GALAXIES

Visible contents of the cosmos

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!

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!)

Galaxies: Shapes

•Elliptical: Football; little starbirth now

•Disk (spiral): Starbirth in disks

•Irregular: No special shape; lots of starbirth

•Peculiar shapes: Usually involve collisions

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)!

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”

“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!)

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?

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!