A white dwarf star, also called a degenerate dwarf, is a stellar core remnant composed mostly out of electron-degenerate matter. A white dwarf is quite dense, its mass may be comparable to that of the Sun, while its volume could be comparable to that of Earth.
Key Facts & Summary
- The first white dwarf that was discovered was in the triple star system of 40 Eridani. This star system contains a main-sequence star, a white dwarf, and a red dwarf star.
- White dwarf stars are composed out of one of the densest forms of matter known in the universe, surpassed by only other compact stars such as neutron stars, black holes, and quark stars.
- White dwarf stars have a prevalence of around 0.4%, and their spectral type is usually D.
- White dwarf stars have temperatures of around 8,000 to 40,000 K, and they have luminosities of around 0.0001 to 100 times that of the Sun.
- A white dwarf’’s faint luminosity comes from the emission of stored thermal energy.
- Usually, white dwarf stars have a mass of around 0.1 to 1.4 that of our Sun, and they continue to live for around 100.000 to 10 billion years.
- White dwarf stars no longer produce energy to counteract their mass.
- Theoretically, white dwarfs cannot exceed 1.4 solar masses, some examples of white dwarf stars are Sirius B, Procyon B, or Van Maanen.
- The nearest known white dwarf to us is Sirius B, at a distance of 8.6 light-years away.
- It is theorized that there are around eight white dwarfs among the hundred star systems nearest to the Sun.
- The term “white dwarf” was first coined by astronomer Willem Luyten in 1922.
- White dwarf stars are thought to be the final evolutionary state of stars whose mass is not high enough to become a neutron star, that of around 10 solar masses.
- It is theorized that our Sun will become a red giant, after which it will morph into a white dwarf star.
It is theorized that white dwarfs represent the endpoint of stellar evolution for main-sequence stars with masses from about 0.07 to 10 solar masses.
The composition of the white dwarf will depend on the initial mass of the star. The central region of a white dwarf star is a thin envelope of helium and usually an even thinner layer of hydrogen.
Few white dwarfs are surrounded by a thin carbon envelope. Only the outermost stellar layers are accessible to astronomical observations. White dwarfs evolve from stars with an initial mass of up to three or four solar masses, possibly even higher.
White dwarfs form when the hot planetary nebula created by a red giant star loses its envelope, which was several times expanded by its red giant phase. When the hot planetary nebula cools down, the star becomes a white dwarf.
White dwarfs have exhausted their nuclear fuel and have no residual nuclear energy source. Their very compact, and because of this, further gravitational contraction is also prevented.
The energy radiated away into the interstellar medium is provided by the residual thermal energy of the nondegenerate ions composing a white dwarf’s core.
This 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 may take up to several billions of 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.
Theoretically, white dwarfs cannot exceed 1.4 solar masses. White dwarf stars have a prevalence in the universe of around 0.4%, and their spectral type is usually D.
White dwarf stars have temperatures of around 8,000 to 40,000 K, and they have luminosities of around 0.0001 to 100 times that of the Sun. A white dwarf’’s faint luminosity comes from the emission of stored thermal energy. Usually, white dwarf stars have a mass of around 0.1 to 1.4 that of our Sun
Observation of White Dwarfs
White dwarfs are somewhat difficult to observe due to their dimness. The first white dwarf discovered was found since its companion star, Sirius is among the brightest.
White dwarfs are also very small and thus hard to detect. They are usually found if they are part of a binary system. The Hubble Space Telescope has observed more than 75 white dwarfs since it was launched.
Some of the observed white dwarfs were so faint that the brightest of them was no more luminous than a 100 watt light bulb seen at the moon’s distance.
White dwarfs should be theoretically prevalent in old globular clusters, such as the M4 globular cluster in the constellation of Scorpius since age is a huge factor in regards to white dwarf populations.
Examples of White Dwarfs
White dwarfs stars are typically found in binary systems, as in the case for the white dwarf companion, Sirius B, which accompanies Sirius A, the brightest star in the night sky.
Though the white dwarf Sirius B is smaller than Earth, it is quite dense. One cubic inch of its material would weigh 13.6 metric tons (15 tons) on Earth.
Another example of a white dwarf is the one situated in the center of the Helix Nebula. The white dwarf there continues to emit large amounts of ultraviolet radiation, which heats up the gases in the nebula and gives it its characteristic colors.
Radiation and Cooling
The degenerate matter that makes up the bulk of a white dwarf has a low opacity yet a high thermal conductivity. As a result, the interior of a white dwarf maintains a uniform temperature, about 107 K.
A white dwarf remains visible for a long period of time, as its tenuous atmosphere of normal matter begins to radiate at 107 K, upon formation, while its greater interior mass is at 107 K yet cannot radiate through its normal matter shell.
A white dwarf, once formed, will remain stable and will continue to cool almost indefinitely, eventually becoming a black dwarf. If the universe continues to expand, it is thought that in 1019 to 1020 years, the galaxies will evaporate as their stars escape into intergalactic space.
White dwarfs should survive galactic dispersion, however, an occasional collision between them may produce a new fusing star of a super-Chandrasekhar mass white dwarf that will explode in a type la supernova.
White dwarfs can also be cannibalized or evaporated by a companion star, thus causing the white dwarf to lose much of its mass, becoming a planetary object, a host star, maybe even a helium or diamond planet.
Did you know?
- Some estimations suggest that around 10% of white dwarfs possess magnetic fields over 1 million gauss / 100 T.
- A white dwarf’s average density of matter is roughly a million times greater than the average density of the Sun.
- The surface gravity of a white dwarf is around 100,000 times greater than that of Earth.
- The more massive a white dwarf is, the smaller it is.
- A white dwarf’s atmosphere is comprised out of light gases such as hydrogen and helium. These elements are pulled very close to a white dwarf’s surface.
- A typical white dwarf has a density between 104 and 107 g/cm3.