ISP205 Lecture #22, March 29, 2001

  1. Announcements:
  2. Interstellar Gas
    1. Stars ~95% of visible matter in Galaxy but lots of space inbetween
    2. Space between stars filled with gas and dust
      (billions of solar masses in our Galaxy)
      99% gas
      1%   dust
    3. Gas density: 1e-3 to 1e5 atoms per cubic centimeter
      in clouds
    4. Temperatures: ~40K to Millions K (parts heated by supernova
      explosion shock waves)
    5. Composition: similar to sun 
    6. Observation of gas:
      1. HII regions
        1. Ionized H recombines emitting various photons, including
          the red 646nm Halpha photons (3rd to 2nd excited state)
        2. Energy source: UV radiation from a closeby star
      2. 21cm radiation from cool HI regions
        Proton and electron spins misalign - transition in lower
        energy state that emits 21cm photons (radio photons)
      3. Additional absorption lines in background stars
  3. Interstellar Dust
    1. Image of Milky Way visible, IR, Radio, .....
    2. Other images of nebulae with lots of dust;
    3. Composition: Water. Methane and Ammonia ices around
      silicate or graphite or iron core.
    4. Grain size: 10-100 nm
    5. Observation of dust:
      1. Blocking star light (Dark Nebulae)
      2. IR observations 
      3. Interstellar reddening (DEMO)
        blue light (~ f4) scatters much more than red light
  4. Clouds
    1. Gas and dust are not evenly distributed but concentrated
      in many clouds
    2. Typical cloud size: 10's of Ly
    3. Biggest clounds: Giant molecular clouds
      1. Densest clouds
      2. 100-1,000,000 solar masses
      3. Interior shielded from radiation by outside dust layers
        therefore cold (10's of K) and molecules can form
        (alcohol, ....)
  5. Cosmic Rays
    1. High speed atomic nuclei, electrons and positrons
      (near the speed of light)
    2. Composition similar to solar system, but some light elements
      enhanced - breakup products from collisions
    3. Charged particles follow magnetic field lines in the Galaxy and
      Earth 
    4. Probably made in Supernova explosions in our Galaxy
      (with the excetion of ultra hig henergy cosmic rays
      that might come from other galaxies)
  6. Birth of a Star
    1. Most stars are born in giant molecular clouds
      (remember formation of the solar system)
    2. Protostar: newly forming star before hydrogen ignition
    3. Protostars start producing radiation (grav. energy) that
      creates a wind blowing away remaining dust
    4. Jets: often wind can only blow in certain directions
      and creates therefore a stream of dust.
    5. Herbig-Haro Objects: Jets hitting nearby material
      producing visible glow
    6. Protostars on the HR diagram
  7. Extrasolar Planets
    1. Observations of planetary disks around newly formed stars
      (picture)
    2. Disks have "doughnut" shape for older proto stars
      Possible explanation: formation of a giant planet took place
      (picture)
    3. Detection of extrasolar planets: detect orbital motion of the star !
      1. Method 1: Detect "wiggle" directly fom changes in stars position
      2. Method 2: Detect motion with doppler effect
    4. First extrasolar planet discovered by M. Mayor and D. Queloz 1995
      orbiting 51 Pegasi (40 Ly away)
    5. So far: (book) 20 planets found around other stars
    6. Surprise: Hot Jupiters
      Giant planets very close to the star (most easy to detect !)
      Maybe they formed further away and moved then closer
      (breaking due to dust)