Astro 1050     Mon. Mar. 28, 2005
   Today: Go over Homework on board
Ch. 11: Neutron Stars, Black Holes

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

Spinning pulsar powers the
 Crab nebula
Red:  Ha
Blue:  “Synchrotron” emission from high speed electrons trapped in magnetic field

Another pic of the Crab, Pulsar

Why a “pulsar?”

“Lighthouse” Model for Pulsars

Another Neutron Star in a SNR

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.

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

Examples:
The Schwarzschild Radius:
Mass in solar masses Rs (km)
10
3
2
1
0.000003 (Earth)

Examples:
The Schwarzschild Radius:
Mass in solar masses Rs (km)
10 30
3 9
2 6
1 3
0.000003 (Earth) 0.9 cm

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

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!).

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

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

Cygnus X-1

More Cool Stuff About Black Holes
Time Dilation – originally “Frozen Stars”
Gravitational Redshift
Wicked Tidal Forces
Hawking Radiation