Download - GALAXIES
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!