What is a White Dwarf?
The relative sizes of Sirius B and the Earth Imagine shrinking the sun by a factor of just over 100, to a body the size of the earth. Since the mass of the sun would remain unchanged the orbit of the earth and the orbits of all the planets would be unaltered. The light emitted by the resulting midget sun, however, would be reduced by over a factor of 10,000! The midday sun would be just a brilliant point of light dimly illuminating the earth. All the planets, even close-in Mercury, would quickly freeze. The laws of physics dictate that such an icy solar system would contain, at its center a most unusual object, distinctly different from all the familiar stars we see when we stare up at the night sky. Nature does not allow the unlimited compression of matter to extreme densities without matter itself suffering dramatic and catastrophic changes in its fundamental character. It came as a series of shocks to astronomers in the early twentieth century that there might be types of matter tha
A white dwarf is a relatively small, dense type of star that is formed when a main sequence star burns all of its hydrogen and helium fuel but lacks the pressure and heat necessary to fuse carbon and oxygen. A white dwarf has a mass typically between 0.5 and 0.7 times that of the Sun, but a volume comparable to that of the Earth. White dwarfhood is the end state of stellar evolution for 97% of known stars. The transformation of a star into a white dwarf begins when a main sequence star, around the mass of our Sun, burns up all its hydrogen fuel and starts being forced to fuse helium into carbon and oxygen. Because its core starts to build up with carbon and oxygen that cannot be fused, the fusion must take place on a shell outside the core. The immense gravity of the core pushes the hydrogen together and causes it to fuse much faster than before, increasing the luminosity of the star by a factor of 1,000 – 10,000 and increasing its radius to something comparable to Mars’ orbit. When al
A white dwarf is a type of star that contains about as much matter as the Sun, but packed into a size comparable to the Earth. The majority of white dwarfs are thought to be made mostly of carbon and oxygen. In stars like the Sun, the inward pull of gravity is balanced by the outward push of the high-temperature hydrogen in the center fusing into helium and releasing energy in the process. There is no nuclear fusion in a white dwarf. Instead, the force that opposes gravity is called “electron degeneracy pressure”.
94% of stars end their lives as white dwarfs. In the last stage of stellar evolution, before turning into a white dwarf, stars begin to shed a large fraction of their mass into the surrounding interstellar space. During this time the star is consuming the last of the fuel used in nuclear fusion, the process which makes stars shine. Once this stage ends only the dense core of the star remains. This core is approximately the size of the Earth but contains about half the mass of the Sun. White dwarfs, because they have no more nuclear fuel and no capacity to fuse elements and therefore generate energy, cool as they age. They may start out at temperatures near 100,000 Kelvin and after 10 to 12 billion years reach temperatures near 4,000 Kelvin. For reference the Sun is approximately 6,000 Kelvin, although it is not cooling off the way white dwarfs do.
