Astronomy 100


Lectures Table of Contents Astro 100

Section 3 Review

If you don't have much time to review the text book, concentrate on Ch.12.1-12.2, 13, 14.2,  15.

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 for 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

    • Harlow 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

    • The age of the Milky Way galaxy is at least about 15 billion years based on observations of old globular clusters.

    • Shape of MW

      • Typical spiral galaxy

      • Disk (like pancake) contains spiral arms, dust clouds, open clusters, young stars, star formation; causes band-like appearance of Milky Way in our sky. Stars have more-or-less circular orbits.

      • Halo = spherical component, including central bulge; has old stars, no star formation. Stars have randomly inclined orbits.

      • Stars in disk orbit in plane in nearly circular orbits (whirlpool)

      • Stars in halo orbit in randomly-oriented orbits (bees)

      • Stars in the disk have more heavy elements because heavy elements were made in previous generations of stars.

    • Size of MW: about 25 kpc (75,000 l.y.) across

    • Spiral Arms

      • Mostly spiral density waves, which induce star formation

    • The Interstellar Composition of MW:

      • Molecular gas (e.g., dark clouds)

      • Atomic gas (emitting 21 cm line)

      • Ionized warm gas (emission nebulae)

      • Hot gas (emitting X-rays)

      • Dust, causing the reddening of star light

      • Cosmic rays and Magnetic field (emitting radio continuum radiation)

    • Star Formation and the Ecosystem of the Galaxy

    • The Center of MW

      •   Is there a massive black hole there?

  • Galaxy Properties

    • Galaxy Types

      • Spirals (like MW) -- live mostly isolated or in small groups. MW lives in the "Local Group"

      • Ellipticals (like MW's halo + bulge) -- live mostly in galaxy clusters and contain proportionally small amount of gas or dust and a large number of old stars. An elliptical galaxy could be formed from the collision and merger of spiral galaxies. 

      • Irregulars, including many starburst and interacting galaxies, typically containing large clouds of gas and dust, both young and old stars, but no obvious spiral arms or nucleus.

    • Distances

      • Measured via Cepheid variables or other standard candle or standard yardstick (supernovae, planetary nebulae...), or by measuring rotation speed of galaxy

      • Nearest spiral galaxy is Andromeda, 2 million l.y. away

      • Farthest galaxies known are 95% of the way to the edge of the observable Universe, 13 or 14 billion l.y. away.

      • The Milky Way galaxy is part of the Local Group.

    • Masses

      • range from 109MSun to 1014MSun.

      • Milky Way has M~1012MSun.

      • 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 galaxies, with more star formation, distorted shapes, smaller sizes

    • Velocities

      • Hubble showed that galaxies are in general receding from each other. But Hubble's law does not work for Local group galaxies. Why?

      • Hubble's Law: V = H0 D -- Universe is exanding. If galaxy A is two times more distant than galaxy B, how faster does galaxy A recede than galaxy B according to the Hubble law?

      • H0 = 65 +/- 10 km/s / Mpc

      • The receding velocity causes the redshift of spectral lines, which can be used to estimate distances to distant galaxies that follow the Hubble's law: D = V/H0.

  • Black Holes and Quasars

  • Quasars are so bright they outshine whole galaxies; brightest objects in Universe; visible to edge of known Universe. Short time variability --> tiny in size. Quasars typically show high redshifts, indicating that are very far away.

  • All big galaxies and quasars are believed to contain center supermassive black holes. In particular, Milky Way might have a dead quasar at center with M=107 MSun

  • 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 (Hubble Law)

        2. relative abundance of elements matches prediction 

        3. cosmic microwave background radiation (relic of the hot big bang when the universe became transparent due to the "recombination" of electrons with protons).

        4. evolving structure (galaxies and large-scale structures appear progressively younger with the lookback time).

      • had no center -- happened everywhere at once

      • predicted by General Relativity

    • Age of Universe

      • From recession of galaxies, age is ~ 1/H0 ~15 billion years

      • From oldest objects (globular star clusters in Milky Way and other galaxies), age is 13-17 billion years.

    • Fate of the Universe

      Depends on gravity, which depends on critical density

      • Open: not enough matter to halt expansion (i.e., the mean density of the Universe is less than the critical density); will expand forever

      • Flat: just enough matter to slow expansion forever...but hangs on knife edge, never recollapses

      • Closed: enough matter in Universe to halt expansion and force collapse --> Big Crunch

      Current data imply Universe is open. Perhaps even accelerating!

Review questions:

  • How do we know that spiral galaxies are rotating?
  • How do we infer the presence of dark matter in galaxies?
  • Where is the center of the Universe?
  • What is meant by the redshift of a galaxy?
  • Why do we think that quasars are both luminous and small?
  • How do we estimate the age of the Universe?
  • What are the origins of elements?
    • Light elements (e.g., H and He)
    • Heavy elements (e.g., C, O, …, Fe)
    • Heavier elements (Co, Ni, Cu, …)
  • Why do young stars contain more heavy elements?

Lectures Table of Contents Astro 100

Houjun Mo Astronomy 100