What is a white dwarf?
A white dwarf is the hot, dense core left behind when a star like the Sun dies. It is about the size of Earth but contains roughly as much mass as the Sun.
After a Sun-like star becomes a red giant and sheds its outer layers, its exposed core remains. This core no longer produces energy through fusion, but it is extremely hot, glowing white from stored heat. A teaspoon of white dwarf material would weigh several tonnes on Earth.
Over billions of years, a white dwarf slowly cools and fades. Scientists predict it will eventually become a cold, dark object called a black dwarf, but the universe is not yet old enough for any black dwarfs to exist. About 97 percent of the stars in the Milky Way will end their lives as white dwarfs.
A famous example
The closest known white dwarf is Sirius B, a small companion to Sirius, the brightest star in the night sky. If a white dwarf in a close pair of stars pulls in too much material from its partner, it can explode as a Type Ia supernova.
Did You Know?
Indian-American astrophysicist Subrahmanyan Chandrasekhar calculated that white dwarfs cannot be more than about 1.4 times the mass of the Sun, a value called the Chandrasekhar limit.