is a SPIRAL galaxy with DISK, central BULGE, and HALO

disk is population I
bulge and halo are pop. II (evolved stars)

spiral arms best observed in 21-cm emission line (electron spin-flip) of neutral hydrogen.

The sun is about 10,000 pc (30,000 ly) from the center.


patterns in proper motions, radial velocities (optical and radio).
sun moves 230 km/s (about 500,000 mph) as measured with respect to globular cluster.
Orbital of the galaxy takes 230 millon years.
Kepler's third law gives mass interior to sun: 100 billion solar masses.
The MW's ROTATION CURVE shows dark matter surrounding the galaxy.

and similarly for other galaxies ==> "missing mass"

(dMs, BDs, WIMPS, MACHOs, neutrinos)

HUBBLE classification for normal galaxies

Ellipticals, spirals, and barred spirals.

there are also dwarf ellipticals, and irregulars (Magellanic clouds).

Average Properties

               Irr      Sc     Sb     Sa        E,S0

C.I.          0.12    0.22    0.41    0.53       0.77
Abs. Mag      -17.3   -19.8   -21.0   -20.6      -19.6
Mass           10^9   2.10^10  10^11  1.6.10^11   2.10^11
%HI             37     20       3       1.3      <0.2
Binary galaxies...mass estimates

Galaxies tend to be found in clusters (local group)

Typical cluster: 1000 members, gravitationally bound, dominant giant ellipticals (20x normal size), hot intergalactic gas, occasional collisions.

Superclusters: clusters of clusters, 100 million ly across
voids: 100-400 million ly across: structure like a sponge

Expansion of the Universe (red-shift): "Hubble Flow"

V. M. Slipher (1914) and Edwin Hubble (1929) uncovered the redshift - distance relation.

Calibrated by various "standard candles": cepheids, globular clusters, H II regions, supernovae (-19: seen at 10 billion ly)

The expansion is uniform and the same everywhere: we are not in the center

Recessional velocity = Ho x distance

so age T is dist/vel = 1/H

at 75 km/sec/Mpc get age of about 13 x 10^9 years (but H may be smaller)


Rudolf Minkowski - 1951 - got spectrum of radio source Cyg A

Redshifts upto z ~ 4, or about 90% c (d ~ 13 billion ly (?))
Peculiar Galaxies

Active galactic nuclei, Seyfert galaxies

Characterized by compact nuclei, strong emission lines, beams and jets, and variability.

Luminosities ~ 10^11 L(sun)

evidence of explosive events, black holes?

Galaxy Formation

Galaxies formed from huge clouds of primordial gas

Gamma-ray bursters (~ 1000, isotropic distribution, a couple of possible repeats, one with proper motion?)

deals with the distribution and behavior of all matter in the universe.

Cosmogony: deals with the origin of the universe (totality, only one)

problem: construct a model describing the structure and evolution of the universe consistent with known physical laws and observation.

Cosmological Principle: Aside from random fluctuations that may occur locally, the universe must appear the same for all observers.