A white dwarf is the exposed core left behind when a star like the Sun runs out of fuel and sheds its outer layers. Roughly 97 percent of stars in the Milky Way, including the Sun, are expected to end as white dwarfs.
Small, dense and hot
A typical white dwarf has about half to two-thirds the Sun’s mass squeezed into a body about the size of Earth. A teaspoon of its material would weigh several tons. It no longer produces energy by fusion. It simply shines from stored heat and cools very slowly over billions of years.
White dwarfs cannot exceed about 1.4 solar masses, the Chandrasekhar limit, named after Subrahmanyan Chandrasekhar. Beyond it, electron pressure can no longer support the star. A white dwarf pushed over the limit in a binary system can explode as a Type Ia supernova, see what a supernova is.
Spectral types
White dwarfs are classified by what their spectrum shows on the surface, using the letter D (for degenerate) followed by another letter:
- DA: hydrogen-rich atmosphere, the most common type
- DB: helium lines
- DO: hot, ionized helium
- DQ: carbon features
- DZ: metal lines
- DC: no strong lines
Their heavy gravity makes heavy elements sink quickly, so the surface usually shows only the lightest element present.
Special kinds
- Polluted white dwarfs: metals in the atmosphere reveal that the star recently swallowed asteroids or planetary debris. They give astronomers a way to study the composition of rocky bodies around other stars.
- Magnetic white dwarfs with very strong fields.
- Pulsating white dwarfs whose brightness varies, used to probe their interiors.
- White dwarfs in binaries, including cataclysmic variables and novae, where material from a companion flares up on the surface.
Famous examples and the Sun’s future
Sirius B, the faint companion of the brightest star in the night sky, was the first white dwarf identified. In about 5 billion years, the Sun will swell into a red giant and then leave a white dwarf behind, see the life cycle of stars. Over enormous times, white dwarfs should cool into cold “black dwarfs”, but the universe is not yet old enough for any to exist, see how the universe might end.
