White dwarf

A white dwarf has a similar diameter with Mars, but half the mass of our sun, Sol. Found with a high frequency in the galaxy, these objects can host a planet. A Habitable Zone can exist.

The star
White dwarfs are remnants of dead stars. This is what will happen to Sol in the far future. When a star becomes a red giant, heavy elements accumulate in its core. The greater the star, the heavier the elements. M - type stars are unable to fuse helium and they end-up as helium white dwarfs. K - type stars are able to pass over helium, but they will get stuck at carbon. Larger, G - type stars, F - type stars and A - type stars will leave behind a white dwarf with nitrogen, oxygen, neon or slightly heavier elements. B - type stars usually are massive enough to continue the fusion cycle to iron. The enormous force developed by a red giant or a small supernova compresses the nucleus of the star so hard, that it suffers a massive implosion. As a result, matter gets so compressed, that atoms become heavily squeezed. Majority of white dwarfs have 50% of the mass of Sol, but only the diameter of Mars! The highest mass of a white dwarf is 1.4 times the mass of Sol. Above this limit, they will degenerate into a neutron star.

As formed, a white dwarf is very hot, with its surface close to 100 000 K. Nuclear reactions no longer occur, so it will slowly cool. At such small size, its gravitational force is huge. On its surface, matter can fall easily (this includes solar wind from a nearby star, tiny asteroids or matter from interstellar environment). This matter generates huge energy while falling, resulting X - rays. Sirius B, the most studied white dwarf, emits more X - rays then its larger companion, Sirius A. As colliding matter accumulates, it gets compressed on the surface. At some point, pressure can be high enough to ignite a nuclear fusion involving the collapsed hydrogen. At that point, deposited matter explodes, creating a nova. However, in case of carbon white dwarfs, since the point of nuclear ignition of carbon is lower, a nova might end into a supernova explosion.

Since white dwarfs are not heated by nuclear fusion, they glow as long as they are hot enough. given their small surface, it will take a lot of time until they will cool enough. In the end, there will result a cold body, named black dwarf.

Usually, white dwarfs rotate very fast (even faster then a second), generating giant magnetic fields. This can protect their planets from solar winds (from other stars) and interstellar environment.