We received such a question, or rather a series of questions from one of our readers:
White dwarfs are formed during the death of stars, whose mass was not enough to form a neutron star. When a star burns up the reserves of hydrogen in its core, it contracts, increasing its temperature, until helium ignites in the core. When helium burns out, the process repeats and carbon begins to burn.
The cycle of contractions and expansions continues until the moment when the star cannot ignite the next chemical element, after which the star sheds the outer layers of matter and leaves in its place a bare core — a white dwarf.
The temperature of the dwarf at this moment can range from several tens to several hundred million degrees, and in a matter of seconds after the shell is dropped, it cools down to hundreds of thousands of degrees. The colder the dwarf gets, the slower it cools.
For several thousand years after its formation, the dwarf will cool down to only tens of thousands of degrees. The oldest white dwarfs, which have existed for about 13 billion years, have cooled to a temperature of just under 4,000 Kelvin. To cool down to 5 Kelvin, they will need about a quadrillion years more, that is, a hundred thousand times more time than has elapsed since the formation of the Universe.
The surface of a white dwarf is a smooth and almost perfect sphere that does not emit light in the visible range. Depending on the mass of the parent star, the white dwarf will consist of various chemical elements, but due to the enormous density, all of them will have to be in a metallized state. I find it difficult to describe the reflective properties of a black dwarf, it is a very specific topic and it is not easy to find data on it.