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Exploring Galaxies: Distances, Shapes, and the Expanding Universe

This overview delves into the fascinating realm of galaxies, examining their various shapes—elliptical, spiral, and irregular—and the significant role of distance indicators like Cepheid variables in determining their distances. Understand the methods for measuring galactic sizes and luminosity through angular diameters and flux. Explore the properties that define galaxies, including mass-luminosity ratios and the mysteries of dark matter. Finally, learn about the expansion of the universe and Hubble's Law, providing insights into the cosmos's age and the formation of clusters and superclusters.

mariko-russell
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Exploring Galaxies: Distances, Shapes, and the Expanding Universe

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Presentation Transcript

  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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