White dwarfs are the corpses of medium-sized stars that ran out of fuel. White dwarfs typically have the mass of the Sun, while being around the size of the Earth. It's no wonder that early twentieth century astronomers were dumbfounded by them.
The first discoveries
When the first white dwarfs were discovered no one realized how weird they were. It took larger telescopes, new techniques and more data to find that out.
40 Eridani B
William Herschel was the first to discover a white dwarf. In 1783 he entered into his catalog of double stars a pair not far from the star 40 Eridani. He didn't know that they were part of a triple star system of which 40 Eridani – now called 40 Eridani A – is the primary star. It was over 125 years later that someone noticed something strange about 40 Eridani B.
The brightest star of the night sky is Sirius. It was one of the stars whose movements German astronomer Friedrich Bessel (1764-1846) studied. He detected a slight wobble in its motion through space and attributed it to an unseen companion with a 50-year orbit. Some sources say that Sirius B was the first white dwarf to be discovered. However Bessel announced his prediction in 1841, and it wasn't until 1862 that Sirius B was seen by Alvan Graham Clark. It happened when Clark and his father, who were American telescope makers, were testing the lens for a large refracting telescope.
Bessel predicted not only an unseen companion for Sirius, but also for Procyon in the constellation Canis Minor. Procyon B remained elusive even longer than Sirius B had. J.M. Schaeberle finally found it in 1896 with Lick Observatory's 36-inch telescope.
In 1910 Henry Norris Russell visited the Harvard College Observatory. He had been measuring the distances to a number of stars, using parallax. With the parallax angle, an astronomer can calculate the distance by geometry. A bigger angle means a closer star, though the angle in the diagram is greatly exaggerated. Even the nearest neighboring star is quite far away.
The observatory was able to give Russell the spectral type of each of the stars he'd studied, using its extensive photographic plate collection. The spectrum of a star tells us a lot about its temperature and luminosity. Then out of curiosity Russell asked about 40 Eridani B. The answer: a type A star. This would make it a luminous white star. But he knew Procyon was only about fifteen light years away, and it was a types G star. So how could its dim companion have the spectrum of a much brighter star?
Afterwards Russell recalled, “I was flabbergasted.” But he probably actually thought it was a mistake until Walter Adams at Mt Wilson observatory confirmed it. Adams also obtained the first spectrum of Sirius B, which also turned out to be small and bright.
Dutch astronomer William Luyten named these little stars white dwarfs.
Looking back on all this, British astrophysicist Arthur Eddington wrote in his 1927 book Stars and Atoms
We learn about the stars by . . . interpreting the messages which their light brings to us. The [decoded] message of the Companion of Sirius . . . ran: "I am composed of material 3,000 times denser than anything you have ever come across; a ton of my material would be a little nugget that you could put in a matchbox." What reply can one make to such a message? The reply which most of us made in 1914 was—"Shut up. Don't talk nonsense."
Quantum physics to the rescue
British physicist Ralph Fowler in 1926 applied quantum physics to the problem, and showed that many things could happen at extremely high temperatures that we don't encounter on Earth. He suggested that white dwarfs were so hot that the atoms were broken apart. Usually most of the volume of an atom is the space between the nucleus and the electrons, but here the nuclei could be much closer together than they are in everyday matter. They would be all jammed together into the smallest possible space while the electrons circulated, kept apart by quantum processes, and creating an outward pressure known as electron degeneracy pressure.
Not quite the end
Not every white dwarf just quietly fades away into the distant future. Some are in close binary systems where they can pull material away from a companion. The dwarf may acquire enough material for a runaway fusion reaction to occur – like a hydrogen bomb. This happens on the surface, and causes the dwarf to brighten for a time in what's called a nova. It can occur more than once, so the white dwarf is a variable of the type known as a cataclysmic variable.
We started off with three white dwarfs, but we now know of around ten thousand. Eventually, they will cool enough so that there's no substantial heat or light coming from them. Then they will be black dwarfs. But this is theoretical. The necessary cooling time is much greater than the present age of the universe, so there aren't any around now.
Yet there can be a more dramatic exit for a white dwarf. If it steals enough material from a binary companion to exceed a certain mass – known as the Chandrasekhar limit – it will blow itself apart in a spectacular explosion called a supernova. These type Ia supernovae were thought to have a similar maximum brightness, making them suitable for measuring enormous distances in space. This works fairly well, but isn't as precise as astronomers had first hoped.