White dwarf

A white dwarf is a very dense type of star. Picture a star that could fit inside a space about the size of the Earth, but it holds as much mass as our Sun! Normal stars shine because of nuclear fusion, but a white dwarf glows because of the leftover heat from long ago.

Many stars, including ones like our Sun, will become white dwarfs after they run out of fuel. They cannot turn into denser objects like a neutron star or a black hole because they do not have enough mass. The closest known white dwarf to us is Sirius B, which is part of the bright binary star system called Sirius.

White dwarfs are made of highly compressed form of matter. Once they form, they stop all fusion reactions and stay standing because of something called electron degeneracy pressure. This makes them extremely dense. Even though they start very hot, white dwarfs cool down slowly over a very long time, sending their energy out into space.

History

Discovery

See also: List of white dwarfs

The first white dwarf was found in the triple star system of 40 Eridani. This system has a bright star called 40 Eridani A and a closer pair of stars, 40 Eridani B (a white dwarf) and 40 Eridani C (a smaller star). Astronomer William Herschel discovered this pair in 1783. In 1910, scientists learned that 40 Eridani B was a very hot white star. This was surprising at the time.

Another famous white dwarf is the companion to the bright star Sirius, called Sirius B. In 1844, astronomer Friedrich Bessel predicted that Sirius had an unseen companion. In 1862, the companion was finally observed. More white dwarfs were found later, and today many thousands are known.

Theory development

Scientists were puzzled by how white dwarfs could be so dense. By studying their movements and light, they found that some white dwarfs had a mass similar to our Sun but were squeezed into a space about the size of Earth. This extreme density was puzzling at first.

Later, scientists realized that white dwarfs are made of a special kind of matter where atoms are broken apart. This allows the stars to be very dense without collapsing further. A maximum mass, called the Chandrasekhar limit, was calculated, showing that a white dwarf cannot be heavier than about 1.4 times the mass of the Sun without changing into a different kind of star.

Occurrence

The Milky Way galaxy has many white dwarfs. We think there are about ten billion of them.

Among the stars closest to the Sun, eight are white dwarfs. The closest and brightest white dwarf is Sirius B. It is part of the Sirius binary star system and is 8.6 light years away.

In the future, more stars will become white dwarfs. Stars that are about 0.07 to 10 times the size of the Sun will become white dwarfs. These stars make up over 97% of the stars in the Milky Way.

Composition and structure

White dwarfs are very dense stars. They have a mass similar to the Sun but fit into a space about the size of Earth. Unlike normal stars, white dwarfs shine because of leftover heat, not because of nuclear reactions.

These stars form when a star like the Sun runs out of fuel and collapses. The material gets squeezed very tightly, making white dwarfs one of the densest types of matter we know. Their interiors are made of tightly packed atoms, mostly carbon and oxygen.

Variability

Main article: Pulsating white dwarf

See also: Cataclysmic variables

Scientists thought that some white dwarfs might get a little brighter and then dimmer every about 10 seconds. But they didn’t see this happen when they looked in the 1960s. The first white dwarf seen to change its brightness was called HL Tau 76. It got brighter and dimmer about every 12.5 minutes in 1965 and 1966. This happens because of gentle pulses inside the star, like how some stars shake a little. There are different kinds of these pulsing white dwarfs, such as ZZ Ceti stars, which have lots of hydrogen, and V777 Her stars, which have more helium. Another group, called GW Vir stars, are stars that are just about to become white dwarfs. All these stars change their brightness just a little bit, which helps scientists learn more about what’s inside white dwarfs.

Formation

After the hydrogen-fusing period of a main-sequence star of low or intermediate mass ends, the star expands to a red giant and fuses helium to carbon and oxygen in its core. If the red giant does not have enough mass to create the very high temperatures needed to fuse carbon, a core made of carbon and oxygen builds up. After the star loses its outer layers and forms a planetary nebula, the leftover core becomes a white dwarf. A white dwarf shines because of its leftover heat, not because of nuclear fusion.

Very small white dwarfs, with less than 25% of a solar mass, are usually found in pairs of stars called binary star systems. For stars with masses between 0.5 and 8 times that of the Sun, their cores become hot enough to fuse helium into carbon and oxygen, but not hot enough to fuse heavier elements. Near the end of their lives, these stars expel their outer layers to form a planetary nebula, leaving behind a carbon-oxygen core which becomes a white dwarf.

Fate

Further information: Black dwarf

Once a white dwarf forms, it stays stable. It will cool very slowly over a long time. The oldest white dwarfs still give off heat. This heat helps us learn about the age of the universe.

In the far future, if the universe keeps expanding, most galaxies will be made up of white dwarfs. Other dim objects will also be there, like brown dwarfs, neutron stars, and black holes.

A white dwarf can last for an incredibly long time—maybe as long as a proton. Protons are thought to last at least 10³⁴ to 10³⁵ years. If protons ever break down, a white dwarf would slowly lose mass and vanish after about 10³⁸ years. Sometimes, a white dwarf can lose mass to a star nearby. It might then become something like a helium planet or a diamond planet orbiting that star.

Debris disks and planets

See also: List of exoplanets and planetary debris around white dwarfs

A white dwarf can hold pieces of the planets and small objects that once moved around its parent star. Scientists find these pieces by seeing metals in the white dwarf's atmosphere. These metals should not be there, and they often come from rocky objects that broke apart and fell onto the white dwarf.

Another way to find clues is by seeing extra heat in infrared light around a white dwarf. This extra heat can mean there is a ring of dust there. This dust comes from rocky objects that broke apart near the white dwarf. Only a few white dwarfs still have giant planets or smaller planets orbiting them. Scientists believe many white dwarfs may have had planets or asteroids that broke apart and fell onto them, leaving metals in their atmospheres.

Habitability

Scientists have wondered if tiny, Earth-like planets could orbit close to white dwarfs — stars that have cooled down — and still be places where life might exist. These planets would need to stay very close to the star, about as far as 0.005 to 0.02 AU. Because the star is small, such planets would always face the same way, like the Moon does to Earth.

However, newer studies suggest this might not work. The strong pull of the star could make the planets too hot, like a runaway greenhouse, making them unable to support life. Also, it is unclear how such planets could end up so close to their stars.

Binary stars and novae

When a white dwarf is part of a pair of stars, it can pull material from the other star. This can cause bright explosions called novae or even stronger explosions known as Type Ia supernovae.

Sometimes, two white dwarfs can get very close and merge together. This can also lead to a Type Ia supernova, where the white dwarf explodes. Scientists study these systems to learn more about how stars change and how these explosions happen.

Nearest white dwarfs

White dwarfs are stars that have stopped shining because their nuclear reactions have ended. They still glow because of the heat they have left. These stars are very small but very heavy, holding a lot of matter in a space about the size of Earth. Scientists have found several white dwarfs close to our solar system. Studying them helps us learn more about these interesting objects.