Chapter 13

The Milky Way Galaxy and Other Galaxies

Our Galaxy:

Visible as the Milky Way

Wright (1759) - a flattened disc

William and Caroline Herschel:
W.H. 1738 - 1822

C.H. 1750 - 1848

Star counts

Sun close to center of flattened disc

Model supported by Kapteyn

Incorrectly, Sun thought to be near center, underestimated size.

Determining the size of the galaxy:
Shapley: globular cluster distribution centered on Sagittarius.

=> Center of galaxy in Sagittarius.

Distance to globular clusters.

Contain RR Lyrae stars.

Identify RR Lyraes in clusters, measure periods, get distances.

More realistic size of galaxy

Previous observers fooled by dust absorption.

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:

Population I - in disk, relatively young, 2-3% metals

Population II - in halo, nuclear bulge, globular clusters, old, 0.1% metals

Galactic Rotation:

Disk stars - circular motion.


Velocity = ~230 km/sec
Period = ~225 x106 years.

Motion of halo objects more random

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 ??

Spiral Structure

Arms delineated by young, bright objects:

O, B stars, open clusters, HII regions, some variables, star formation

Radio maps - radio waves not obscured by dust.

Neutral H clouds (21 cm)

Giant molecular clouds (CO)

Arms irregular, interrupted.

2 or 4 arms?

Galactic Nucleus:

Infrared => large numbers of stars, densely packed.

Sagittarius A* - radio source at galactic center, smaller than solar system

Gas clouds, molecules, ionized gas

Black hole at center?

Gas orbiting rapidly.

Velocities of stars => black hole

Radio observations consistent with hypothesis.

Black hole most likely explanation of observations.


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

Elliptical Galaxies:

Spherical or elliptical, smooth light distribution

E0 - E7 : E0 round

Dwarf ellipticals (dE) common

Giant ellipticals and cD galaxies in galaxy clusters

Spiral Galaxies (S):

Central nucleus and gas, dust, hot, young stars in spiral pattern.

Sa, Sb, Sc

Depends on nuclear bulge, tightness of spiral, amount of gas, dust.

Barred spirals (SB):

'Bar' through nucleus.

Arms originate in bar.

SBa, SBb, SBc, : same criteria as Sa, Sb, Sc.

Milky Way probably SBbc

Lenticular galaxies (S0, SB0):

Nucleus and disk, no spiral arms

Intermediate between E, spirals.

Irregular Galaxies (Irr ): No obvious nucleus, spiral arms.

Closer Look at Spiral Structure

Flocculent spirals - chaotic arms

Grand design spirals - well defined arms

What maintains spiral structure?

Self-Sustaining Star Formation:

Formation of hot stars triggers further star formation.

Different rotation periods drag inner edge of clump ahead - forming "spiral arm".

Produces broken, "flocculent" spirals.

Density Wave Theory:

Compression moving through disc

Could be produced by gravitational interactions of stars in disc

Enhanced gravity attracts gas, dust into spiral arms

Shock wave compresses gas, initiates star formation.

Stars of all masses form in spiral arms: O, B stars most visible.

Density waves could be sustained by gravitational disturbances due to central bar and/or galaxy-galaxy interactions

Compositions and Masses

20-30% galaxies are spirals

Elliptical galaxies: mostly Population II stars, little gas

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:

Fewer galaxies

Structure irregular

Little stripping, few collisions.

More spirals than rich clusters.

Superclusters - aggregates of clusters, groups

100 - 250 x 106 light years (l.y.)

Connected by filaments of galaxies, clusters.

Voids (100 - 150 x 106 l.y.) in between

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