White dwarfs are the glowing lumps of carbon left over after stars have used up all their fuel. They are hot, dense and small, typically with the mass of the Sun packed into the volume of the Earth. The structure of these objects is complex. Astronomers cannot see the glowing carbon embers because white dwarfs are always surrounded by a thin, dense layer of gas, drawn in by the star’s intense gravity. It is this gas that glows with an intense white light at temperatures usually between 8000K and 16,000K–by comparison the Sun’s atmosphere is about 6000K.