have been discovered
Omega Centauri, globular cluster (NGC 5139)
Cluster Summary
Type Open Cluster Globular Cluster
Stars Dozens to thousands (Population I)
Hundreds to millions (Population II)
Location Within galactic disk
Orbit galaxy in halo
Age Young, thousands to millions of years
Among oldest known stars (billions of years)
Open Cluster HR Diagrams
•This HR diagram plots the stars for many open clusters
•As clusters age, their main sequence stars appear lower on the HR diagram
•Pleiades are younger than M67
Open Clusters
Pleiades—M45M67
Which cluster is oldest? Note the star colors of the main stars in each cluster
Globular Cluster M55
Colors indicate temperatures, red (cool) to blue (hot)
The “turn off” area on the main sequence represents the cluster’s age
Dark Matter
Galaxies consist of ordinary stars, gas, and dust
Observed motions of stars in a galaxy indicate unseen (dark) matter
Due to dark matter, galaxies rotate faster than the observable matter allows
Visible galaxies surrounded by large, nonluminous galactic halos
http://www.pparc.ac.uk/Ps/aac/images/image33.gif
Dark Matter in NGC 3198
http://bustard.phys.nd.edu/Phys171/lectures/ngc3198_opt_rad_rot.jpg
Between the Stars
Space between the stars in a galaxy is a vacuum, or empty space
Also termed interstellar mediumMatter between the stars is of gas and dustInterstellar matter 99% gas, 1% dustGas consists of 75% hydrogen and 25% heliumInterstellar dust similar in size to cigarette smokeIn spiral galaxies, gas and dust is concentrated
in the disk and spiral arms
Molecular Clouds
> 100 interstellar molecules have been detected in molecular clouds between stars
Water vapor, amino acids, and other organic molecules (urea, alcohol)
Ingredients of life can be found in the galaxy’s molecular clouds
Cosmic origin of life?
Nebulas
Historically, a nebula meant any type of hazy, glowing patch of light in the night sky
Andromeda Galaxy once termed the Andromeda Nebula
Today, nebula means interstellar gas and dust
Emission Nebulas
Emission nebulas glow due to excitement from nearby stars
Also called diffuse nebulas or HII regionsOrion Nebula (M42), famous example of
an emission nebula (next slide)
Pillars of Creation (M16)•In this iconic photo from the Hubble Space Telescope, a small portion near the center of M16, the Eagle Nebula, is revealed
•Note the pink, newly formed stars within the nebula
Dark Nebulas
Dark nebulas, or absorption nebulas, molecular clouds that block the light of distant stars
E.E. Barnard used photography to map the galaxy’s dark nebulas in the early 20th century
Dark nebulas today bear “B” numbers in his honor
Barnard’s S Nebula
http://www.astropix.com/HTML/D_SUM_S/B72.HTM
A dark nebula represents interstellar dust that blocks the light of stars from behind
Horsehead Nebula
Famous dark nebula (B33) located near the Belt of Orion
Star Populations
Walter Baade (1944) divided a galaxy’s stars into two categories:
Population I Hot, young, and luminousFound in galactic diskContain heavy elements beyond H and He
Population IIOld, cool stars, some first stars in universeFound in globular clusters and galactic nucleusAlmost all H and He
Catalogs
Messier Catalog of 110 objects (1784) by French astronomer Charles Messier
Dreyer’s New General and Index Catalogs of thousands of objects (1895-1908)
NGC contains 7840 objects“M” or “NGC”, or “IC” numbersObjects, mainly galaxies, star clusters,
and nebulae
Messier Catalog
All of the Messier objects are shown in this montage
M1 starts the upper left, M110 ends the lower right
Formation of the Milky Way
Milky Way appears to be about 13 billion years old
MW likely represents one of the original galaxies created after the Big Bang
Sun and planets are 4.5 billion years oldBig Bang origin of universe 13.7 bya
Other Galaxies
Until 1924, Milky Way was the “universe”Edwin Hubble proved that galaxies other than
our Milky Way existHubble used Cepheid variable stars to measure
the distances to galaxiesHubble also photographed stars in the
Andromeda “Nebula”The universe contains an estimated 100 billion
galaxies. Each of these galaxies contains about 100 billion stars
Standard Candles
An astronomical object whose absolute magnitude is know from its observed characteristics
Can be used to determine distances to galaxies up to 10 million LY from earth
Examples: Cepheid variable starsGlobular clustersSupernovas
Andromeda Galaxy
Our nearest large neighborSimilar in size and shape to Milky WayAndromeda Galaxy is visible to the naked
eye as a glowing spot in the constellation of Andromeda
2.3 million light years distant (wave!)
Andromeda Galaxy (M31, M32, M110)
Andromeda Constellation
Locate M31 and M33
Triangulum Galaxy (M33)
M33
The pink glow of emission nebulas can be seen in this photo
Virgo
Virgo Supercluster About 200 galaxies
are visible in small telescopes
Galaxies range from Ursa Major through Coma Berenices to Virgo
Sombrero Galaxy
Center of Virgo Cluster
Virgo Cluster near M84, M86
M86
Sombrero Galaxy (M104) from HST
Sheets and Voids
Superclusters are located in thin sheets of galaxies
Sheets border empty voidsVoids resemble gigantic bubblesThe universe resembles a sponge
Large-Scale Structure of the Universe
http://www.astro.princeton.edu/~cen/PROJECTS/p1/DARKallz0.jpeg
Sheets & Voids:http://heasarc.nasa.gov/docs/cosmic/sheets_voids.html
Active Galaxies (AGN)
An active galaxy emits large amounts of energy from its nucleus
AGNs exhibit jets of hydrogen racing outward along an axis
Massive black holes?Mysterious quasars might represent
extremely active galaxies
Galactic Cannibalism
Galaxies can merge together to form larger galaxies
Elliptical galaxies may be the result of multiple galaxy collisions
NGC 2207 (left) and IC 2163 (HST)
Quasars
Resemble faint stars, are strong radio sourcesAcronym for Quasi-Stellar Radio SourceSmall objects, yet emit energy egual to
thousands of galaxies combinedMost distant and powerful objects knownMight represent early stage of galaxy growthSeyfert galaxies are active galaxies with nuclei
that resemble quasars
3C 273-Quasar
HST image of 3C 273 showing disk details (quasar blocked)
3C 273 Starfield in Virgo
3C 273 is one of the first identified quasars, it resembles an 11th magnitude star in Virgo and is visible in small amateur
telescopes