White Dwarf - astronomy.

White Dwarf - astronomy. White Dwarf, old star that has exhausted its available nuclear fuel and collapsed, yet continues to radiate light from thermal energy (heat energy) trapped in it during its collapse. This is the final luminous phase in the evolution of low- to medium-mass stars. White dwarf stars are common throughout the Earth's galaxy, the Milky Way. The first few stages in the evolution of a white dwarf are the same as for other stars. A cloud of interstellar hydrogen gas and dust particles condenses under the mutually attractive force of gravitation until the temperature at the center of the cloud is high enough to cause the fusion of hydrogen atoms to form helium (see Atom). Hydrogen fusion releases electromagnetic radiation, which produces an outward pressure. When the outward radiation pressure and the inward gravitational force reach equilibrium, the star stabilizes as a main-sequence star--the longest phase in the life of any star (see Star: Evolution of Stars). After the hydrogen in a star's core has been converted to helium, hydrogen fusion slows down and stops, and the radiation released by fusion, which had supported the star against gravity, dissipates. Without the outward pressure of radiation, the star collapses under the gravitational pressure of its own weight. Small stars will no longer support fusion reactions of any kind, but the collapse of a massive star will compress the helium-enriched core enough to cause helium fusion. Depending on the mass of the star, it may go through many successive stages using helium and then heavier elements for fuel, but all stars eventually reach a stage where they can no longer support fusion reactions of any kind. When a star is unable to sustain nuclear reactions, it collapses a final time. If the mass of the collapsed star's core is greater than a critical value known as the Chandrasekhar limit, which is equal to the mass of about 1.4 suns, the core will collapse into a neutron star or perhaps a black hole (see Chandrasekhar, Subrahmanyan). Most stars that collapse, however, have cores with masses less than the Chandrasekhar limit. The cores of these stars collapse to an intermediate state called a degenerate electron state. The collapse stops at this point because the inward gravitational force is balanced by the mutual electrical repulsion of the negatively-charged electrons of the core's atoms. The degenerate electron core is known as a white dwarf. The outer envelope of the collapsed star--up to 90 percent of the total mass of the star--is blown away during the process of collapse into a planetary nebula--an expanding sphere of glowing matter surrounding the collapsing star. White dwarfs exhibit an unusual relationship between mass and size. Adding mass to a white dwarf increases the overall gravitational force holding it together. The additional inward pressure caused by the additional mass squeezes the atoms together, compressing them even tighter, with the net result that as matter is added to a white dwarf, it gets smaller, not larger. Thus, the least massive white dwarfs are the largest, and the most massive ones are the smallest. The density of a white dwarf could range from 107 to 1011 kg/m3--from 5,000 to 50,000,000 times more dense than the Earth. A star the size of the Earth's Sun would collapse into a sphere the size of the Earth and would have a density of about 109 kg/m3, or about 500,000 times the density of the Earth. Compression of all of the matter and thermal energy of a star's core causes the temperature of the core to soar. The final temperature of a white dwarf of mass near the Chandrasekhar limit can exceed 80,000° C (144,000° F). Objects this hot give off electromagnetic radiation, or light, with a brilliant blue-white color (see Blackbody). As it radiates light, the star loses energy and cools, slowly changing color from blue-white, to white, to yellow, to orange, and finally, at a temperature of about 4000° C, to a dull red. If a white dwarf cooled beyond this temperature, it would cease to emit visible light and would become what is known as a black dwarf. Since the most massive white dwarfs are also the smallest, they cool at the slowest rate--astronomers estimate that the most massive white dwarfs, those near the Chandrasekhar limit--would take several times the age of the universe to cool down to the black dwarf stage. Astronomers estimate that white dwarfs are quite numerous--perhaps comparable to the number of visible stars. Although all white dwarfs are incredibly hot in their early phases, they are so small that only the brightest and nearest ones are visible through the most powerful telescopes. Instead, white dwarfs are most commonly detected as unseen companions in binary stars--systems of two gravitationally bound stars. In some cases, a white dwarf's intense gravitational field pulls material from its companion star onto the white dwarf. When this happens, the fall of material through the white dwarf's gravitational field and onto its surface causes bursts of highly energetic radiation that Earth-based observers may recognize as a nova or recurrent nova. If the white dwarf accumulates enough material through this process to cross the Chandrasekhar limit, it will collapse again and become a neutron star. Contributed By: Dennis L. Mammana Microsoft ® Encarta ® 2009. © 1993-2008 Microsoft Corporation. All rights reserved.