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.
For more information about White dwarf, read the full article at
Wikipedia.
This text uses material from Wikipedia and is available under the GNU Free Documentation License.
News tagged with white dwarfs
Ticking stellar time bomb identified (w/ Video)
Nov 17, 2009 |
4.7 / 5 (11) |
0
(PhysOrg.com) -- "One of the major problems in modern astrophysics is the fact that we still do not know exactly what kinds of stellar system explode as a Type Ia supernova," says Patrick Woudt, from the University ...
Search results for white dwarfs
Myopia appears to have become more common
6 hours ago |
not rated yet |
0
Myopia (nearsightedness) may have been more common in Americans from 1999 to 2004 than it was 30 years ago, according to a report in the December issue of Archives of Ophthalmology.
Antidepressants may increase risk of stroke and death
Medicine & Health / Medications
7 hours ago |
not rated yet |
0
Postmenopausal women who take antidepressants face a small but statistically significant increased risk for stroke and death compared with those who do not take the drugs. The new findings are from the federally-funded, multi-institution, ...
Fossils on the Edge of Forever
Space & Earth / Earth Sciences
7 hours ago |
4.8 / 5 (5) |
0
Astrobiologists have not yet found alien life on other planets. But the fossil record has evidence of aliens of another sort: the Ediacarans that lived on Earth millions of years ago.
Developing countries end boycott at climate talks
10 hours ago |
not rated yet |
2
(AP) -- Poor countries ended a boycott of U.N. climate talks Monday after getting assurances that rich nations were not conspiring to soften their commitments to cutting greenhouse gases, European officials ...
Racial differences in medication use
10 hours ago |
not rated yet |
0
Older adults experience a number of medication-related problems, including under treatment, suboptimal drugs, suboptimal dosing, and non-adherence, which can have negative effects on their quality of life. Now new research, ...
Study finds racial disparities exist in radiation therapy rates for early stage breast cancer
17 hours ago |
not rated yet |
0
Black women are less likely than white women to receive radiation therapy after a lumpectomy, the standard of care for early stage breast cancer, according to a new study by researchers at The University of Texas M. D. Anderson ...
26 operations, 13 kidneys: hope to few with little
18 hours ago |
5 / 5 (1) |
0
(AP) -- Twenty-six operations put healthy kidneys into 13 desperately ill people: Doctors in the nation's capital just performed a record-setting kidney swap, part of a pioneering effort to expand transplants ...
Decades-old dioxins pollute river, divide US community
Dec 13, 2009 |
5 / 5 (2) |
0
The signs posted along Michigan's Tittabawassee River warning of dangerous dioxin levels don't really worry fisherman David Mitchell.
Endangered rhinos return to wild
Dec 11, 2009 |
not rated yet |
0
A Czech zoo is to transfer four endangered Northern White rhinos to a Kenyan reserve in a last-ditch attempt to ensure the survival of the species.
Computing with a wave of the hand (w/ Video)
Dec 11, 2009 |
4.9 / 5 (14) |
7
(PhysOrg.com) -- The iPhone’s familiar touch screen display uses capacitive sensing, where the proximity of a finger disrupts the electrical connection between sensors in the screen. A competing approach, ...
List of search results for white dwarfs
