ISP205 Lecture #24, April 5, 2001

  1. Announcements:
    1. New Homework Assignment: Set 8
      Due date extended: April 17, 2001
    2. Previous assignment: Set 7
      Still due: April 10, 2001
  2. Review: Life of stars also on HR diagram
  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. Younger
        2. Located in galactic disk
        3. 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 (transparency)
  4. Summary: Lifecycle of stars and Nucleosynthesis (transparency)
  5. Neutron Star:
    1. Size: ~6 miles
    2. Mass: 1.4 - 3 (?) solar masses
    3. Very dense: 1 Tsp = 700 Million tons !
    4. New born neutron stars are pulsars
      1. Discovered by Jocelyn Bell as grad student, Cambridge
        1967 (her advisor, Anthony Hewish got the Nobel Prize)
        (picture)
      2. Rapidly rotating neutron star (periods of ms to seconds)
        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
          
  6. Reviving neutron stars and white dwarfs:
    1. Mass transfer (transparency)
      Neutron stars and white dwarfs in binary systems orbit another star.
      They can suck matter from the companion and shine again !
    2. Novae (transparency)
      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
      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
  7. Relativity
    1. The speed of light in vacuum is the highest possible speed
      (example)
      This is a well established fact recognized everyday in 
      particle accelerators.
         
    2. The speed of light is constant (example)
      1. Consequences: times and lengths of events differ depending
        on the motion of the observer (Relativity)
      2. This is the basis of "Special Relativity"
              
    3. Equivalence principle (example)
      1. Consequence 1: Gravity bends space
      2. Consequence 2: Gravity makes time run slower
      3. Together this means: Gravity bends space time
      4. This is the basis of "General Relativity"
    4. Observational evidence for general relativity
      1. Orbit of Mercury - Perihelion moves additional 43 arcsec per century
        (on top of 531 arcsec from the gravity pull of other planets)
        picture
      2. Starlight bend by sun during eclipse (1.75 arcsec) (picture)
      3. Clocks run slower on top floor than in basement
        1. demonstrated by Robert Pound and Glenn Rebka 1959 
          at the Harvard physics building using an atomic clock.
        2. Clocks on rocket confirm general relativity to 0.02%
      4. Gravitational redshift
      5. Flightpaths of spacecraft (need relativity to calculate them)
  8. Black Holes
    1. Iron core more massive than ~ 3 solar masses collapses into point
      (nothing can stop collapse)
    2. Event horizon: an imaginary  sphere around the black hole
      1. Defines the "Point of no return" 
        Nothing inside the event horizon can escape the black hole
        because space is bent backwards (that's why its black)
      2. Radius of event horizon ~3 km for 1 solar mass black hole
    3. Black holes have the same gravitational attraction than any 
      other object with the same mass (at the same distance from 
      center)
    4. Properties of black holes: Mass, Spin, Charge
      Nothing else !
    5. Falling into a black hole - as seen from outside
    6. Falling into a black hole - as seen from inside
    7. Observational evidence for black holes
      1. So far not much  "direct" evidence (event horizon)
      2. Black hole X-ray binaries
        Compact objects with masses larger than 3 solar masses
        have been found as part of a binary system (2 stars orbiting 
        each other)
        As they cannot be neutron stars, the only thing we know they 
        can be are black holes
      3. Supermassive black holes in the center of the galaxy
        (Millions of solar masses)