White dwarfs are dense, Earth-sized remnants of stars that have exhausted their nuclear fuel and have collapsed under their own gravity. They are composed mostly of carbon and oxygen, with a thin layer of hydrogen or helium on their surface. White dwarfs are incredibly hot when they first form, but they gradually cool down over billions of years, eventually becoming cold, dark objects known as black dwarfs. White dwarfs are important objects of study in astrophysics because they represent the final stage of evolution for most stars, including our Sun. By studying white dwarfs, astronomers can learn more about the processes of stellar evolution, nuclear fusion, and the fate of planetary systems around dying stars. White dwarfs are also used as standard candles in cosmology to measure distances to other galaxies and determine the age of the universe.