![]() ![]() That energy slowly diffuses outward through the insulating stellar envelope, and the white dwarf slowly cools down. The energy radiated away into the interstellar medium is thus provided by the residual thermal energy of the nondegenerate ions composing its core. Their compact structure also prevents further gravitational contraction. White dwarfs have exhausted all their nuclear fuel and so have no residual nuclear energy sources. The hot planetary-nebula nucleus left behind has a mass of 0.5–1.0 solar mass and will eventually cool down to become a white dwarf. During the entire course of its evolution, which typically takes several billion years, the star will lose a major fraction of its original mass through stellar winds in the giant phases and through its ejected envelope. Near the end of this second red-giant phase, the star loses its extended envelope in a catastrophic event, leaving behind a dense, hot, and luminous core surrounded by a glowing spherical shell. After quiescent phases of hydrogen and helium burning in its core-separated by a first red-giant phase-the star becomes a red giant for a second time. White dwarfs evolve from stars with an initial mass of up to three or four solar masses or even possibly higher. Only the outermost stellar layers are accessible to astronomical observations. ![]() A very few white dwarf stars are surrounded by a thin carbon envelope. Surrounding this core is a thin envelope of helium and, in most cases, an even thinner layer of hydrogen. The central region of a typical white dwarf star is composed of a mixture of carbon and oxygen. Both predictions are in excellent agreement with observations of white dwarf stars. This limiting mass, known as the Chandrasekhar limit, is on the order of 1.4 solar masses. Furthermore, the existence of a limiting mass is predicted, above which no stable white dwarf star can exist. The application of the so-called Fermi-Dirac statistics and of special relativity to the study of the equilibrium structure of white dwarf stars leads to the existence of a mass-radius relationship through which a unique radius is assigned to a white dwarf of a given mass the larger the mass, the smaller the radius. Degeneracy pressure is the increased resistance exerted by electrons composing the gas, as a result of stellar contraction ( see degenerate gas). Unlike most other stars that are supported against their own gravitation by normal gas pressure, white dwarf stars are supported by the degeneracy pressure of the electron gas in their interior. Learn about the different types of stars categorized according to their mass and temperature - red dwarfs, red giants, supergiants, white, and brown dwarf stars See all videos for this article SpaceNext50 Britannica presents SpaceNext50, From the race to the Moon to space stewardship, we explore a wide range of subjects that feed our curiosity about space!.Learn about the major environmental problems facing our planet and what can be done about them! Saving Earth Britannica Presents Earth’s To-Do List for the 21st Century.100 Women Britannica celebrates the centennial of the Nineteenth Amendment, highlighting suffragists and history-making politicians. ![]() COVID-19 Portal While this global health crisis continues to evolve, it can be useful to look to past pandemics to better understand how to respond today.Student Portal Britannica is the ultimate student resource for key school subjects like history, government, literature, and more. ![]() This Time in History In these videos, find out what happened this month (or any month!) in history.#WTFact Videos In #WTFact Britannica shares some of the most bizarre facts we can find.Demystified Videos In Demystified, Britannica has all the answers to your burning questions.Britannica Classics Check out these retro videos from Encyclopedia Britannica’s archives.Britannica Explains In these videos, Britannica explains a variety of topics and answers frequently asked questions. ![]()
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