Eventually, over tens or even hundreds of billions of years, a white dwarf cools down to become a black dwarf that does not emit energy. Since the oldest stars in the universe are only 10 to 20 billion years old, no black dwarf is yet known. If a white dwarf crossed the Chandrasekhar boundary and no nuclear reaction took place, the pressure exerted by the electrons would no longer be able to balance gravity and it would collapse into a denser object called a neutron star. [47] Carbon-oxygen white dwarfs, accumulating the mass of a nearby star, undergo an uncontrolled nuclear fusion reaction that results in a Type Ia supernova explosion in which the white dwarf can be destroyed before reaching the limit mass. [48] Although most white dwarfs are thought to consist of carbon and oxygen, spectroscopy generally shows that their emitted light comes from an atmosphere dominated by hydrogen or helium. The dominant element is usually at least 1,000 times more common than all other elements. As Schatzman explained in the 1940s, high surface gravity is thought to cause this purity by gravitationally separating the atmosphere so that heavy elements are at the bottom and the lightest at the top. [83] [84]: §§5–6 This atmosphere, the only part of the white dwarf visible to us, is considered the tip of an envelope that is a remnant of the star`s envelope in the AGB phase and may also contain accreted matter from the interstellar medium. The envelope is thought to consist of a helium-rich layer with a mass not exceeding 1⁄100 of the total mass of the star, which, when the atmosphere is dominated by hydrogen, is covered by a hydrogen-rich layer with a mass of about 1⁄10,000 of the total mass of the star. [61] [85]: §§4–5 When a white dwarf is in a binary system and accumulates matter from its companion, a variety of phenomena can occur, including novae and Type Ia supernovae.

It can also be a super soft X-ray source if it is able to pick up material from its companion fast enough to keep fusion on its surface. [181] On the other hand, phenomena in binary systems such as tidal interaction and star-disk interaction, moderated by magnetic fields or not, act on the rotation of accreting white dwarfs. In fact, the fastest white dwarfs, certainly known, are members of binary star systems (the CTCV J2056-3014 white dwarf being the fastest). [182] A system of binary stars close to two white dwarfs can emit energy in the form of gravitational waves, causing their mutual orbits to shrink steadily until the stars merge. [183] [184] The white dwarf WD 0145+234 shows brightening in the mid-infrared, as shown by NEOWISE data. The clarification is not visible until 2018. It is interpreted as the tidal disturbance of an exoasteroid, the first time such an event has been observed. [172] But not all white dwarfs will spend millennia cooling their heels.

Those in a binary star system may have a gravitational pull strong enough to collect material from a nearby star. When a white dwarf gains enough mass in this way, it reaches a level called the Chandrasekhar limit. At this point, the pressure at its center will become so great that an uncontrolled merger will take place and the star will explode in a thermonuclear supernova. The magnetic fields of a white dwarf could allow the existence of a new type of chemical bond, the perpendicular paramagnetic bond, in addition to ionic and covalent bonds, leading to what was initially called „magnetized matter“ in research published in 2012. [104] There were rumors of screams and lightning coming from the fountain and reports of a figure in white. No one else would dare to show themselves exposed to a foreigner, a white man. The relationship between mass and radius of white dwarfs can be derived with an energy minimization argument. The energy of the white dwarf can be approximated by taking it as the sum of its potential gravitational energy and kinetic energy. The potential gravitational energy of a unit mass piece of the white dwarf, Eg, is of the order of −G M/R, where G is the gravitational constant, M is the mass of the white dwarf and R is its radius. As soon as the star lacks hydrogen at its center, the star fuses helium with carbon and oxygen. The fusion of hydrogen moves to a shell that surrounds the nucleus. The star swells and becomes a red giant.

For most stars – including our Sun – this is the beginning of the end. As the star expands and stellar winds blow at an increasingly violent rate, the outer layers of the star escape the relentless gravitational pull of gravity. Estimating how long white dwarfs cooled can help astronomers learn a lot about the age of the universe. They can be called pre-white dwarfs. [106] [110] These variables all show small variations (1% to 30%) in light output resulting from an overlay of vibrational modes with periods of hundreds to thousands of seconds. Observation of these variations provides asteroseismological clues inside white dwarfs. [111] The metal-rich white dwarf WD 1145+017 is the first white dwarf observed with a decaying minor planet passing through the star. [177] [169] The disintegration of the planetesimal creates a cloud of debris that passes in front of the star every 4.5 hours, causing a 5-minute discoloration of the star`s optical brightness.

[169] The depth of transit varies widely. [169] One possibility is that the added mass could cause it to collapse into a much denser neutron star. White dwarfs are born when a star stops. A star spends most of its life in a precarious balance between gravity and the outward pressure of the gas. The weight of a few octalions of tons of gas pressing on the stellar core results in densities and temperatures high enough to trigger nuclear fusion: the fusion of hydrogen nuclei into helium. The steady release of thermonuclear energy prevents the star from collapsing in on itself. The material of a white dwarf no longer undergoes fusion reactions, so the star no longer has an energy source. As a result, it cannot sustain itself against gravitational collapse by the heat generated by fusion, but is only supported by electronic degeneracy pressure, making it extremely dense. The physics of degeneracy gives a maximum mass for a non-rotating white dwarf, the Chandrasekhar limit – about 1.44 times M☉ – beyond which it cannot be supported by the electron degeneracy pressure.

A carbon-oxygen white dwarf approaching this mass limit, usually by mass transfer from a companion star, can explode as a Type Ia supernova through a process known as carbon detonation; [1] [5] SN 1006 is considered a famous example. White dwarf temperatures can exceed 100,000 Kelvin, according to NASA (opens in a new window) (about 179,500 degrees Fahrenheit). Despite these sweltering temperatures, white dwarfs have low light because they are so small, according to NMSU. White dwarfs are the hot, dense remnants of long-dead stars. These are the stellar cores left behind after a star has exhausted its fuel supply and blown most of its gas and dust into space.