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  • Writer's pictureSven Piper

White Dwarfs

Updated: 6 days ago


Artist's impression of two white dwarfs merging
Artist's impression of two white dwarfs merging (Credit: ESO/L. Calçada)

A white dwarf is formed when a star with a maximum of 1.4 solar masses dies. It consists of the hot core of the star, which has previously expanded into a red giant and in the final stage, when helium is burned inside, has stripped off its outer shell to form a planetary nebula.


Most stars, including the Sun, will eventually become white dwarfs after they exhaust their nuclear fuel and shed their outer layers. In addition, most white dwarfs are only the size of the Earth, but have a mass of about 0.2 to 1.4 solar masses, with most falling within a narrow range of about 0.5 to 0.7 solar masses. But a sugar-cube-sized amount of white dwarf material would have a mass of about 1 ton!


They have a surface temperature of 10,000 to 100,000 Kelvin, which explains their color. They consist mainly of carbon and oxygen and under this extreme pressure, the gas is in a state known as degenerate matter. Only neutron stars and the hypothetical quark stars have a higher density.


As they no longer undergo fusion processes and have no other source of energy, they slowly cool down over time and eventually end up as black dwarfs, although our current universe is not yet old enough to harbor black dwarfs.


If they are part of a binary star system, white dwarfs can lead to novae or end in a type Ia supernova. But Astronomers discovered also several white dwarfs with planets orbiting them, which is surprising given the intense heat and radiation that white dwarfs emit.


Some white dwarfs have extremely strong magnetic fields, which can be millions of times stronger than the Earth's magnetic field.


History


The first white dwarf discovered, but not recognized as such, was 40 Eridani B in the triple star system, 40 Eridani. Therefore, the first official white dwarf to be discovered and recognized was in the Sirius system in 1862. To the surprise of astronomers, Alvan Graham Clark discovered a small companion (Sirius B) of the bright Sirius star. With a surface temperature of 25,000 Kelvin and a diameter similar to that of the Earth, it is almost as massive as the Sun.


The second white dwarf, which was discovered in 1917, was named Van Maanen's Star after its discoverer, Adriaan Van Maanen, and is located 14.4 light years away in the constellation of Pisces.


It was not until 1930 that Subrahmanyan Chandrasekhar discovered what a white dwarf really is and that no white dwarf can have more than 1.4 solar masses, as otherwise the star would continue to collapse and a neutron star or black hole would form. Subrahmanyan Chandrasekhar was awarded the Nobel Prize for this in 1983.


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