20. Why are white dwarfs hot?
a. They are fusing carbon into magnesium in their cores
b. They were the cores of stars that were fusing helium into carbon, and still have leftover heat from that fusion
c. They are gaining mass, and contracting due to degeneracy effects
d. They are heated by strong surface magnetic fields

Bonus Questions

B1. Why doesn't the sun explode like a hydrogen bomb?
a. any excess energy produced is turned into gravitational potential energy
b. the sun produces much less energy per second than a hydrogen bomb
c. neutrinos carry any excess energy out of the sun's interior, acting as a safety valve
d. any excess energy produced is immediately radiated away as gamma photons

B2. Why does the degenerate core of a red giant explode like a fusion bomb?
a. the core isn't fusing, so the red giant's envelope collapses in on it, causing a huge explosion
b. explosive helium fusion is easier to create than explosive hydrogen fusion
c. it gains mass from the red giant until it is too massive for electron degeneracy to prevent it from exploding
d. electron degeneracy pressure is independent of temperature, so when the temperature rises there is no thermal safety valve to prevent explosion