Chapter 13 Jane Lucas

Jane Lucas
Astronomy 100-Week 14 – Chapter 13
White Dwarfs
Neutron Stars
Black Holes
The Origin of Gamma-Ray Bursts
White Dwarfs – A white dwarf is the core left over from a low-mass star, and they are supported against the crush of gravity by electron degeneracy pressure.  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.  It is about the same size as our planet Earth.  They shrink when you add mass to them because their gravity gets stronger.  A white dwarf in a close binary system can acquire hydrogen from its companion through an accretion disk.  As hydrogen builds up on the white dwarfs surface, it may ignite with nuclear fusion to make a nova.  
Neutron Stars – A neutron is the ball of neutrons created by the collapse of the iron core in a massive star supernova.  Degeneracy pressure of neutrons supports a neutron star against gravity.  Electron degeneracy pressure goes away because electrons combine with protons, making neutrons and neutrinos.  Neutrons collapse to the center, forming a neutron star.  The first neutron star was discovered by Bell Burnell in 1967.  Matter that falls towards a neutron star forms an accretion disk, just as in a white-dwarf binary.  Accretion matter adds angular momentum to a neutron star, increasing its spin.  Episodes of fusion on the surface lead to X-ray bursts.  
Black Holes – A black hole is an object whose gravity is so powerful that not even light can escape it.  The surface of a black hole is the radius at which the escape velocity equals the speed of light.  The spherical surface is known as the evident horizon.  The radius of the event horizon is known as the Schwarzchild radius.  A black hole’s mass strongly warps space and time in the vicinity of event horizon.  Nothing can escape from with the event horizon because nothing can go faster than light.  
The Origin of Gamma-Ray Bursts – Gamma-ray bursts occur in distant galaxies and are most powerful bursts of energy we observe anywhere in the universe.  Some gamma-ray bursts appear to come from unusually powerful supernovae.  
In conclusion for Chapter 13 we learned dwarfs, neutron stars, and black holes.  They can all have close stellar companions from which they accrete matter.  These binary systems produce events in our universe such as: novae, white dwarf supernovae, and X-ray bursters.  Black holes are a bottomless pit in space time in which nothing can escape.