When do white dwarfs formed

But in a white dwarf, the density is much higher, and all of the electrons are much closer together. This is referred to as a "degenerate" gas, meaning that all the energy levels in its atoms are filled up with electrons. For gravity to compress the white dwarf further, it must force electrons where they cannot go.

Once a star is degenerate, gravity cannot compress it any more, because quantum mechanics dictates that there is no more available space to be taken up. So our white dwarf survives, not by internal fusion, but by quantum mechanical principles that prevent its complete collapse.

Degenerate matter has other unusual properties. For example, the more massive a white dwarf is, the smaller it is. This is because the more mass a white dwarf has, the more its electrons must squeeze together to maintain enough outward pressure to support the extra mass. However, there is a limit on the amount of mass a white dwarf can have.

Subrahmanyan Chandrasekhar discovered this limit to be 1. This is appropriately known as the "Chandrasekhar limit. With a surface gravity of , times that of Earth, the atmosphere of a white dwarf is very strange.

This continued radiation from the white dwarf, coupled with the lack of an internal energy source, means that the white dwarf begins to cool. Eventually, after hundreds of billions of years, the white dwarf will cool to temperatures at which it is no longer visible and it will become a black dwarf.

With such long timescales for cooling due mostly to the small surface area through which the star radiates , and with the age of the Universe currently estimated at Due to their high temperatures and small size, white dwarfs are found below the main sequence in the Hertzsprung-Russell diagram.

White dwarf stars are extreme objects that are roughly the same size as the Earth. The easiest way to picture this is to imagine squeezing the mass of the Sun into an object about the size of the Earth! The result is that gravity at the surface of the white dwarf is over , times what we experience here on Earth, and this pulls the atmosphere of the star into an extremely thin surface layer only a few hundred metres high. White dwarfs in the globular cluster M4, are much fainter than the dominant red and yellow stars.

Many white dwarfs fade away into relative obscurity, eventually radiating away all of their energy and becoming so-called black dwarfs , but those that share a system with companion stars may suffer a different fate. If the white dwarf is part of a binary system, it may be able to pull material from its companion onto its surface.

Increasing the white dwarf's mass can have some interesting results. One possibility is that the added mass could cause it to collapse into a much denser neutron star. A far more explosive result is the Type 1a supernova. As the white dwarf pulls material from a companion star, the temperature increases, eventually triggering a runaway reaction that detonates in a violent supernova that destroys the white dwarf.

This process is known as a "single-degenerate model" of a Type 1a supernova. In , researchers were able to closely observe the complex shells of gas surrounding one Type 1a supernova in fine detail. If the companion is another white dwarf instead of an active star, the two stellar corpses merge together to kick off the fireworks. This process is known as a "double-degenerate model" of a Type 1a supernova.

At other times, the white dwarf may pull just enough material from its companion to briefly ignite in a nova, a far smaller explosion. Because the white dwarf remains intact, it can repeat the process several times when it reaches that critical point, breathing life back into the dying star over and over again. Join our Space Forums to keep talking space on the latest missions, night sky and more! And if you have a news tip, correction or comment, let us know at: community space.