Visible contents of the cosmos 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!