Instructor:

Neal Katz
532 Graduate Research Tower
Phone: 545-2085
Office Hours: Tu/Th 2:15 - 3:15 or by appointment

Following are what I consider the most important topics covered in the third section of the semester, since the last exam:

Galaxies and Cosmology

• The Milky Way: Our Galactic Home
  • Contains about 100 billion stars, including the Sun
  • Contains all the stars visible to the naked eye (except Andromeda Galaxy, Magellanic Clouds)
  • Cepheid variable stars: Period-luminosity relation --> more luminous stars vary more slowly --> can use period (blink rate) to measure luminosity --> can determine distance from apparent brightness
  • Shapley used Cepheid variables to find distances to globular clusters of stars; found Sun is not at Galactic center, but 8.5 kiloparsecs = 8,500 pc = 25,000 l.y. from center
  • Shape of MW
    • Typical spiral galaxy
    • Disk (like pancake) contains spiral arms, dust clouds, open clusters, young stars, star formation, metal rich population I stars; causes band-like appearance of Milky Way in our sky
    • Halo = spherical component, including central bulge; has old stars, no star formation, metal poor population II stars
    • Stars in disk orbit in plane in nearly circular orbits (whirlpool)
    • Stars in halo orbit in randomly-oriented orbits (bees)
  • Size of MW: about 25 kpc (75,000 l.y.) across
  • Origin of MW:
    1. Top down or
    2. Bottom up?
• Galaxy Properties
  • Galaxy Types
    • Spirals (mostly like MW disk) -- live mostly isolated or in small groups. MW lives in the "Local Group"
    • Ellipticals (like MW's halo + bulge) -- live mostly in galaxy clusters
    • Irregulars, including starburst and interacting galaxies
  • Distances
    • Measured via Cepheid variables or other standard candle or standard yardstick (supernovae, planetary nebulae...)
    • Nearest galaxy is Andromeda, 2 million l.y. away
    • Farthest galaxies known are 95% of the way to the edge of the observable Universe, 13 billion l.y. away.
  • Masses
    • range from 10⁸M_Sun to 10¹³M_Sun.
    • Milky Way has M~10¹²M_Sun.
    • Measured by rotation curves; stars make up only 10% of total --> 90% of Universe is "dark matter"
  • Evolution determined by looking far away = back in time; see younger Universe of galaxies, with more star formation, distorted shapes, smaller sizes
  • Velocities
    • Hubble showed that galaxies are all receding from each other
    • Hubble's Law: v = H₀ x r -- Universe is expanding
    • H₀ = 70 +/- 10 km/s / Mpc
• Quasars and Black Holes
  • Quasars
    • Discovered by radio, then optical, in 1960's
    • Looked like stars, but with strange spectra --> "quasi-stellar objects" = QSO = quasar
    • Spectra show enormous redshifts (many % of speed of light) --> huge recession velocity --> very distant (many billions of light years away) --> we are looking far back in time
    • Variability over few days to weeks implies small size (few light days or light weeks across), since most of quasar must be changing in luminosity, and QSO would be unable to coordinate variation on larger structure than light can traverse.
    • QSOs appear to live in the nuclei of galaxies, which are then called active galactic nuclei (AGN).
    • QSOs important because
      • Among brightest, most distant objects known
      • Give clues to galaxy formation and evolution
      • Shine through intervening matter (more clues)
      • Powered by special energy source
  • Black Holes That Shine -- Gravity Power
    • QSOs and other AGN probably powered by intense emission from accretion disk of matter just about to be devoured by a massive (10⁸ M_Sun) black hole. As matter swirls faster and faster, it heats up from friction, eventually emits X-rays, etc.
    • Huge, long jets (like a fire hose spray) often shoot out along the axes of the disk's rotation.
    • Many normal galaxies today (including the Milky Way) have hosted a QSO long ago; right now there still is a massive black hole at the Milky Way's center.
• The Big Bang and the Fate of Our Expanding Universe
  • The Hot Big Bang
    • Beginning of the Universe as we know it
    • Main evidence:
      1. expanding Universe
      2. relative abundance of elements matches prediction
      3. cosmic microwave background radiation
    • had no center -- happened everywhere at once
    • predicted by General Relativity
  • Age of Universe
    • From recession of galaxies, age is ~ 14 billion years
    • From oldest objects (globular star clusters in Milky Way and other galaxies), age is 13 billion years.
  • Fate of the Universe

    Depends on gravity, which depends on critical density

    1. Open: not enough matter to halt expansion; will expand forever
    2. Flat: just enough matter to slow expansion forever...but hangs on knife edge, never recollapses
    3. Closed: enough matter in Universe to halt expansion and force collapse --> Big Crunch

    Current data imply Universe is open. Perhaps even accelerating!