1. Visible contents of the cosmos
GALAXIES
Visible contents of the cosmos

2. Galaxies: Local or Distant?
Angular diameters span ratio of about 100
Angular diameter => actual diameter to distance ratio; large ratio => large distances
Clincher: Cepheid variables in Andromeda galaxy (1922); 2 million ly!

3. Galaxies-Distances
Distance indicators: Objects whose luminosities estimated (cepheids/P-L main method)
Measure flux from whole galaxy, apply inverse-square law for light, get a distance (with an error, sometimes large!)

4. Galaxies: Shapes Elliptical: Football; little starbirth now
Disk (spiral): Starbirth in disks
Irregular: No special shape; lots of starbirth
Peculiar shapes: Usually involve collisions

5. Galaxies-Properties Actual diameter from angular diameter and distance
Luminosity from flux and distance, apply inverse-square law for light
Mass: Rotation curve (disks) from Doppler shifts (Newton’s version Kepler’s 3rd)!

6. Hubble Law Measure red shifts (radial velocities) and distances
Find direct proportion: Greater distances, larger red shifts (radial velocities) implies the universe is expanding!
Expansion rate now: about 20 km/s/Mly = Hubble “constant”

7. “Age” of Cosmos Time = distance/rate (rate = H)
If H = 30 km/s/Mly, age about 10 billion years (whoops!)
If H = 20 km/s/Mly, age about 15 billion years (OK)
If H = 15 km/s/Mly, age about 20 billion years (better yet!)

8. Mass/Luminosity Ratio
Divide mass (M in solar masses) by luminosity (L in solar luminosities); for sun, M/L = 1; near sun, M/L a few
Add mass but not luminosity (dark matter) increases M/L
M/L up to 100 in galaxies; what/where is the dark matter?

9. Clusters & Superclusters
Red shifts as proxy for distances
Contain: few hundred to few thousand galaxies
Size: up to billion light years
M/L: a few hundred (lots of dark matter)
Superclusters largest entities in cosmos!