Notes
Slide Show
Outline
1
Astro 1050     Fri. Nov. 1, 2002
  •    Today: Articles
  •                Ch. 11: Neutron Stars &                     Black Holes
2
The Crab Nebula – Supernova from 1050 AD
  • Can see expansion between 1973 and 2001
    • Kitt Peak National Observatory Images
3
What happens to the collapsing core?
  • Neutron star (more in next chapter)
    • 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,  ~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
4
Spinning pulsar powers the
 Crab nebula
  • Red:  Ha


  • Blue:  “Synchrotron” emission from high speed electrons trapped in magnetic field
5
Another pic of the Crab, Pulsar
6
Why a “pulsar?”
7
“Lighthouse” Model for Pulsars
8
Another Neutron Star in a SNR
9
Other cool stuff about Neutron Stars
  • Novel Dragon’s Egg by Robert L. Forward
  • Short Story “Neutron Star” by Larry Niven


  • Binary Pulsars
    • Gamma Ray Bursts?


  • Pulsar Planets


  • Gravity bends light!
10
Black Holes -- basics
  • Nothing we know of can stop collapse after neutron pressure fails


  • Consider escape velocity from the 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
11
Examples:
  • The Schwarzschild Radius:
    • Mass in solar masses Rs (km)
      • 10
      • 3
      • 2
      • 1
      • 0.000003 (Earth)
12
Examples:
  • The Schwarzschild Radius:
    • Mass in solar masses Rs (km)
      • 10 30
      • 3 9
      • 2 6
      • 1 3
      • 0.000003 (Earth) 0.9 cm
13
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


  • 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.
14
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


  • 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 will find MASSIVE black holes in centers of most galaxies
15
Cygnus X-1
16
More Cool Stuff About Black Holes
  • Time Dilation – originally “Frozen Stars”


  • Gravitational Redshift


  • Wicked Tidal Forces


  • Hawking Radiation


  • A good online black hole FAQ: http://cosmology.berkeley.edu/Education/BHfaq.html


  • Virtual trips to neutron stars and black holes: http://antwrp.gsfc.nasa.gov/htmltest/rjn_bht.html