White dwarf stars are the form that an average or small star, meaning that it has a mass of >8M, takes at the end of their lifetime. Which happens when they have used up all their nuclear fuel. 1M = 1 solar mass, is equal to the mass of our closest star, our sun. More specific 1,9891*10^30 kg.

White dwarf stars are one of the most dense forms of matter in the Universe. The only forms of matter that have an even higher density are neutron stars and black holes, which form at the end of a massive star's life cycle. White dwarfs have a density of 1 x 109 kg/m3. For reference, the density of earth is 5.4 x 103 kg/m3.

We can use our knowledge of white dwarfs as a contributing factor to calculate the age of the universe. We can do that by estimating how long it takes for a white dwarf star to turn into a black dwarf. By doing that we would get a lower limit on the age of the Universe and our galaxy, the milky way. But according to theory, the Universe is too young for white dwarfs to have had time to cool since the process of a white dwarf turning into a black dwarf takes billions of years.