Most stars don't go out with a bang. They spend billions of years fusing hydrogen into helium, swell into red giants as their fuel runs low, shed their outer layers into space, and leave behind a small, dense, slowly cooling remnant called a white dwarf. It's the most common fate for stars in the universe -- and it's what our own Sun will eventually become.
How a White Dwarf Forms
When a star similar in mass to the Sun exhausts the hydrogen in its core, it expands into a red giant. During that phase, the outer layers of the star are only loosely bound and begin drifting away over thousands of years, forming an expanding shell of gas called a planetary nebula. What's left at the center, once the gas has dispersed, is the exposed core of the original star -- the white dwarf.
The process strips the star down to its essence. What remains is an object roughly the mass of the Sun compressed into a sphere about the size of Earth. It produces no new energy. It generates no fusion. It simply radiates away the heat it has stored, cooling over billions of years.
What White Dwarfs Are Made Of
White dwarfs are composed primarily of carbon and oxygen -- the ash left over from the nuclear fusion that powered the star during its lifetime. The material is extraordinarily dense. A teaspoon of white dwarf material would weigh roughly five tons on Earth.
That density is maintained by a quantum mechanical effect called electron degeneracy pressure -- the resistance of electrons to being compressed into the same space. Unlike a main sequence star, which is held up by the outward pressure of fusion, a white dwarf is held up by a purely quantum mechanical force. It will never collapse further under its own gravity, no matter how much it cools.
How Hot and Bright Are They
Newly formed white dwarfs are intensely hot -- surface temperatures can reach 100,000 Kelvin or more, compared to the Sun's roughly 5,800 Kelvin. That heat gives them a blue-white color and makes them genuinely luminous despite their small size. As they age, they cool and dim, shifting from blue-white to yellow-white to orange over billions of years.
The end state of a sufficiently old white dwarf -- one that has cooled completely over hundreds of billions of years -- would be a black dwarf: a cold, dark, inert sphere of carbon and oxygen. The universe isn't old enough for any black dwarfs to exist yet. Every white dwarf that has ever formed is still warm.
Famous White Dwarfs
The most well-known white dwarf visible from Earth is Sirius B -- the companion to Sirius, the brightest star in the night sky. Sirius appears as a single brilliant point of light, but it's actually a binary system. Sirius B, orbiting close by, is a white dwarf with roughly the mass of the Sun compressed into a sphere about the size of Earth. It's too faint to see without a telescope, but it was the first white dwarf ever identified, confirmed in 1915.
Procyon, another bright nearby star, also has a white dwarf companion -- Procyon B -- discovered later that same year. These nearby systems gave astronomers their first real look at what stellar remnants actually look like.
What Happens When a White Dwarf Has a Companion
A white dwarf in a binary system can have a second life. If it orbits close enough to a companion star, its gravity can pull material from that companion onto its surface. As hydrogen accumulates and heats up, it can ignite in a sudden thermonuclear explosion visible across the galaxy -- a nova. The white dwarf survives, the accumulated material burns off, and the process can repeat.
If the white dwarf accumulates enough total mass -- approaching about 1.4 times the mass of the Sun -- it can trigger a far more violent event: a Type Ia supernova. These explosions are so consistent in their brightness that astronomers use them as standard candles to measure distances across the universe. Some of what we know about the expansion of the universe comes from watching white dwarfs explode.
The Sun's Future
In about five billion years, the Sun will exhaust its core hydrogen and begin its red giant phase, expanding to swallow the inner planets. Eventually it will shed its outer layers into a planetary nebula and leave behind a white dwarf -- a cooling Earth-sized sphere of carbon and oxygen, the compressed remains of the star that made life on this planet possible.
That white dwarf will outlast almost everything. It will still be cooling, slowly, long after the Milky Way and Andromeda galaxies have merged, long after most stars have burned out, long after any record of anything that happened on Earth has faded entirely.