Chapter 13 Quiz: Olivia Ward

1. What is a white dwarf?
   
   A white dwarf is the left over core from a dead low-mass star. White dwarfs are supported by electron degeneracy pressure against the crush of gravity. They typically have the same mass as the Sun, but are no larger than Earth. No white dwarf can have a mass greater than 1.4 MSun and higher mass white dwarfs are typically smaller in size.
   
   
2. What can happen to a white dwarf in a close binary system?
   
   A white dwarf in a close binary system can acquire hydrogen from its companion through an accretion disk. A star that started with less mass gains mass from its companion. As hydrogen builds up on the white dwarf's surface, it may begin nuclear fusion and cause a nova. In some extreme cases, accretion may continue until the white dwarf's mass exceeds the white dwarf limit, causing it to explode as a white dwarf supernova.
   
3. What is a neutron star?
   
   A neutron star is a ball of neutrons created by the collapse of the iron core in a massive supernova.
   
4. How were neutron stars discovered?
   
   Neutron stars were discovered through pulsars, which were the first evidence of neutron stars. Neutron stars spin rapidly when they are born. Their strong magnetic fields can direct beams of radiation through space. while the neutron star rotates. We see these neutrons stars as pulsars.
   
5. What can happen to a neutron star in a close binary system?
   
   Neutron stars in a close binary system can accrete hydrogen from their companions, forming accretion disks. Accreting matter adds angular momentum to the neutron star, increasing its spinning. This matter can eventually become hot enough for helium to fuse, causing x-ray bursts.
   
6. What is a black hole?
   
   A black whole is an object whose gravity is so powerful that not even light can escape it. The spherical surface, the event horizon, of the black whole is the radius at which the escape velocity is the speed of light. Nothing can escape a black whole because nothing can move faster than the speed of light.
   
   
7. What would it be like to visit a black hole?
   
   You could orbit a black hole because there is no sucking action. Time would seem to run slowly for objects falling toward the black hole. Light would be red-shifted as the object approached the hole.
   
8. Do black holes really exist?
   
   Some x-ray binaries include compact objects that are far too massive, mass exceeding 3 MSun, to be neutron stars, making it likely that they are black holes.
   
   
9. What causes gamma ray bursts?
   
   The cause of gamma ray bursts is not exactly known, but the energy required to make a burst suggests that some gamma ray bursts appear to come from powerful supernova explosions that create black holes. Other gamma ray bursts may come from mergers of neutron stars in close binary systems, causing black holes.
   
   
10. What did Jocelyn Bell discover?
    
    In 1967, Jocelyn Bell discovered a regular pulse (pulsars) of neutron stars using a radio telescope.