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A white dwarf is what stars like our Sun become when they have exhausted their nuclear fuel. Near the end of its nuclear burning stage, such a star expels most of its outer material (creating a planetary nebula), until only the hot core remains, which then settles down to become a very hot (T > 100,000K) young white dwarf. Since a white dwarf has no way to keep itself hot unless it is accreting matter from a nearby star (see Cataclysmic Variables), it cools down over the course of the next billion years or so. Many nearby, young white dwarfs have been detected as sources of soft X-rays (i.e. lower-energy X-rays); soft X-ray and extreme ultraviolet observations enable astronomers to study the composition and structure of the thin atmosphere of these stars.
The first white dwarf star was detected in 1862, but its spectrum was not obtained until 1914. Called Sirius B, it forms a binary system with Sirius, the brightest-appearing star in the sky. It eluded discovery and analysis for a long time because it is very, very faint. Although only 8 LY away, the white dwarf companion of Sirius is quite difficult to see without a rather large telescope.
We have now found hundreds of white dwarfs. A good example of typical white dwarf is the nearby star 40 Eridani B. Its surface temperature is a relatively hot 12,000 K, but its luminosity is only 1/275 LSun. Calculations show that its radius is only 1.4% of the Sun's, or about the same as that of the Earth, and its volume is 2.5 X 10-6 that of the Sun. Its mass, however, is 0.43 times the Sun's mass, just a little less than half. To fit such a huge mass into so tiny a volume, the star's density must be about 170,000 times the density of the Sun, or more than 200,000 g/cm3. A teaspoonful of this stuff would have a mass of about 50 tons! Wow. At such densities, matter can't exist in its usual state.
Because a white dwarf is no longer able to create internal pressure, gravity unopposedly crushes it down until even the very electrons that make up a white dwarf's atoms are mashed together. In normal circumstances, identical electrons (those with the same "spin") are not allowed to occupy the same energy level. Since there are only two ways an electron can spin, only two electrons can occupy a single energy level. This is what's know in physics as the Pauli Exclusion Principle. And in a normal gas, this isn't a problem; there aren't enough electrons floating around to completely fill up all the energy levels. But in a white dwarf, all of its electrons are forced close together; soon all the energy levels in its atoms are filled up with electrons. Well, if all the energy levels are filled, and it is impossible to put more than two electrons in each level, than our white dwarf has become degenerate. For gravity to compress the white dwarf anymore, it must force electrons where they cannot go. Once a star is degenerate, gravity cannot compress it any more because quantum mechanics tells us there is no more available space to be taken up. So our white dwarf survives, not by internal combustion, 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. There is a limit on the amount of mass a white dwarf can have, however. It was found by Subrahmanyan Chandrasekhar to be 1.4 times the mass of our Sun, and is is call the "Chandrasekhar limit" after its discoverer.
With a surface gravity of 100,000 times that of the earth, the atmosphere of a white dwarf is very strange. The heavier atoms in its atmosphere sink and the lighter ones remain at the surface. Some white dwarfs have almost pure hydrogen or helium atmospheres, the lightest of elements. Also, the very strong gravity pulls the atmosphere close around it in a very thin layer, that, if were it on earth, would be lower than the tops of our skyscrapers!
Underneath the atmosphere, scientists believe there is a 50 km thick crust, the bottom of which is a crystalline lattice of carbon and oxygen atoms. One might make the comparison between a cool carbon/oxygen white dwarf and a diamond! (After all, a diamond is just crystallized carbon!) make the comparison between a cool carbon/oxygen white dwarf and a diamond! (After all, a diamond is just crystallized carbon!)
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A brown dwarf is a failed star. A star shines because of the thermonuclear reactions in its core, which release enormous amounts of energy by fusing hydrogen into helium. For the fusion reactions to occur, though, the temperature in the star's core must reach at least three million kelvins. And because core temperature rises with gravitational pressure, the star must have a minimum mass: about 75 times the mass of the planet Jupiter, or about 7 percent of the mass of our sun. A brown dwarf just misses that mark-it is heavier than a gas-giant planet but not quite massive enough to be a star.
In the mid-1980s astronomers began an intensive search for brown dwarfs, but their early efforts were unsuccessful. It was not until 1995 that they found the first evidence of their existence. Since then, researchers have detected dozens of the objects. Now observers and theorists are tackling a host of intriguing questions: How many brown dwarfs are there? What is their range of masses? Is there a continuum of objects all the way down to the mass of Jupiter? And did they all originate in the same way?
The search for brown dwarfs was long and difficult because they are so faint. All astrophysical objects-including stars, planets and brown dwarfs-emit light during their formation because of the energy released. In a star, the glow is eventually supplanted by the thermonuclear radiation from hydrogen fusion; once it begins, the star's size and luminosity stay constant, in most cases for billions of years. A brown dwarf, however, cannot sustain hydrogen fusion, and its light steadily fades as it shrinks. The light from brown dwarfs is primarily in the near-infrared part of the spectrum. Because brown dwarfs are faint from the start and dim with time, some scientists speculated that they were an important part of "dark matter," the mysterious invisible mass that greatly outweighs the luminous mass in the universe. |
