White dwarf stars are believed to represent the final evolutionary state of stars that are not massive enough to end their lives in super or hyper novae events. Also known as “degenerate stars”, white dwarfs consist of electron-degenerate matter that no longer produces fusion energy. Instead, white dwarfs radiate their stored thermal energy as a faint luminosity, but so slowly that the Universe has not existed long enough for any white dwarf stars to have emitted all of their heat. Below are 10 more interesting facts about white dwarf stars you may not have known.
White dwarf stars are relatively rare
There are only eight known white dwarf stars in the 100 star systems that are closest to us, with the closest known white dwarf star to us being Sirius B, the companion star of Sirius A in the Sirius binary system, which is located 8.6 light years away in the constellation Canis Major.
About 97% of all Milky Way stars will become white dwarfs
While only ten thousand or so white dwarfs have been found, more than 97% of the stars in the Milky Way, including the Sun, are not massive enough to become anything else than white dwarf stars when they end their lives. Taken to extremes, this means that once all the stars in the Milky Way have evolved into white dwarfs and have cooled down sufficiently to become black dwarfs, the Milky Way will for all intents and purposes become invisible, except perhaps for the few neutron stars that may outlive both the white dwarfs and the dispersion of the galaxy.
Almost all white dwarfs stars have the same mass
While white dwarf stars fall into a wide range of masses, from as little as 0.17 to as much as 1.3 times the Sun’ mass, most white dwarfs weigh in at between 50% to 70% of the Sun’s mass, with an average of around 60%. In practice, this means that while white dwarf stars are typically about as big as the Earth, they are in general terms about as massive as the Sun, meaning the density of white dwarfs can be as much as 1 million times higher than that of the Sun. This in turn means that 1 cubic cm of a white dwarf can weigh as much as one metric ton, with only black holes, neutron stars, and possibly quark stars being denser.
White dwarf stars cannot exceed 1.4 solar masses
Due to the nature of degeneracy pressure, which is what supports a white dwarf against gravitational collapse into a neutron star, a white dwarf can never exceed 1.4 solar masses, a limit that is known as the “Chandrasekhar Limit”, after the Indian astronomer who first calculated this limit in 1930. However, this figure assumes that the star is not rotating, but if it does, the limit increases slightly. Nonetheless, in cases where a white dwarf is rotating in a non-uniform manner and the viscosity of the star is not taken into account, there is no upper mass limit at which a (hypothetical) white dwarf can be in hydrostatic equilibrium.
White dwarf stars cool down more slowly as they age
Studies have shown that since white dwarf stars do not generate energy to replace the heat that is lost through radiation, the rate at which these stars cool down slows as they age. The following example illustrates the point: a white dwarf with a mass of 0.59 times that of the Sun that has a helium atmosphere and a surface temperature of 8,000K, will take about 1.5 billion years to cool down to 7,140K. Cooling down another 500K will take about 0.3 billion years, while cooling down to 6,000K, and then another 500K will take 0.4 billion and 1.1 billion years respectively.
White dwarf stars have atmospheres
Spectroscopic studies have revealed that much of a white dwarf star’s luminosity derives from its atmosphere, which can consist of either hydrogen or helium. While both elements are usually present in the atmosphere of a white dwarf, one always predominates by a factor of at least 1,000 as compared to all other elements in the stellar atmosphere. Most investigators agree that this is the result of a process in which gravity separates the elements in the atmosphere, with the most massive molecules accumulating at or near the surface of the star, with the lighter elements stacked onto this layer in order of their mass. In the case of hydrogen-rich atmospheres, the total mass of the hydrogen component may be as massive as 1/10,000th of the stars’ total mass.
Some white dwarf stars are metal-rich
The fact that the spectra of some white dwarf stars show strong metal lines came as a surprise to astronomers, since these heavy elements should have gravitated towards the star’s core soon after its formation. While there is no certainty as to the origin of the metals in some spectra, it is thought that in the case of the white dwarf designated Ton 345 at least, the metal abundances in its spectrum derives from the remains of a planet that was destroyed by the progenitor star during its asymptotic giant branch phase.
White dwarf stars will outlive their host galaxies
Although white dwarf stars are considered to be stable after their formation, they will eventually cool down to become cold black dwarfs. However, due to the opacity, or resistance of their outer layers to radiation, it is estimated that white dwarfs will take about 1034–1035 years to reach this state. This exceedingly long life time is based on the known life time of protons, which is far longer than it will take for galaxies to disperse, or “evaporate”, which process is expected to be completed in a mere 1019 to 1020 years.
Some white dwarf stars host planets
While there is some debate as to how planets may form around white dwarfs, many white dwarfs are nevertheless orbited either by planets, as in the case of two circumbinary planets around a curious white dwarf/red dwarf binary system designated NN Serpentis, or by dense dust/debris discs. Most investigators subscribe to the theory that planets orbiting white dwarfs are the remains of planets that were destroyed by the creation of the white dwarf, such as would happen when our Sun swells up during its red-giant phase. In our case, Earth might end up as a disintegrating rocky body orbiting the Sun in its white dwarf phase.
White dwarf stars can explode several times, and yet survive
While some processes can destroy a white dwarf star in a supernova explosion, many white dwarf stars survive repeated, but less cataclysmic thermonuclear explosions of accreted hydrogen-rich material on their surfaces. Provided the stars’ core remains intact, a white dwarf can survive as many explosions on its surface as it takes to deplete the source of in-falling material.