ISP205 Lecture #23, April 3, 2001

  1. Announcements:
    1. New Homework Assignment: Set 8
      Due: April 10, 2001
    2. Previous assignment: Set 7
      Due: April 10, 2001
  2. Review: 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
        (example for planet discoveries)
    4. First extrasolar planet discovered by M. Mayor and D. Queloz 1995 orbiting 51 Pegasi (40 Ly away)
    5. So far: ~30 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)
  3. Review: Protostars on the HR diagram (picture 20.12)
    1. ZAMS: Zero Age Main Sequence: the position in the HR diagram 
      where a newborn star starts fusing hydrogen into helium and begins
      its life on the main sequence.
    2. Brown Dwarfs: 
      Stars with less than 0.08 solar masses that cannot ignite hydrogen.
  1. Life beyond the Main Sequence
    1. Main sequence lifetimes:
      Mass in solar masses Main sequence lifetime
      0.4 200 billion years
      0.8 14 billion years
      1.1 9 billion years
      1.7 2.7 billion years
      3.3 500 million years
      16 10 million years
      40 1 million years

       

    2. Red Giant Phase
      1. Contracting helium core and hydrogen shell burning
      2. Hydrogen burns faster - more energy is generated
        - outer layers of the star expand but cool
        - star becomes bigger and redder
      3. Red Giants can become several AU's large
        Sun: will become a red giant in 5 billion years and
                will incorporate earth
        Betelgeuse (picture)
      4. Red Giants on the HR diagram (picture)
              
    3. Star Clusters:
      1. Types of star clusters:
        1. Globular clusters (picture)
          1. Very old: Formed before disk of the Galaxy
          2. 100,000 - million stars
        2. Open clusters (picture)
          1. Located in galactic disk
          2. 50-100 stars
        3. OB Associations (picture)
          1. Very young, massive O,B stars in starforming region
          2. 100-10000 stars
      2. Clusters are a sample of different stars born at the same time
      3. HR diagrams of clusters with different ages (pictures)
    4. Life of a low mass star (less than 8 solar masses)
      1. Helium burning (picture)
        (Sun: only 100 Million years)
      2. Carbon and Oxygen core
      3. Planetary nebula:
        No further burning possible - CO core contracts
        and the generated energy triggers a wind that
        blows away the remaining outer layers in a
        "Planetary nebula" (pictures)
      4. White dwarf: (picture, example to be seen in sky)
        CO core contracts until electrons get so densely packed
        that they are not allowed to move closer anymore
        (Quantum mechanics - Pauli principle)
      5. White dwarf properties:
        1. Size: ~ earth
        2. Mass: up to 1.4 solar masses
        3. High density: 1 Tsp weighs 15 tons ! (truck)
        4. Doesn't do much except for cooling
    5. Life of a massive star (more than 8 solar masses)
      1. After helium burning it survives burning the products of the 
        previous burning stage successively:

        Example: 25 solar mass star, core burning stages
        Stage Duration Product Name Product Z Product N
        Hydrogen burning 7 Mio yr Helium 2 2
        Helium burning 700,000 yr Carbon,Oxygen 6,8 6,8
        Carbon burning 400 yr Oxygen, Neon 8,10 8,10
        Neon burning 1 yr Oxygen, Magnesium, Silicon 8,12,14 8,12,14
        Oxygen burning 8 month Silicon, Sulfur 14,16 14,16
        Silicon burning 1 day ~Iron, Nickel ~24-28 ~24-28

        Z = number of protons in nucleus
        N = number of neutrons in nucleus
           

      2. Iron is the most stable nucleus -
        fusion of two iron nuclei does not generate energy
        anymore
      3. Structure of the star after silicon burning: the onion
        (picture)
      4. Death: Supernova explosion
        1. Explosion shines brighter than a galaxy
        2. Example: 1987 A (pictures AAST, story)
        3. Supernova remnants (1987A rings, older remnants pictures)
      5. How does a supernova work (Movie)
        1. Iron core collapses (electrons cannot hold it)
        2. Collapse stops when protons and neutrons get
          too densely packed
          (same as for electrons stabilizing the white dwarf)
        3. Rebounce leads to explosion (Demo)
        4. Neutrino wind helps
        5. Light generated by decay of radioactive nuclei
          (light curve picture)
      6. Such a supernova is called type II Supernova
        (lots of hydrogen lines)
      7. Collapsed core forms Neutron star or Black hole
      8. Neutron Star:
        1. Size: ~6 miles
        2. Mass: 1.4 - 3 (?) solar masses
        3. Very dense: 1 Tsp = 700 Million tons !
          (= 7000 air craft carriers)
        4. New born neutron stars are pulsars
          1. Discovered by Jocelyn Bell as grad student, Cambridge
            1967 (her advisor, Antony Hewish got the Nobel Prize)
            (picture)
          2. Rapidly rotating neutron star (periods ...)
            that emits a beam of radio radiation (picture)
            (lighthouse effect - DEMO)
          3. Crab pulsar in supernova remnant (pictures)
            powers the nebulas emission and slows down !
            Crab: 33ms, AD1054 SN (chinese astronomers in taurus)
            (6500 Ly away, spin down 0.01ms/year)
          4. Pulsars live for ~10 Million years until too slow to 
            create radio radiation 
            (1350 known today)
        5. Neutron stars usually get a kick during the supernova explosion
          and move away from the remnant
      9. Black holes: next lecture
    6. Summary: Lifecycle of stars and Nucleosynthesis
    7. Reviving neutron stars and white dwarfs:
      1. Mass transfer (picture)
        Neutron stars and white dwarfs in binary systems orbit another star.
        They can suck matter from the companion and shine again !
      2. Novae (picture)
        1. White dwarfs accumulate material from the companion
          for 20 - 100,000 years
        2. It explodes within days as a Nova
        3. Burned material is ejected and white dwarf stars
          again to accumulate material - goto 1.
                  
      3. Type Ia Supernova
        1. White dwarfs the accumulate too much and become
          heavier than 1.4 solar masses explode in a supernova
          powered by the fusion of carbon and oxygen
        2. The star is completely disrupted
              
      4. X-ray bursters and ms pulsars (picture)
        1. Neutron stars accumulate material from companion
          for hours - days
        2. Frequent explosions seen as X-ray bursts
        3. The accumulation (accretion) spins the neutron star up
          and it can become a pulsar again
        4. Pulsar periods as short as a few milliseconds are possible