During the first quarter of this century astronomers found that the brightest star in the sky, Sirius, was orbited by a much fainter companion. Analysis of the orbit yielded a mass for the companion similar to that of the sun while an analysis of its light suggested that its size was approximately the same as Earth's. Further observation revealed that these small massive stars are reasonably common but had gone undetected because they are so faint. The stars of this unique class are called white dwarfs. They range in mass from perhaps a third to just under one and a half times the mass of the sun. We now know that these stars have exhausted their supply of nuclear fuel, which would enable them to shine like the sun and other ordinary stars. Under the weight of their own matter, they have collapsed to roughly a hundredth the size of the sun. A fundamental law of quantum mechanics (i.e., the Pauli exclusion principle) limits the ability of the crushing gravity to pack electrons into an ever decreasing volume. The pressure of these highly packed electrons balances the weight of the overlying stellar material, stopping any further collapse. Since the electron pressure results from a simple packing of the electrons in a particular volume, there is no need for them to be heated by nuclear fusion, as is the case in normal stars (the nuclear fusion produces the pressure to balance the weight of overlying material). We call such densely packed matter electron-degenerate. A teaspoon of such matter would weigh 40 tons (36.3 tonnes) on Earth. Thus, white dwarfs are stable stars which slowly cool off, becoming dark stellar cinders.
Since two thirds of all stars occur in binary star systems where the components orbit one another, it is not surprising that many white dwarfs are found in binary systems. As the companion of the white dwarf ages it will expand and begin to lose matter to the gravitational pull of the white dwarf. This results in the injection of hydrogen-rich matter from the normal companion into the outer layers of the white dwarf. After a certain amount of this matter accumulates, it will undergo a nuclear fusion explosion which will blow off the outer ten-thousandth of the white dwarf. The energy released during this explosion will cause the system to brighten perhaps a million times or more, forming what we call a nova. The extent of the explosion critically depends on aspects of the donor star as well as the white dwarf. Smaller explosions may be called dwarf novae. This material will be blown clear of the system and the expanding shell will become visible to later observers.
There is a limit in mass to which the white dwarf can grow because there is a limit to the extent that electrons can resist the increased gravitational forces. Depending on the chemical composition of the white dwarf, this limit occurs when it reaches between 1.2 and 1.4 times the mass of the sun. In binary systems where donated mass from the companion forces the white dwarf beyond this limit, the white dwarf will collapse by a factor of one thousand to become a neutron star. The circumstances of the collapse may be so violent as to result in an explosion called a Type I Supernova. Such an explosion is far more violent than a nova and may result in the disruption of the binary system.
See also Stellar evolution.