White Dwarf
A white dwarf is the final stage in the life cycle of most stars, including our Sun. After exhausting its nuclear fuel and ejecting its outer layers as a planetary nebula, theremaining core contracts into a compact object about the size of Earth, but with a mass close to that of the Sun.
Structure and Properties
A white dwarf is essentially a crystal of carbon and oxygen, held againstgravitational collapse by electron degeneracy pressure — a quantum property that prevents two electrons from occupying the same state. Its density is enormous: a coffee spoonful of white dwarf matter would weigh about one metric ton. It no longer produces energy through fusion, but radiates the heat accumulated over billions of years.
A very slow cooling process
White dwarfs cool very slowly, over tens of billions of years. The coldest known ones have surface temperatures below 4,000 K. It is theorized that black dwarfs exist— completely cooled-down white dwarfs — but the universe is not yet old enough for them to have formed.
Type Ia supernovae
In a binary system, a white dwarf can accrete matter from a companion star. If its mass exceeds the Chandrasekhar limit (1.44 solar masses), the degeneracy pressure can no longer support it: it explodes as a Type Ia supernova. These explosions, whose luminosity can be predicted, are used as standard candles to measure cosmic distances.
Did you know?
The first white dwarf discovered was Sirius B, the companion of Sirius. In 1915, Walter Adams measured its temperature and luminosity and realized that it must have an unimaginable density. This discovery was one of the first observational confirmations of electron degeneracy.
Frequently Asked Questions
Will the Sun become a white dwarf?
Yes. In about 5 billion years, the Sun will swell into a red giant, eject its outer layers, and itsresidual core will contract into a white dwarf with a mass of about 0.6 solar masses, roughly the size of Earth.
What is the Chandrasekhar limit?
It is the maximum mass a white dwarf can have without collapsing, which is about 1.44 solar masses. Beyond that, electron degeneracy pressure is no longer sufficient, and the star either explodes as a supernova or collapses into a neutron star.
How long does it take for a white dwarf to cool down?
Tens of billions of years. No white dwarf has yet had time to cool completely since the formation of the universe. The coldest ones observed have surface temperatures of a few thousand kelvin.