What is the composition of a white dwarf or neutron star and why does it still shine?
Answers
A white dwarf is an extremely dense small star. It is the remains of a star that has gone nova. The force of gravity is what creates it's high density (think mass of the sun compressed into the volume of the Earth). Essentially it is electrons and protons all packed together tightly with essentially no space between them.
A white dwarf shines not because it is generating any energy, but because the nova process has heated it up to such a high temperature. The "shine" is the heat slowly dissipating into space. This process is thought to take trillions of years and once a white dwarf has radiated all that heat away it will be a black dwarf.
A neutron star comes from a much bigger star than makes a white dwarf, but not a giant star. Due to the larger mass the force of gravity is greater and the particles are not just packed together but we actually have protons and electrons fusing to form neutrons. The density of these stars is much greater even than a white dwarf (think twice the mass of the sun in the volume about the same as the asteroid that wiped out the dinosaurs).
A neutron star also doesn't generate any new energy, so in theory it could also become a black dwarf. That being said, neutron stars can rotate extremely fast due to the law of conservation of angular motion. If you think of a spinning figure skater bringing their arms in causes them to speed up rotation, now apply that on a massive scale and you can end up with a rotating neutron star. In this situation it generates a vast magnetic field which causes it to emit radiation from the poles. Since it is spinning the radiation it emits comes out in pulses. This is a pulsar.
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