Chapter 17 Responses
"What is the difference between electron degeneracy pressure and neutron degeneracy pressure? Which type supports a white dwarf? Why are neutron stars so much smaller in size than white dwarfs?"
Electron degeneracy pressure is pressure that comes from densely packed electrons that can't condense further whereas with neutron degeneracy pressure the pressure comes from densely packed neutrons. White dwarfs are supported by electron degeneracy pressure. Neutron stars are more massive than white dwarfs so their cores collapse to be smaller.
"With degeneracy pressure, how are the speeds of electrons (or neutrons) related to the mass of the degenerate object? Explain why this relationship between speed and mass implied upper limits on the possible masses of white dwarfs and neutron stars. What is the white dwarf limit? What is the neutron star limit?"
The more massive the object the faster the speed of electrons or neutrons. White dwarfs can't be more massive than 1.4 solar mass ("white dwarf limit") because anything higher and the electrons would need to travel faster than the speed of light. Since that isn't possible, at this point the electron degeneracy pressure loses the fight against gravity and the star collapses into a neutron star. The neutron star limit is less than 3 solar masses. Anything greater and neutron degeneracy pressure fails, and the collapse creates a black hole.
"What is an accretion disk? Under what conditions does an accretion disk form? Explain how the accretion disk provides a white dwarf with a new source of energy that we can detect from Earth."
An accretion disk is "a rapidly rotating disk of material that gradually falls inward as it orbits a starlike object" (CP Glossary). Accretion disks can form in close binary systems when one star is a white dwarf or neutron star and the other is a main sequence or giant. The stars must be close enough for gravity to encourage the transfer of gas between them. Clumps of gas from the main sequence or giant star fall toward the white dwarf/neutron star and form a disk around it. Slower gases rub against faster gases in the accretion disks which causes friction-generated heat. The disk becomes hot enough for us to detect its ultraviolet or X-ray radiation.
"Describe the process of a nova. Why do novae only occur in close binary star systems? Can the same star system undergo a nova event more than once? Explain."
When accretion takes place and gas is added to white dwarf, the star once again has a hydrogen shell. (Hydrogen because the gas is coming from the upper layers of the other star.) If enough accretion takes place the temperature and pressure will increase to the point where fusion will begin in the shell. During the time when the star is burning brightly because of this process it is a nova.
Novae only take place in close binary star systems because a) there needs to be another star present to contribute the gas to the white dwarf and b) the two stars need to be close enough so there is sufficient gravity to cause the transfer to take place. The same star system can keep experiencing nova events as long as the gravity is there and there is gas to be shared.
"Contrast the process of a white dwarf supernova with that of a massive star supernova. Observationally, how can we distinguish between these two types of supernovae?"
A white dwarf supernova takes place after the star has already "died," while a massive star's supernova is part of its dying process. White dwarf supernovae take place when through accretion the white dwarf reaches the 1.4 solar mass limit. Degeneracy pressure isn't able to stop gravity so the star collapses, experiences carbon fusion and flash, and explodes completely. Not all white dwarfs go through a supernova process but all massive stars eventually do. The supernovae of massive stars occur when the core collapses at the end of its life and the outer layers are ejected, leaving behind a neutron star or black hole depending upon the remaining mass of the core. White dwarf supernovae don't leave behind anything.
We can distinguish between the two types of supernovae by observing their light. White dwarf supernovae don't have hydrogen, massive star supernovae do. White dwarf supernovae fade steadily and massive star supernovae fade in two stages. Also, all white dwarf supernovae have nearly identical light curves and maximum luminosities because white dwarf supernovae always occur at the same mass.
"What is a pulsar? Briefly describe how pulsars were discovered and how they get their name. How do we know that pulsars must also be neutron stars?"
Pulsars are neutron stars that direct beams of radiation along the magnetic poles. They get their name because the beams are like a lighthouse beam, sweeping across the field of view as they quickly rotate, giving a pulsing effect. Pulsars were discovered in 1967 when Jocelyn Bell at Cambridge University noticed that regular pulses of radio waves were coming from the Cygnus constellation. Pulsars are known to be neutron stars because only neutron stars are small and massive enough to spin so quickly. Also, pulsars have been found at the center of supernovae explosions.
"Explain why neutron stars in close binary systems are often called X-ray binaries."
Neutron stars in close binary systems are often called X-ray binaries because there is so much gravitational energy in a neutron star's accretion disk that the X-ray emissions are very intense.
"Briefly explain the process by which the core of a very high mass star can collapse to form a black hole."
Black holes occur when the collapsing core of a high mass star is greater than three solar masses. The boosted temperature and energy from the star's collapse aren't strong enough to fight off gravity. Instead they act as additional mass and help the core to collapse without end.
"What is the event horizon of a black hole? How does it get its name? How is it related to the Schwarzschild radius?"
The event horizon of a black hole is the boundary between the inside of the black hole, where nothing can be seen because the escape velocity is greater than the speed of light, and the outside universe where we are. It gets its name because information about events occurring inside the black hole can't get past this point to reach us. The Schwarzschild radius is the radius of the event horizon. The radius depends only on the black hole's mass, and is measured according to the radius the event horizon would have if it were flat.
"What do we mean by the singularity of a black hole? How do we know that our current theories are inadequate to explain what happens at the singularity?"
The singularity of a black hole is the center of the black hole where gravity has crushed all matter to an infinitely tiny and dense point. We know our current theories are inadequate to explain what happens at the singularity because we can't receive any information from within the event horizon and theoretical predictions conflict with one another. General relativity says that spacetime should grow infinitely curved at the singularity while quantum physics says spacetime should fluctuate chaotically.
"Briefly describe the observational evidence supporting the idea that Cygnus X-1 contains a black hole."
Cygnus X-1 seems to consist of an 18 solar mass star orbiting an unseen 10 solar mass star. The latter star is over the 3 solar mass limit of neutron stars, thus qualifying to be a black hole. Also, variations in the system's X-ray emission suggests that the 10 solar mass star is too small to be an ordinary star.
"What are gamma-ray bursts? Why are they so mysterious?"
Gamma ray bursts are the most powerful bursts of energy in the universe. Their cause is unknown, but they may come from the collision of two neutron stars or, for a more popular theory, they may come from extremely powerful supernovae resulting in black holes, called hypernovae. Gamma ray bursts are mysterious because these theories remain unsuccessfully proven so, despite being the most powerful events in the universe, we don't know what causes them.
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