Jessica Horn: Quiz 13

1. What is a white dwarf? A white dwarf is the core left over from a low mass star, supported against the crush of gravity by electron degeneracy pressure. A white dwarf typically has the mass of the Sun compressed into a size no larger than Earth. No white dwarf can have a mass greater than 1.4M Sun.
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 that swirls toward the white dwarf's surface. As hydrogen builds  up on the white dwarf's surface, it may begin nuclear fusion and cause a nova that, for a few weeks, may shine as rightly as 100,000 Suns. 
3. What is a neutron star?A neutron star is the ball of neutrons created by the collapse of the iron core in a massive star supernova. It resembles a giant atomic nucleus 10 km in radius and with more mass than the Sun.
4. How were neutron stars discovered? Neutron stars spin rapidly when they are born, and their strong magnetic fields can direct beams of radiation that sweep through space as the neutron star spins,. We see such neutron stars as pulsars, and these pulsars provided the first direct evidence for the existence f neutron stars.
5. What can happen to a neutron star in a close binary system? Neutron stars in close binary systems can accrete hydrogen from their companions, forming dense, hot accretion disks. The hot gas emits strongly in X rays, so we see these systems as X-ray binaries. In some of these systems, frequent bursts of helium fusion occur on the neutron star's surface, causing X-ray bursts.
6. What is a black hole? A black hold is a place where gravity has crushed matter into oblivion, creating a hole in the universe from which nothing can ever escape, not even light. The event horizon marks the boundary between our observable universe and the inside of the black hold; the black hole's Schwarzschild radius is the size of the event horizon.
7. What would it be like to visit a black hole? You could orbit a black hole just like you could any other object of the same mass. However, you's see strange effects for an object falling toward the black hole: Time would seem to run slowly for the object, and its light would be increasingly red-shifted as it approached the black hole. The object would never quite reach the event horizon, but it would soon disappear from view as its light became so redshifted that no instrument could detect it.
8. Do black holes really exist? No known force can stop the collapse of a stellar corpse with a mass above the neutron star limit of 2 to 3 solar masses, and theoretical studies of supernovae suggest that such objects should sometimes form. Observational evidence supports this idea: Some X-ray binaries include compact objects far too massive to be neutron stars, making it likely that they are black holes.
9. What causes gamma ray bursts? No one knows exactly how gamma ray bursts are produced, but the energy required to make one suggests that gamma ray bursts occur when certain kinds of black holes are formed. At least some gamma ray bursts appear to come from unusually powerful supernova explosions that may create black holes. Others may come from the creation of black holes through mergers of neutron stars in close binary systems. 
10. What did Jocelyn Bell discover? She discovered the first radio pulsars with her thesis supervisor Antony Hewish.