White dwarfs

In about 5 billion years the Sun will run out of fuel. It will swell into a red giant, then blow its outer layers away into space. What is left in the middle will be a white dwarf.

A white dwarf is the bare core of a dead star. It is tiny for a star, only about as big as the Earth. But it holds about half as much stuff as the whole Sun, so it is squeezed very tight. A piece the size of a sugar cube would weigh about a tonne.

A white dwarf has no fuel left to burn. It starts out white hot, and from then on it only cools, like a coal taken out of a fire. That takes billions of years.

Any star lighter than about eight Suns ends this way. Only the heavier ones do something else.

The first white dwarf ever found goes round Sirius, the brightest star in the night sky.

A white dwarf is what a star with less than about 8 times the Sun’s mass leaves behind. The Sun is one of them.

How one is made. Late in its life such a star is a red giant, fusing helium into carbon in its core. When the helium is gone, the core is not hot enough to fuse the carbon. The star grows unsteady and sheds its outer layers, which drift off as a glowing shell called a planetary nebula. The core is left bare. It begins hotter than 100,000 degrees, and it will never make energy again.

How dense it is. A typical white dwarf has about half the Sun’s mass and is only slightly bigger than the Earth.

	The Earth	A white dwarf the size of the Earth
Density, in kg per cubic metre	5,513	1,000,000,000
A sugar cube of it (one cubic centimetre)	5.5 grams	A tonne
Gravity at the surface	1	100,000

That is nearly 200,000 times as dense as the Earth. Only neutron stars and black holes pack matter tighter. The sugar cube row is our sum from the densities.

What holds it up. In an ordinary star, the heat from fusion pushes outwards and balances gravity. A white dwarf has no fusion, so gravity squeezes it until the electrons in it are jammed together. There the squeezing stops, because of a rule of quantum physics: no two electrons may be in exactly the same state. Once every low state is taken, the electrons cannot be packed closer without being forced into states that are already full, and they push back. That push does not depend on heat, so the white dwarf can go cold and still stand.

Heavier means smaller. Add mass to a white dwarf and it shrinks, because the electrons have to be squeezed tighter to carry the extra weight. Sirius B shows it. It has as much mass as the Sun (1.018 times), about twice the typical white dwarf, and its radius is 0.0081 of the Sun’s. Worked out, that is about 5,630 km, a little smaller than the Earth, and a density of about 2.7 tonnes in every cubic centimetre.

The limit. This cannot go on for ever. Subrahmanyan Chandrasekhar worked out that no white dwarf can be heavier than 1.4 times the Sun. Above that, the electrons cannot hold the weight. He shared the Nobel Prize in Physics in 1983 for his work on the structure of stars. A dead core heavier than the limit cannot be a white dwarf. It collapses further, into a neutron star or a black hole.

When a white dwarf is fed. A white dwarf on its own is finished. One with a close partner star is not. Its gravity can pull gas across from the partner, and a white dwarf that gains mass this way can blow up as a supernova as bright as 10 billion Suns.

The surface. Gravity at the surface is 100,000 times the Earth’s. The heavy atoms sink and the light ones float, so many white dwarfs show almost pure hydrogen or helium on top. The whole atmosphere is pulled down into a layer so thin that on the Earth it would not reach the tops of skyscrapers. Below it, scientists think, is a crust about 50 km thick, and under that carbon and oxygen locked into a crystal. Diamond is crystal carbon, which is where the idea of a star turning into a diamond comes from. The crust and the crystal are a model. Nobody has measured them directly.

The long cooling. With no new energy, a white dwarf fades from white hot towards a cold, dark lump that would be called a black dwarf. That takes so many billions of years that the universe is thought to be too young to hold many black dwarfs yet, if it holds any.

How the first was found. In 1844 Friedrich Bessel noticed that Sirius wobbles slightly as it moves across the sky, as if something unseen were swinging it about. In 1862 the telescope makers Alvan G. Clark and his father saw the culprit, a faint point beside the brilliant star. The two go round each other every 50 years. White dwarfs are small and faint, so a visible partner is still one of the best ways to find them. In 1995 the Hubble telescope picked out more than 75 in a single cluster of old stars, M4, about 7,000 light years away. The brightest of them shone no more strongly than a 100 watt light bulb seen from as far away as the Moon.

What the 3D view shows. Sirius is among the Sun’s neighbours, 8.6 light years away. Its white dwarf is not drawn as a separate point. On a view light years across, the two are far too close together to tell apart, and the bright star would drown it in any case.