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Astro 1050     Mon. Mar. 28, 2005
  •    Today: Go over Homework on board
  • Ch. 11: Neutron Stars, Black Holes
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Chapter 11: Neutron Stars and Black Holes
What happens to the collapsing core?
  • Neutron star
    • Quantum rules also resist neutron packing
      • Densities much higher than white dwarfs allowed
        • R ~ 5 km      r ~ 1014 gm/cm3   (similar to nucleus)
      • M limit uncertain,  ~2 or ~3 MSun before it collapses


    • Spins very fast (by conservation of angular momentum)


    • Trapped spinning magnetic field makes it:
      • Act like a “lighthouse” beaming out E-M radiation (radio, light)
        • pulsars
      • Accelerates nearby charged particles
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Spinning pulsar powers the
 Crab nebula
  • Red:  Ha


  • Blue:  “Synchrotron” emission from high speed electrons trapped in magnetic field
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Another pic of the Crab, Pulsar
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Why a “pulsar?”
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“Lighthouse” Model for Pulsars
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Another Neutron Star in a SNR
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Other cool stuff about Neutron Stars
  • Novel Dragon’s Egg by Robert L. Forward
  • Short Story “Neutron Star” by Larry Niven (available at www.fictionwise.com)


  • Binary Pulsars
    • Tests of general relativity
  • Pulsar Planets
    • Variation in pulsar times indicates a wobble from a planet in orbit around the pulsar.
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Black Holes -- basics
  • Nothing can stop collapse after neutron pressure fails


  • Escape velocity from a surface at radius R:


  • As R shrinks (but M is fixed), Vescape gets larger and larger


  • At some point VEscape= c  (speed of light)


    • Happens at Schwarzschild radius:


    • Not even light can escape from within this radius
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Examples:
  • The Schwarzschild Radius:
    • Mass in solar masses Rs (km)
      • 10
      • 3
      • 2
      • 1
      • 0.000003 (Earth)
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Examples:
  • The Schwarzschild Radius:
    • Mass in solar masses Rs (km)
      • 10 30
      • 3 9
      • 2 6
      • 1 3
      • 0.000003 (Earth) 0.9 cm
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Black Holes -- details

  • Remember – gravity is same as before, away from mass
    • Black holes do NOT necessarily pull all nearby material in
    • A planet orbiting a new black hole would just keep on orbiting as before (assuming the ejected material or radiated energy didn’t have an effect)


  • Any mass can potentially be made into a black hole – if you can compress it to a size smaller than RS = 2GM/c2
    • 1 MSun: 3.0 km        106 MSun 3´106 km         1 MEarth 8.9 mm
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Black Holes -- details


  • If you do make material fall into a black hole, material will be falling at close to the speed of light when it reaches RS
    • If that falling gas collides with and heats other gas before it reaches RS, then light from that hot material (outside RS) can escape (important in quasars!).
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Black Holes – detection

  • By definition – can’t see light from black hole itself


  • Can see large amounts of energy released by falling material just before it crosses RS


  • Can see motion of nearby objects caused by gravity of black hole
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Black Holes – detection


  • Example: Like White Dwarf accretion disk but w/ black hole instead
    • Gas from red giant companion spills over towards black hole
    • Gas spirals in toward black hole, through accretion disk
      • Gas will be much hotter because it falls further, to very small RS
    • Gas will be moving at very high velocity
      • Much faster than with white dwarf since much closer  (P2 µ a3)


  • Signature of black hole:  Very high energy release, very high velocity


  • We find MASSIVE black holes in centers of most galaxies
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Cygnus X-1
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More Cool Stuff About Black Holes
  • Time Dilation – originally “Frozen Stars”


  • Gravitational Redshift


  • Wicked Tidal Forces


  • Hawking Radiation