White Dwarfs
A hydrogen-burning star (like the sun) is kept in equilibrium by a balance between radiation pressure and gravitational attraction. When the hydrogen burns out, the star goes through a number of changes during which it sheds about half its mass. If the mass of the star is less than a critical value, called the Chandresekhar limit, after its discoverer, then this contraction to a smaller size is halted by a quantum mechanical effect called degeneracy. If the mass is greater than this the pressure developed by the electronic degeneracy is insufficient to prevent gravitational collapse to a neutron star or even a black hole. White dwarfs have typically the same masses as neutron stars, but are about the size of the earth, with densities on the order of 10^6 g*cm^(-3).
Although white dwarfs generate no nuclear energy, they further evolve into black dwarfs as their luminosities (L=4*ð*(R^2)* ó*T^4) diminish. Since their surface is still very hot, the surface electrons have enough energy to be in higher energy states. These electrons radiate energy, and with no thermonuclear reactions to replenish it, fall into lower energy states. Once an electron is in the lowest energy state possible it can no longer radiate energy, and with time, becomes dark and cold. It is believed that the universe is not yet old enough to contain any black dwarfs.
The first white dwarf to be discovered was found as a companion star to Sirius, a bright star near the constellation Canis Major. In 1844, astronomer Friedrich Bessel noticed that Sirius had a slight wobble (an effect attributed to an orbiting object). In 1863, Alvan Clark located the object and it was found to be a white dwarf. This pair is now referred to as S...
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...r the mass of a white dwarf, beyond which degeneracy pressure can no longer support the star against further gravitational collapse. MCh ~3.2 . Using the somewhat smaller value for the white dwarf radius gives MCh ~ 1.5 which is close to the generally accepted limit .
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Reuteman, Rob. “Vocational School Enrollment Booms Amongst White Collar Bust.” Cnbc.com. CNBC, 3 Dec. 2009. Web. 29 Jan. 2012.
A white dwarf uses electron degeneracy pressure to support itself. It is because of the electron degeneracy pressure that white dwarfs have a small size relative to other types of stars.
McIntyre, M. & McDonald, C. (2014). Nursing Philosophies, Theories, Concepts, Frameworks, and Models. In Koizer, B., Erb, G., Breman, A., Snyder, S., Buck, M., Yiu, L., & Stamler, L. (Eds.), Fundamentals of Canadian nursing (3rd ed.). (pp.59-74). Toronto, Canada: Pearson.
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