Chapter 12
Collapsing, Exploding, Interacting Stars, and Stellar Black Holes
Iron core - fusion stops
Iron fusion is endothermic
Core contracts, heats
Shell fusion continues
Calculations suggest core collapse at iron mass ~ 2 Msun
Core more massive than Chandrasekhar limit
Electron degeneracy cannot stop collapse
Electrons + protons => neutrons + neutrinos
Neutron degeneracy pressure stops collapse if remaining mass less than 2-3 solar masses
Neutron star - radius ~ 10 km
density ~ 4 x 1017 kg/m3
Outer layers fall inward
"Bounce" on hot core
Supernova Remnant
Neutron Stars and Pulsars
Discovered 1967 - Bell, Hewish
Radio sources emitting short, regular pulses (pulsars)
From conservation of angular momentum, neutron stars rotate ~ 1 rotation/sec
Magnetic field strength also conserved => very strong magnetic fields on neutron stars
Tsurface ~ 106 K
Lighthouse theory:
Millisecond pulsars - old neutron stars in close binaries
Spun up by mass transfer from evolving companion
Collapsed Objects in Close Binaries
Neutron stars in binary systems
Binary Pulsars
Energy lost due to gravitational waves
First observation of effect of gravity waves
Black Holes
Region of space where escape velocity greater than speed of light.
Receive no information from inside
Produced by large mass in small volume
Schwarzschild radius - characteristic size of black hole
Rs = 2 x G x M/c2
Example: 1 Msun black hole, R = 3 km.
Generally: R (km) = 3 x M (solar masses)
Mass of collapsing core greater than maximum mass of neutron star, ~3Msun
Degenerate electrons, neutrons can't support
Collapses to singularity
Infinite density, gravity
Zero radius.
BUT PHYSICS NOT UNDERSTOOD.
Singularity would be INSIDE black hole.
The Event Horizon
Theoretical black hole properties: mass, angular momentum, electric charge
Schwarzschild black holes only have mass
Event horizon - sphere with Schwarzschild radius
Nothing escapes from within event horizon
Material can fall in to black hole
Tidal effects strong at event horizon of stellar mass black holes
Time dilation - clocks run more slowly in strong gravitational fields, as observed remotely
To remote observer, in principle, infalling material appears to linger at edge 'forever' while material in fact falls in.
But due to large gravitational redshift, material disappears anyway.
Kerr black holes rotate
Matter falling into black holes emits large amounts of energy - x-rays
Detecting black holes
Detected by impact on nearby matter
E.g. binary star with massive, invisible companion
In close binary, accretion disk emitting x-rays suggests compact object, possibly a black hole, if mass > 3 solar masses.
Look for binary stars emitting X-rays
Mass estimates difficult
Several stellar mass black hole candidates
Prof. Donna Weistrop
University of Nevada, Las Vegas