White dwarf star
News •
-
What is a white dwarf star?
A white dwarf star is a faint star that represents the final evolutionary state of a low to intermediate mass star. They are characterized by low luminosity, a mass similar to the Sun, and a radius comparable to Earth. White dwarf stars are very dense, with densities approaching 1,000,000 times that of water.
These stars have exhausted their nuclear fuel and radiate energy from the residual thermal energy of the ions in their core, slowly cooling over billions of years until they become cold, inert stellar remnants called black dwarfs. A white dwarf is supported against its own gravitation by the degeneracy pressure of the electron gas in its interior. The maximum mass that a white dwarf can have is 1.4 solar masses, which is known as the Chandrasekhar limit.
4 Britannica SourcesCreated from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles. -
How does a white dwarf star form?
White dwarf stars are the dense, compact remains of low- to intermediate-mass stars, typically with a mass comparable to the Sun but a radius similar to Earth's. After a star has exhausted its hydrogen and helium fuel through nuclear fusion, it enters a second red-giant phase and loses its outer layers in a catastrophic event. This results in a planetary nebula phase, leaving behind a hot, luminous core with a mass of 0.5–1.0 solar mass.
This core, composed mainly of carbon and oxygen, eventually cools to become a white dwarf. White dwarfs lack nuclear energy sources and are supported by electron degeneracy pressure, not normal gas pressure. They slowly radiate their residual thermal energy into space, eventually becoming cold, inert remnants called black dwarfs. A white dwarf cannot exist stably above the Chandrasekhar limit of approximately 1.4 solar masses.
4 Britannica SourcesCreated from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles. -
What is the size and mass of a typical white dwarf star?
A typical white dwarf star has a mass comparable to that of the Sun but is approximately the size of Earth. White dwarf stars are incredibly dense, with densities averaging 1,000,000 times that of water.
White dwarfs evolve from stars with an initial mass of up to three or four solar masses. 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. If the residual mass in the core of a star is less than 1.4 solar masses (the Chandrasekhar limit), the stellar remnant will become a white dwarf.
3 Britannica SourcesCreated from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles. -
Why do white dwarf stars no longer produce energy through fusion?
White dwarf stars no longer produce energy through nuclear fusion because they have exhausted their nuclear fuel. After a star has gone through its red giant phase, it loses its envelope, leaving behind a hot core primarily made of carbon and oxygen.
The compact structure of white dwarfs prevents further gravitational contraction. Energy is radiated into space from the residual thermal energy of the ions in the core, causing the white dwarf to slowly cool. Eventually, after billions of years, the white dwarf will stop radiating and become a black dwarf.
3 Britannica SourcesCreated from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles. -
What happens to a white dwarf star as it cools over time?
As a white dwarf star cools over billions of years, it exhausts its nuclear fuel and can no longer contract gravitationally. The star's residual thermal energy radiates into space, causing it to gradually cool. After this thermal energy is depleted, the white dwarf becomes a cold, inert remnant called a black dwarf.
1 Britannica SourceCreated from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles. -
How do white dwarf stars help astronomers learn about the age and evolution of our galaxy?
White dwarf stars help astronomers study the age and evolution of the galaxy because they represent the endpoint in the life cycle of low- to intermediate-mass stars. After these stars exhaust their nuclear fuel, they become red giants, eject their outer layers, and leave behind a hot core that cools into a white dwarf.
The rate at which a white dwarf cools is predictable. By observing the temperature and luminosity of white dwarfs, astronomers can estimate their age. Since the oldest white dwarfs have been cooling for billions of years, their age provides a lower limit for the age of the galaxy itself. Additionally, the distribution and characteristics of white dwarfs in different parts of the galaxy offer insights into the history of star formation and the evolution of stellar populations.
2 Britannica SourcesCreated from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.
White dwarf star, any of a class of faint stars representing the endpoint of the evolution of intermediate- and low-mass stars. White dwarf stars, so called because of the white colour of the first few that were discovered, are characterized by a low luminosity, a mass on the order of that of the Sun, and a radius comparable to that of Earth. Because of their large mass and small dimensions, such stars are dense and compact objects with average densities approaching 1,000,000 times that of water.
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. Degeneracy pressure is the increased resistance exerted by electrons composing the gas, as a result of stellar contraction (see degenerate gas). 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. Furthermore, the existence of a limiting mass is predicted, above which no stable white dwarf star can exist. This limiting mass, known as the Chandrasekhar limit, is on the order of 1.4 solar masses. Both predictions are in excellent agreement with observations of white dwarf stars.
The central region of a typical white dwarf star is composed of a mixture of carbon and oxygen. Surrounding this core is a thin envelope of helium and, in most cases, an even thinner layer of hydrogen. A very few white dwarf stars are surrounded by a thin carbon envelope. Only the outermost stellar layers are accessible to astronomical observations.
" height="20" alt="HEW Logo">
White dwarfs evolve from stars with an initial mass of up to three or four solar masses or even possibly higher. 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. 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. This is the planetary-nebula phase. 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. 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.
White dwarfs have exhausted all their nuclear fuel and so have no residual nuclear energy sources. Their compact structure also prevents further gravitational contraction. The energy radiated away into the interstellar medium is thus provided by the residual thermal energy of the nondegenerate ions composing its core. That energy slowly diffuses outward through the insulating stellar envelope, and the white dwarf slowly cools down. Following the complete exhaustion of this reservoir of thermal energy, a process that takes several additional billion years, the white dwarf stops radiating and has by then reached the final stage of its evolution and becomes a cold and inert stellar remnant. Such an object is sometimes called a black dwarf.
White dwarf stars are occasionally found in binary systems, as is the case for the white dwarf companion to the brightest star in the night sky, Sirius. White dwarf stars also play an essential role in Type Ia supernovae and in the outbursts of novae and of other cataclysmic variable stars.