White dwarf facts for kids

A white dwarf is a very small and dense star. Imagine taking something as heavy as our Sun and squishing it down to the size of the Earth! That's how dense a white dwarf is. This happens because gravity pulls all the atoms in the star very close together, and they even lose their electrons.

White dwarfs are what most stars become after they run out of fuel. Over 97% of the stars in our Milky Way galaxy will end up as white dwarfs. Our own Sun will become one too!

Here's how it works:

• A star like our Sun spends most of its life burning hydrogen into helium in its core. This is called a main-sequence star.
• When the hydrogen fuel runs low, the star expands into a red giant. During this stage, it starts burning helium into carbon and oxygen.
• If the star isn't big enough to burn carbon, it will eventually shed its outer layers. These layers form a beautiful cloud called a planetary nebula.
• What's left behind is the hot, dense core – the white dwarf.

A white dwarf doesn't create new energy through nuclear fusion anymore. It's like a hot ember slowly cooling down. It shines because of the heat it still has stored inside.

How White Dwarfs Were Found

Scientists first discovered white dwarfs a long time ago, in the 1700s.

• The first one, called 40 Eridani B, was found on January 31, 1783, by William Herschel. It's part of a system with three stars.
• The second white dwarf was found in 1862. It was a small star near the bright star Sirius. This companion star is called Sirius B. Even though Sirius B was very hot (about 25,000 degrees Celsius), it was much fainter than its partner, Sirius A. Scientists realized that Sirius B had almost the same mass as our Sun, but it was incredibly tiny.
• In 1917, another white dwarf named Van Maanen 2 was discovered. It's one of the closest white dwarfs to Earth, besides Sirius B.

Heat and Light of White Dwarfs

When a white dwarf first forms, it's extremely hot. Its center can be millions of degrees Celsius, and its surface is still thousands of degrees.

• Because it has no new energy source, a white dwarf slowly loses its heat and cools down over billions of years.
• At first, they glow with a blue or white light. As they cool, they become dimmer and their light fades.
• White dwarfs cool very slowly for two reasons:

* They have a tiny surface area to release heat from. * The material inside them is so dense that heat and light can't easily escape.

Eventually, after an incredibly long time, a white dwarf will cool down completely and stop giving off light. When this happens, it's called a black dwarf. However, the universe isn't old enough for any black dwarfs to exist yet!

When White Dwarfs Explode

Sometimes, a white dwarf can "re-ignite" and explode as a supernova.

• There's a limit to how much mass a white dwarf can have before it becomes unstable. This is called the Chandrasekhar limit.
• If a white dwarf is orbiting another star, it might pull material from that star. If it gathers enough extra mass to go over the Chandrasekhar limit, the carbon inside it can suddenly start fusing again.
• Astronomers believe this kind of explosion creates a very powerful type of supernova called a Type Ia supernova.

Images for kids

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  Image of Sirius A and Sirius B taken by the Hubble Space Telescope. Sirius B, which is a white dwarf, can be seen as a faint point of light to the lower left of the much brighter Sirius A.

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  A comparison between the white dwarf IK Pegasi B (center), its A-class companion IK Pegasi A (left) and the Sun (right). This white dwarf has a surface temperature of 35,500 K.

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  The white dwarf cooling sequence seen by ESA's Gaia mission

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  Artist's impression of debris around a white dwarf

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  The merger process of two co-orbiting white dwarfs produces gravitational waves

See also

In Spanish: Enana blanca para niños