When a low-mass star runs out of fuel, it becomes a white dwarf. This is the bright, hot, dense core which remains after a planetary nebula event. About 6% of all known stars in our part of the Milky Way are white dwarfs.
White dwarfs are made of carbon and oxygen. This was created by the star by nuclear fusion during its main sequence and red giant phases. This material is compacted into a relatively small space, which makes white dwarfs very dense. Imagine the mass of the Sun, squashed to the size of the Earth. A matchbox of white dwarf material would weigh the same as fifteen elephants!
New white dwarfs have some of the hottest surface temperatures of any star. They can be over 100,000 °C! However, because they are small, they look quite faint from a distance.
White dwarfs do not release energy through nuclear fusion reactions. The light and heat they emit is just left over from previous stages of their evolution. Over time, this energy will gradually radiate away. Over many thousands of millions of years, they will stop glowing completely and become cold black dwarfs.