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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!.

mariko-russell
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GALAXIES

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  1. 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 angulardiameter and distance • Luminosity from flux and distance, apply inverse-square law for light • Mass: Rotation curve (disks) from Dopplershifts (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!

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