A white dwarf is the remaining compact core of a low-mass star that has come to the end of its lifetime following a planetary nebula event. They are thought to make up roughly 6% of all known stars in the Sun's neighbourhood.
White dwarfs are made of highly compressed carbon and oxygen material, and are so dense that their mass is comparable to that of the Sun, even though their size is similar to that of the Earth. A matchbox of white dwarf material would weigh the same as fifteen elephants.
Newly created white dwarfs have some of the hottest surface temperatures of any star, at over 100,000°C, but because of their small size, they appear quite faint from a distance.
As nuclear reactions no longer occur in white dwarfs, they have to rely on their thermal store of energy for all heat and light. Over time this will gradually radiate away, allowing them to cool down and change colour. Eventually, they will disappear from sight to become cold black dwarfs.