A white dwarf is made from a medium or small-sized star (any star less than nine or ten times the size of the sun) when it uses all of its fuel. After the star uses its fuel, it is a very tight ball of gasses. Then, it gets colder and colder, until it stops making light and heat, when it becomes a black dwarf. Scientists are not sure how long this takes. Some think it takes 1,000,000 billion years, but others think it could be longer. Scientists do not believe that black dwarf stars can exist right now because the universe is only 13.8 billion years old.