Related topics: stars
White dwarf
hideA white dwarf, also called a degenerate dwarf, is a small star composed mostly of electron-degenerate matter. Because a white dwarf's mass is comparable to that of the Sun and its volume is comparable to that of the Earth, it is very dense. Its faint luminosity comes from the emission of stored thermal energy. White dwarfs comprise roughly 6% of all known stars in the solar neighborhood. The unusual faintness of white dwarfs was first recognized in 1910 by Henry Norris Russell, Edward Charles Pickering, and Williamina Fleming;, p. 1 the name white dwarf was coined by Willem Luyten in 1922.
White dwarfs are thought to be the final evolutionary state of all stars whose mass is not too high—over 97% of the stars in our galaxy., §1. After the hydrogen-fusing lifetime of a main-sequence star of low or medium mass ends, it will expand to a red giant which fuses helium to carbon and oxygen in its core by the triple-alpha process. If a red giant has insufficient mass to generate the core temperatures required to fuse carbon, an inert mass of carbon and oxygen will build up at its center. After shedding its outer layers to form a planetary nebula, it will leave behind this core, which forms the remnant white dwarf. Usually, therefore, white dwarfs are composed of carbon and oxygen. It is also possible that core temperatures suffice to fuse carbon but not neon, in which case an oxygen-neon-magnesium white dwarf may be formed. Also, some helium white dwarfs appear to have been formed by mass loss in binary systems.
The material in a white dwarf no longer undergoes fusion reactions, so the star has no source of energy, nor is it supported against gravitational collapse by the heat generated by fusion. It is supported only by electron degeneracy pressure, causing it to be extremely dense. The physics of degeneracy yields a maximum mass for a nonrotating white dwarf, the Chandrasekhar limit—approximately 1.4 solar masses—beyond which it cannot be supported by degeneracy pressure. A carbon-oxygen white dwarf that approaches this mass limit, typically by mass transfer from a companion star, may explode as a Type Ia supernova via a process known as carbon detonation. (SN 1006 is thought to be a famous example.)
A white dwarf is very hot when it is formed but since it has no source of energy, it will gradually radiate away its energy and cool down. This means that its radiation, which initially has a high color temperature, will lessen and redden with time. Over a very long time, a white dwarf will cool to temperatures at which it will no longer be visible, and become a cold black dwarf. However, since no white dwarf can be older than the age of the Universe (approximately 13.7 billion years), even the oldest white dwarfs still radiate at temperatures of a few thousand kelvins, and no black dwarfs are thought to exist yet.
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News tagged with white dwarfs
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Dec 16, 2009 |
4.9 / 5 (15) |
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(PhysOrg.com) -- A team of Chinese astronomers have discovered a giant planet close to the exotic binary star system QS Virginis. Although dormant now, in the future the two stars will one day erupt in a violent ...
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Senate Dems look to close deal on health overhaul
23 hours ago |
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(AP) -- Outnumbered Republicans are vowing to delay passage of historic health care legislation as long as possible after jubilant Democrats locked in Nebraska Sen. Ben Nelson as the 60th and decisive vote.
Obama hails 60th Senate vote for health care
Dec 19, 2009 |
2.3 / 5 (3) |
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(AP) -- Jubilant Democrats locked in Nebraska Sen. Ben Nelson as the 60th and decisive vote for historic health care legislation Saturday, putting President Barack Obama's signature issue firmly on a path ...
Video game watchdog shuts down, victim of economy
Dec 18, 2009 |
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(AP) -- David Walsh said when he was assembling his first report card on video game violence 13 years ago, children were attacking on-screen monsters or aliens with imaginary chain saws and guns.
China Building 30-Mile Bridge Connecting Hong Kong to Guangdong Province
Dec 18, 2009 |
4.8 / 5 (10) |
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China Daily reports the commencement of the 30-mile Hong Kong-Zhuhai-Macao Bridge, the longest sea bridge under construction world-wide. The six-lane expressway will cut travel time from three-hours to around ...
Hackers steal SKorean-US military secrets
Dec 18, 2009 |
1 / 5 (1) |
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(AP) -- South Korea's military said Friday it was investigating a hacking attack that netted secret defense plans with the United States and may have been carried out by North Korea.
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Medicine & Health / Psychology & Psychiatry
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Space & Earth / Space Exploration
Dec 17, 2009 |
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(PhysOrg.com) -- It looked like a light show in Elkton, Md., on Tuesday, Dec. 15, as NASA ground tested a full-scale attitude control motor, or ACM. The motor operated with precision as its elaborate eight-valve ...
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In the new blockbuster Avatar, humans visit the habitable - and inhabited - alien moon called Pandora. Life-bearing moons like Pandora or the Star Wars forest moon of Endor are a staple of science fiction. ...
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Medicine & Health / Psychology & Psychiatry
Dec 17, 2009 |
3.2 / 5 (9) |
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Subtle patterns of nonverbal behavior that appear on popular television programs influence racial bias among viewers, according to research from Tufts University to appear in the December 18, 2009, issue of the journal Science.
Breathlessness eased in patients with rare, often fatal disease
Dec 17, 2009 |
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Patients with a rare, deadly disease that mostly affects young women felt a dramatic reduction in breathlessness using an approved drug, according to study results published online today in The Journal of Heart and Lung Tr ...
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