Among the many bright stars visible in the winter sky, two are particularly worthy of our attention. They are also among the nearest, with Sirius at 2.7 parsec and Procyon at 3.5 parsec. They are interesting because both are double stars and each has a white dwarf as a companion.
Stars become white dwarfs at the ends of their active careers, when their fuel is used up.
They are very small and dense and derive the small amount of energy which they are still able to radiate from slow gravitational contraction. The rules of quantum mechanics require that, when matter is very densely packed, the particles must have high velocities, which means that it must be very hot. But this energy is not available, because if it were radiated away, the particles would have to slow down, which is not allowed. Further contraction requires even more heat energy, so, except for the more massive stars, the contraction must stop at some point, and most white dwarfs are nearing this point. If we suppose that both stars of each pair were formed at about the same time, then the stars which are now white dwarfs evolved more quickly, which means that they were originally the more massive of the pair.