The Existence Of Which Of These Star Types Is Only, At Present, Hypothetical?
Answer: Black Dwarf
In astronomy, there is a broad class of stars known as dwarf stars, defined by their relatively small size and low level of luminosity. The term was coined at the start of the 20th century by Danish astronomer Ejnar Hertzsprung in relation to his observations of stars much brighter and much fainter than our Sun (which he called “giants” and “dwarfs”, respectively). Over the next century, the astronomy community built on his term by labeling different dwarf stars based on their characteristics.
White dwarfs like Sirius B (the tiny neighbor of the much brighter Sirius A seen here) are very dense, and emit light not by continued fusion, but by the stored thermal energy from the time when they were active stars. Brown dwarfs are large stars, smaller than our sun but bigger than the largest planets in our solar system. They aren’t massive enough to fuse hydrogen into helium (like our Sun), but still emit light, believed to be generated by the fusion of deuterium. Red dwarfs, the most common type of star in the Milky Way, but so dim as to not be visible to the naked eye on Earth, undergo fusion at such a slow pace that they have expected lifespans measured in the trillions of years.
If that bit about trillions of years has you contemplating the age of the universe, then good: because that brings us to the black dwarf, a star that has never been observed and currently exists only in scientific theory. White dwarfs are dying stars that are only emitting energy because they’re radiating the last of their heat into space, like a pot cooling down on a stove after the burner has been turned off. Eventually, a white dwarf will give up so much of its energy that the temperature of the star will drop to the same temperature as the cosmic microwave background (CMB), the thermal radiation left over from the birth of the universe, and become a black dwarf.
These black dwarfs are only theoretical at the moment, however, because it takes billions of years for a white dwarf to form, and billions more for it to cool down. Based on the estimated age of the universe and our observations of the coolest known white dwarf (which is still nearly 3,000K), we’ve got a long wait ahead of us to see if the black dwarf theory shakes out.