Astro Exam 3

• Galaxy


  Great wheel-shaped star system, the Milky Way
• Milky Way


  • Over 200 billion stars
  • One of billions of galaxies in the universe
  • Huge collection of stars, dust and gas
• The Milky Way is about 100,000 light-years wide
• Milky Way galaxy


  Large barred spiral galaxy
• Parts of the Milky Way


  • Disk Component
  • Spherical Component
  • Nucleus (Supermassive black hole)
• Disk Component


  • Contains spiral arms, open clusters, young stars, gas and dust
• Spherical Component


  • Contains Halo (globular clusters), Nuclear Bulge (cross between disk and halo), old stars, little gas and dust
• How the size of the Milky Way was found


  1. Harlow Shapley used variable stars to find distance to star clusters

  2. Noticed open clusters lie near Milky Way

  3. Globular clusters concentrated on one side of sky in Sagittarius

  4. Used RR Lyrae variable stars to find distance to globular clusters

  5. Postulated the center of the galaxy was in the direction of Sagittarius

  6. Quadrupled the measured width of the galaxy
• Variable stars


  Giant stars that go through a variable star phase, where the star pulsates in size, temperature, and brightness
• Variable stars


  • Have a period-luminosity relationship
  • 3 types with known period-luminosity relationship: RR Lyrae Stars, Type I and Type II Cepheids
• Use of variable stars


  Once a star's actual brightness is known, its distance can be estimated from the star's apparent brightness
• Spiral arms


  • Consist of bright young stars, clusters, gas, and dust clouds
  • Contain a concentration of young stars
• Population I vs Population II stars


  • Population I: 2nd, 3rd, and 4th generations stars, contain 1.5-3% heavy elements, primarily found in the disk
  • Population II: "first" generation stars, almost entirely Hydrogen and Helium, less than 1% heavy elements, found in halo and globular clusters
• Sun


  A Population I star
• Young and old parts of the galaxy


  • Young: Disk component, spiral arms
  • Old: Spherical component (Halo and Nuclear Bulge)
• Galactic nucleus


  • Unobservable at visible wavelengths
  • Radio, infrared, X-ray, and gamma-ray observations suggest violent, energetic activity
  • Central parsec crowded with over a million stars
  • Stars in high-velocity orbit around a supermassive black hole
• 3 major types of galaxies


  • Elliptical
  • Spiral
  • Irregular
• Elliptical Galaxies


  • Round or oval in shape
  • Have no visible gas or dust
  • No hot bright stars or spiral patterns or emission nebulae, composed of old, red, low mass population II stars (lacks any star formation)
  • Similar in composition and shape to the halo of the Milky Way
  • Both the largest and smallest galaxies are ellipticals
• Spiral Galaxies


  • Disk component with the young population I star, hot bright stars, gas and dust, spiral structure
  • 2 subclasses: Normal Spirals (arms wind from the nucleus), Barred Spirals (arms wind from an elongated nuclear "bar")
• Irregular Galaxies


  • Chaotic Appearance, no definite structure
  • Large gas and dust clouds (star formation)
  • Small: 5 to 25 % the diameter of the Milky Way
  • Composition similar to the disk of spiral galaxies
• Spiral galaxies are the most plentiful
• Evidence for dark matter


  • Outermost stars of galaxies orbiting much faster than expected
  • Galaxies in clusters moving too fast to be kept in the cluster by the gravity of "luminous matter"
  • Gravitational mass must be 5x greater than the detectable mass for clusters to be gravitationally bound
• Mapping dark matter


  Gravitational Lensing: Deflection of light from distant galaxies around foreground galaxies
• Mundane dark matter


  Dim, low-mass stars, brown dwarfs, and gas-giant particles
• Exotic dark matter


  Black Holes, massive unknown elementary particles
• Nonbaryonic matter


  Suspected component of dark matter composed of matter that does not contain protons and neutrons
• WIMPs


  Weakly interactive massive particles, a hypothetical type of subatomic particle of which dark matter could be composed of
• Hot Dark Matter


  Invisible matter in the universe composed of low-mass, high-velocity particles such as neutrinos
• Cold Dark Matter


  Invisible matter in the universe composed of heavy slow-moving particles such as WIMPs
• Standard Candles


  Distance indicators to other galaxies, objects of known brightness that astronomers use to find the distance
• Cepheid variable stars and type 1A supernova

  Two of the most important standard candles
• Quasar


  Ultra-active cores of distant galaxies
• We know quasars are not stars because they have unusual spectral properties, huge redshifts, and rapid variability in brightness
• Why we think quasars are far away


  They look like starlike points of light, have huge redshifts indicating enormous distance, and have enormous luminosity
• Why high quasar luminosity implies large mass


  There is a maximum luminosity an object of a given mass can have before it blows itself apart by radiation pressure, so more luminous objects need more mass to hold themselves together
• Why quasar active regions are thought to be small

  Their rapid fluctuations in brightness imply a small size, as an object can't change its brightness in less than the light travel time to cross the object
• Possible power source of quasars

  Supermassive black hole with an accretion disk of matter
• Relationship between quasars and active galaxies


  They both have supermassive black holes at their center and enormous luminosity
• Hubble Law


  The more distant the galaxy, the larger its redshift and thus, the faster its velocity away from us
• How Hubble Law implies expansion


  There is a linear relation between the distance to a galaxy and its radial velocity, most galaxies have redshifts, and the faintest galaxies have larger redshifts, implying galaxies are receding from each other