The Milky Way Galaxy and Other Galaxies
Visible as the Milky Way
Wright (1759) - a flattened disc
The Modern View
Disc shaped system
Diameter ~80,000 - 100,000 light years
Sun ~ 25,000 light years from center
Disc ~ 2000 light years thick
Gas, dust, young and old stars - open clusters. Star formation.
Nuclear bulge - elongated bar
Mostly old stars, some gas, young stars
Halo - spherical, globular clusters, old stars, little gas or dust
Two stellar populations:
Mass of the Galaxy
Calculate mass inside sun's orbit from orbital radius, speed
Total mass greater than 100 x 109 solar masses
Rotation curve - graph of velocity of rotation against distance from center.
Rotation curve flat or increasing beyond Sun.
Suggests mass in outer parts of galaxy - unseen.
Halo - radius ~200,000 light years ??
Mass ~ 1012 solar masses ??
Arms delineated by young, bright objects:
Radio maps - radio waves not obscured by dust.
Distances critical, difficult
Standard candles - objects of known brightness, e.g. Cepheids
Standard rulers - objects of known size, e.g. globular clusters
Galaxies of Many Kinds (Morphology)
Hubble sequence: Ellipticals, Spirals, Barred Spirals, Irregulars
Barred spirals (SB):
Milky Way probably SBbc
Lenticular galaxies (S0, SB0):
Nucleus and disk, no spiral arms
Intermediate between E, spirals.
Closer Look at Spiral Structure
Flocculent spirals - chaotic arms
Grand design spirals - well defined arms
What maintains spiral structure?
Self-Sustaining Star Formation:
Density Wave Theory:
Enhanced gravity attracts gas, dust into spiral arms
Compositions and Masses
20-30% galaxies are spirals
Spiral galaxies: Population I and II, gas, spiral arms dominated by hot young stars, HII regions
Irregular galaxies: young stars, lots of gas
Masses: 106 - 1013 solar masses
Luminosities: 2 x 105 - 1012 solar lumin.
Diameters: ~5000 - 5 x 106 light years
Rotation curves of spirals suggest presence of unseen matter
Evolution of Galaxies
Determined by angular momentum and rate of star formation in gas cloud
Little rotation, rapid star formation produces ellipticals
More rapid rotation, slower star formation produces spirals, lenticulars
BUT: galaxy interactions important.
Trigger star formation (starburst galaxies)
Galaxy mergers (cannibalism)
Do galaxies form from smaller pieces?
The Local Group of Galaxies
Milky Way is 1 of 3 dominant spirals
~25 galaxies, mostly dwarfs
M31 (Andromeda Galaxy) largest, bright spiral
Several satellite galaxies, possible black hole at center
Milky Way - second largest, SBbc
Several satellites - Large and Small Magellanic Clouds
M33 (in Triangulum) - spiral, smallest of 3, possible central black hole
Members of Local Group still being discovered
Clustering on Various Scales
Most galaxies in clusters.
Groups - small, few dozen members
Regular (Rich) clusters:
> 1000 galaxies, spheroidal distrib.
Diameter ~ 3 Mpc (3 x106 pc)
Concentrated to center.
Giant E (cD) at center.
X-ray emitting metal-rich gas
Stripping and collisions important.
Mostly E, S0 galaxies.
Virgo, Coma clusters
Irregular (Poor) clusters:
Superclusters - aggregates of clusters, groups
Dark Matter in Clusters
If clusters gravitationally bound, must contain undetected matter (dark matter)
Gravitational lensing => presence of dark matter
If hot x-ray emitting gas confined to cluster => presence of dark matter
Prof. Donna Weistrop
University of Nevada, Las Vegas