White dwarf stars are the small, dense remnants left behind when stars like the Sun reach the end of their lives. Although a typical white dwarf is only about the size of Earth, it can contain more than half the mass of the Sun, making it one of the densest forms of matter in the Universe.
White dwarfs no longer produce energy through sustained nuclear fusion. Instead, they gradually radiate away their remaining heat over billions of years. Along the way, they reveal some remarkable physics — from matter supported by quantum effects to interiors that can crystallize as they cool.
White Dwarfs Are the Remains of Sun-Like Stars
A white dwarf forms after a low- or intermediate-mass star exhausts the fuel in its core and reaches the final stages of its evolution. The star expands into a red giant and eventually sheds its outer layers into space, leaving behind the hot, compact core that becomes a white dwarf.
Most stars in the Milky Way are expected to end their lives as white dwarfs because they never become massive enough to undergo the core collapse that can produce a neutron star or black hole. A white dwarf is therefore not an ordinary faint star, but a stellar remnant — the exposed remains of a star whose normal fusion-powered life has ended.
They Are Earth-Sized but Almost as Massive as the Sun
White dwarfs are remarkably compact. A typical one is roughly the size of Earth but contains around 60% of the Sun’s mass, while some approach the mass of the Sun itself. Sirius B, the white dwarf companion of Sirius, is slightly smaller than Earth despite having about the same mass as the Sun.
Packing so much mass into such a small volume creates extraordinary densities. A teaspoon of typical white dwarf material would weigh several tons on Earth, while the gravity at its surface can be hundreds of thousands of times stronger than Earth’s.
Electron Pressure Stops White Dwarfs From Collapsing
With so much mass packed into such a small space, a white dwarf’s own gravity is constantly squeezing it inward. Yet the star does not simply collapse. As its electrons are forced closer and closer together, a quantum-mechanical effect called electron degeneracy pressure creates resistance to further compression. This provides the outward pressure needed to balance gravity, even though nuclear fusion has stopped.
The balance changes as a white dwarf becomes more massive. Adding mass strengthens its gravity, which squeezes the star more tightly before electron degeneracy pressure can balance it again. As a result, white dwarfs behave in a surprising way: the more massive they are, the smaller they become.
But there is a point beyond which this balancing act cannot continue. At roughly 1.4 times the mass of the Sun, known as the Chandrasekhar limit, electron degeneracy pressure can no longer support a stable white dwarf against gravity. A white dwarf therefore cannot simply keep gaining mass and becoming smaller indefinitely.
White Dwarfs Are Hot but Surprisingly Faint
A newly formed white dwarf can have a surface temperature of more than 100,000 K, making it far hotter than the Sun. Yet despite this extreme temperature, white dwarfs are relatively faint because they are so small. With only about an Earth-sized surface from which to radiate energy, they produce far less light than a much larger star.
Unlike an ordinary star, a white dwarf is no longer generating a steady supply of energy through nuclear fusion. Its glow comes mainly from heat left over from its earlier life, which slowly escapes into space. As that stored heat is lost, the white dwarf gradually becomes cooler and dimmer.
Their Interiors Can Eventually Crystallize
As a white dwarf cools, something remarkable begins to happen deep inside it. Most white dwarfs contain a dense core made largely of carbon and oxygen, and at sufficiently low temperatures these atomic nuclei can begin arranging themselves into an ordered structure rather than remaining in a more fluid state. In other words, part of the star begins to crystallize.
This does not mean the white dwarf turns into an enormous diamond. Its interior remains an extraordinarily dense form of matter unlike anything encountered under everyday conditions. Crystallization also releases energy, slowing the white dwarf’s cooling and giving astronomers another way to understand the ages and internal structure of these ancient stellar remnants.
White Dwarfs Can Produce Supernovae
A white dwarf normally spends the rest of its existence slowly cooling, but life in a binary system can dramatically change its fate. If it pulls gas from a close companion star, the material can accumulate on its surface until runaway nuclear fusion produces a nova. The eruption blasts much of this material into space without destroying the white dwarf, allowing the process to happen again if more gas accumulates.
Under different circumstances, however, interaction with a companion — or the merger of two white dwarfs — can lead to a Type Ia supernova, a runaway thermonuclear explosion that destroys the white dwarf itself. Remarkably, this means some white dwarfs can produce a supernova even though the stars they came from were not massive enough to explode as supernovae during their normal lives. Instead of the gravitational core collapse of a massive star, the explosion results from the thermonuclear destruction of the white dwarf remnant.
White Dwarfs Slowly Cool Toward Black Dwarfs
For most white dwarfs, the future is much quieter. With no sustained nuclear fusion replacing the energy they radiate into space, they continue cooling and fading for an almost unimaginable length of time. As their stored heat gradually escapes, they become colder and increasingly difficult to detect.
Given enough time, a white dwarf should eventually cool until it no longer gives off significant visible light, becoming a theoretical object known as a black dwarf. But no black dwarfs are expected to exist yet. Even the oldest white dwarfs have been cooling for only billions of years, while the Universe itself is just 13.8 billion years old — nowhere near long enough for any of them to have reached this final state.
The Sun Will Eventually Become a White Dwarf
Our own Sun will eventually follow the same path. In about 5 billion years, it will exhaust the hydrogen in its core and expand into a red giant. Later, it will shed its outer layers into space, leaving behind its hot, dense core as a white dwarf. The remnant will contain much of the Sun’s remaining mass but be compressed to roughly the size of Earth. With nuclear fusion over, it will then begin an extraordinarily long cooling process, gradually becoming colder and fainter over billions of years.
The Sun is not massive enough to end its normal life in a core-collapse supernova. Its future is much quieter: red giant, white dwarf, and an almost unimaginably long decline into darkness.