Allison Thompson Chapter 13 notes

Chapter 13 The bizarre stellar grave yard



13.1
What is a white dwarf?
-White dwarfs are the remaining cores of dead stars.
-Electron degeneracy pressure supports them against gravity.
-White dwarfs cool off and grow dimmer with time.
-White dwarfs with the same mass as the sun are the same size of Earth.
-Higher mass white dwarfs are smaller.
-Quantum mechanics says that electrons must move faster as they are squeezed into a very small space.
-As a white dwarfs mass approaches 1.4m its electrons must move at nearly the speed of light.
-Because nothing can move faster than light a white dwarf cannot be more massive than 1.4.

What can happen to a white dwarf in a close binary system?
-A star started with less mass gains mass from its companion.
-Mass falling toward a white dwarf from its close because. has Some angular motion.
-The matter therefore orbits the white dwarf in an acceleration disk.
-The temperature of accreted matter eventually becomes hot enough for hydrogen fusion.
-Fusion begins suddenly and explosively causing a nova.
-The nova star system temporarily appears Much brighter.
-The explosion drives accreted matter out into space.
-Massive star supernovas have iron cores of massive star reaches. white dwarf limit and collapses into a neutron star causing an explosion.
-White dwarf supernova is carbon fused suddenly begins as a white dwarf in close binary system reaches white dwarf limit causing Total explosion.
-One way to tell supernova types apart is with a light curve showing How luminosity changes with time.
-Supernova are Much Much more luminous Than novae.
-Nova is H to He fusion of layer of accretes matter white dwarf left intact.
-Supernova a complete explosion if white dwarfs.
-Light curves differ
-Spectra differ exploding white dwarfs don't have hydrogen absorption lines.

13.2
What is a neutron star?
-A neutron star is the ball of neutrons left behind by a massive star supernova.
-The degeneracy pressure of neutrons supports a neutron star against gravity.
-Electron degeneracy pressure goes away because electrons combines with protons making neutrons and neutrinos.
-Neutrons collapse to the center forming a neutron star.
-A neutron star is the same size as a small city.

How were neutron stars discovered?
-Using a radon telescope in 1967 Jocelyn Bell noticed a regular pulse.
-Pulsar at center of Crab nebula pluses 30 times per second.
-A pulsar is a neutron star that beams radiation along a magnetic axis that is not aligned with the rotation axis.
-The radiation beam sweep through space like a light house beams as the neutron star rotates.

What can happen to a neutron star in a close binary system?
- Matter falling toward a neutron star forms an acceleration disk.
-Accreting matter adds angular momentum to a neutron star increasing spinning.
-Matter accreting onto neutron star can eventually become hot enough for helium to fuse.
-Quantum mechanics says that neutrons in the same place cannot be in the same state.
-Neutron degeneracy pressure can no longer support a neutron star against gravity if its mass exceeds about 3M.

13.3
What is a black hole?
-A black hole is an object that’s so powerful that nit even light can escape.
-light would not be able to Escape earths surface if you could shrink it.
-The surface of a black hole is the radius at which the escape of velocity equals the speed of light.
-The spherical surface is known as the event horizon.
-The radius of the event horizon is known as Schwarzschild radius
-the event horizon of 3m black hole is also about as bug as a small city.
-A black holes mass strongly wraps space and time in vicinity of the event horizon.
-Nothing can Escape from within the event horizon because nothing can to faster than light.
-No Escape means there is no contact with something that falls in. it increases the holes mass changes its spin or charge but otherwise loses its identity.
-Beyond the neutron star limit no known force can resist the crush of gravity.
-As far as we know gravity crushes all the matter into a single point known as a singularity.

What would it be like to visit a black hole?
-If the sun shrank into a black hole its gravity would be different only near the event horizon.
-Light waves take extra time to climb put of a deep hole I'm space-time leading to gravitational red shift.
-Time passes more slowly near the event horizon.
-Tidal force near the event horizon of a 3M black hole would be lethal to humans.
-Tidal forces would be gentler near a super massive black hole because its radius is much bigger.

Do black holes really exist?
-Needs to measure mass. use orbital properties of companion measure velocity and distance of orbiting gas.
-If its a black hole if its not a star its mass exceeds the neutron star limit .
-Some x ray binaries contain compact objects of mass exceeding 3m.
-One famous x-ray binary with a black hole is in a constellation.

13.4
What causes gamma rays to burst.
-Brief bursts of gamma rays coming from space were first detects in the 1960s.
-Observations in the 1990s showed that gamma rays bursts were coming from very distant galaxies.
-They must be among the most powerful explosions.
-Observations show that at least Some gamma rays bursts are produced by supernova explosions.