White dwarf

A white dwarf is a very dense type of star. Imagine a star that fits into a space about the size of the Earth, but it has as much mass as our Sun! Unlike normal stars that shine because of nuclear fusion, a white dwarf glows because of the leftover heat from its past.

Many stars, including ones like our Sun, will eventually become white dwarfs after they finish burning their fuel. They cannot become denser objects like a neutron star or a black hole because they don’t 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 up only because of something called electron degeneracy pressure. This makes them extremely dense. Even though they start very hot, white dwarfs slowly cool down over a very long time, radiating their energy into space.

History

Discovery

See also: List of white dwarfs

The first white dwarf was found in the triple star system of 40 Eridani, which 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 realized that 40 Eridani B, though dim, 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 over time, 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 calculated 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 puzzled astronomers 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 is thought to contain about ten billion white dwarfs. Among the one hundred star systems nearest the Sun, there are eight white dwarfs. The closest and brightest known white dwarf is Sirius B, which is part of the Sirius binary star system and is located 8.6 light years away.

In the future, the number of white dwarfs is expected to increase. Stars with masses from about 0.07 to 10 times that of the Sun, which make up over 97% of the stars in the Milky Way, will eventually become white dwarfs.

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 once thought that some white dwarfs might shine a little brighter and then dimmer every about 10 seconds, but they couldn’t see this happening when they looked in the 1960s. The first white dwarf that was seen to change its brightness was called HL Tau 76. It was noticed to get brighter and dimmer about every 12.5 minutes in 1965 and 1966. This happens because of gentle pulses inside the star, similar to 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 by 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. These tiny white dwarfs have helium cores. Over a very long time, much longer than the age of the universe, such a star might burn its hydrogen and become a special type called a blue dwarf, ending as a helium white dwarf. However, most of these small white dwarfs are thought to come from losing material in binary 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 formed, a white dwarf is stable and will continue to cool slowly over a very long time. The oldest white dwarfs we know of still give off heat, which helps us understand how old the universe might be. In the far future, if the universe keeps expanding, most galaxies will be made up of white dwarfs, along with other dim objects like brown dwarfs, neutron stars, and black holes.

A white dwarf can exist for an incredibly long time—perhaps as long as the life of a proton, which is estimated to be at least 10³⁴ to 10³⁵ years. If protons eventually break down, a white dwarf would slowly lose mass and eventually disappear after about 10³⁸ years. Sometimes, a white dwarf can lose mass to a nearby star and transform into 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 keep remnants of the planets and other small bodies that once orbited its parent star. One way scientists find these remnants is by noticing metals in the white dwarf's atmosphere. These metals, which shouldn't be there, often come from rocky bodies that broke apart and fell onto the white dwarf.

Another clue is finding extra heat in infrared light around a white dwarf, which can mean there is a ring of dust there. This dust comes from rocky bodies that broke apart near the white dwarf. Only a few white dwarfs have giant planets or smaller planets still orbiting them. Scientists think that many white dwarfs might have had planets or asteroids that broke apart and fell onto them, leaving traces of 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 about as far from the star as 0.005 to 0.02 AU, which is very close. Because the star is small, like a planet, 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 without losing too much heat and becoming lifeless.

Binary stars and novae

When a white dwarf is part of a binary star system and pulls matter from its companion star, interesting events can happen. This can lead to explosions called novae or even more powerful explosions known as Type Ia supernovae. In some cases, the white dwarf can become a strong source of X-ray energy if it pulls material fast enough to keep fusion reactions going on its surface.

Close pairs of white dwarfs can lose energy and spiral toward each other, eventually merging due to the release of energy in the form of gravitational waves. This process can sometimes lead to a Type Ia supernova, where the white dwarf explodes. Scientists study these systems to understand how stars evolve and how such explosions occur.

Nearest white dwarfs

White dwarfs are nearby stars that have stopped shining through nuclear fusion but still glow because of their leftover heat. They are very small and very heavy, packing a lot of mass into a space about the size of Earth. Scientists have identified several white dwarfs that are close to our solar system, making them important for studying these fascinating objects.