White dwarf. Stellar remnant that is generated when a star of mass less than 9-10 solar masses has exhausted its nuclear fuel. In fact, it is a stage of stellar evolution that will pass through 97% of the stars we know, including the Sun. White dwarfs are, along with red dwarfs, the most abundant stars in the universe.
Summary
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- 1 History
- 1 Composition
- 2 Training
- 3 Radiation and cooling
- 4 Atmosphere
- 5 Magnetic field
- 2 Sources
History
The first white dwarf was discovered in the 40 Eridani triple star system, which is comprised of the main sequence star 40 Eridani A orbiting the binary system formed by the white dwarf 40 Eridani B, and 40 Eridani C, a red sequence dwarf. principal. This binary system was discovered by William Herschel on January 31, 1783. The same binary star was later observed by Friedrich Georg Wilhelm von Struve and Otto Wilhelm von Struve in 1825 and 1851 respectively. In 1910, Henry Norris Russell, Edward Charles Pickering, and William Fleming discovered that, despite being a faint star, 40 Eridani B was spectral type A, or white. The spectral type of 40 Eridani B was officially confirmed in 1914 by Walter Adams.
During the 19th century, techniques for measuring the position of stars became precise enough to be able to detect very small changes in the position of some of them. Friedrich Bessel, in 1844, using these techniques, perceived that the stars Sirius (α Canis Majoris) and Procyon (α Canis Minoris) were varying their positions, for which he deduced that these position changes were due to a star invisible until then. Bessel estimated that the period of this star would be about half a century. CHFPeters calculated an orbit for this star in 1851.
The mentioned star is none other than Sirius B, also known as the Cub, the second discovered white dwarf. It has a surface temperature of about 25,000 K, which includes it within hot stars. On January 31, 1862, Alvan Graham Clark observed a kind of dark star near Sirius that had not been previously sighted, and was later identified as the star predicted by Bessel. Regardless, Sirius B turned out to be 10,000 times less luminous than the main star Sirius A. Since it had to have a high brightness per unit area, Sirius B had to be much smaller than Sirius A. calculations determined a radius roughly equal to that of Earth. Analysis of the orbit of the Sirius star system showed that the mass of this strange star was approximately the same as that of the Sun. This implied that Sirius B must be hundreds of times denser than lead, something that could not be explained hydrostatically. . The mystery remained unsolved for quite some time, with Sirius B being considered a rarity impossible to explain.Walter Adams announced in 1915 that he had discovered that the spectrum of Sirius B was similar to that of its companion.
The first white dwarfs discovered after the classic three were in the 1930s . In 1939, 18 white dwarfs were discovered. Several scientists, including Luyten, continued to search for white dwarfs in the 940s. By 1950, about a hundred white dwarfs were known, and by 1999, the number was already around 2,000. Since then, the Sloan Digital Sky Survey has found 9,000 new white dwarfs.
Composition
They are composed of atoms in a plasma state; Since thermonuclear reactions no longer take place in its core, the star does not have any source of energy to balance the gravitational collapse, so the white dwarf is compressing on itself due to its own weight. The distance between the atoms within it decreases radically, so the electrons have less room to move (in other words, the density increases a lot, up to orders of 106 g / cm3, several tons per cubic centimeter).
At these densities the Heisenberg indeterminacy principle and the Pauli exclusion principle come into play for electrons, who are forced to move at very high speeds, generating the so-called electronic degeneracy pressure, which is the one that effectively opposes the collapse of the star. This electron degeneracy pressure is a radically different phenomenon from thermal pressure, which is what generally maintains “normal stars.” The densities mentioned are so enormous that a mass similar to that of the Sun would fit in a volume like that of the Earth., and are only surpassed by the densities of neutron stars and black holes. White dwarfs emit only stored thermal energy, and therefore have very weak luminosities.
Low and intermediate mass stars (masses less than 8-10 solar masses), upon completion of hydrogen fusion during their life in the main sequence, expand like red giants, and proceed to fuse helium into carbon and oxygen in their nucleus. . If the red giant does not have enough temperature to then fuse carbon and oxygen in turn , its nucleus is compressed by gravity and its envelope is expelled in a series of thermal pulses during the giant phase in the asymptotic branch, producing thus a planetary nebula that surrounds a stellar remnant: the white dwarf.
99% of white dwarfs are basically made up of carbon and oxygen, which are the residues of helium fusion. However, on the surface is a layer of compressed and partially degenerated hydrogen and helium, which forms the atmosphere of the white dwarf. Only a few are made up entirely of helium as it has not been burned, or of oxygen, neon and magnesium, products of carbon combustion.
Newly formed, white dwarfs have very high temperatures, but since they do not produce energy, they gradually cool down. In theory, white dwarfs will cool over time until they no longer emit detectable radiation, then become black dwarfs. However, the cooling process is so slow that the age of the universe since the Big Bang is too short to host one of these black dwarfs at this time. In fact, the coldest known white dwarfs have temperatures of several thousand K. The term white dwarf was coined by Willem Luyten in 1922, although the most appropriate name for objects of this nature is degenerate stars.
Training
The origin of these bodies is progressive and smooth. In mature stars the outermost layers are greatly expanded in their transformations to stars of the giant asymptotic branch and little by little they detach from their exhausted nucleus. When the fusion reactions end, the nucleus contracts and heats up, although without reaching the ignition temperature of the next phase. Before reaching this temperature, the electrons degenerate and stop the process. Thus a white dwarf is formed with a starting temperature in its core of between 100 and 200 million degrees that will gradually cool down. The detached material will, in turn, form a planetary nebula in the center of which will be the white dwarf.
The white dwarf, once formed, gradually cools and fades, from an intense blue color it will turn to a reddish color, and then it will go to infrared, over time the temperature will equalize with the background radiation of the universe until, hypothetically, it ends. being a black dwarf, and wandering through space indefinitely. To become aware of the slowness of the cooling of white dwarfs, it should be borne in mind that the universe continues to expand, and it is estimated that in a matter of 1019 to 1020 years, the galaxies will vanish, since the stars from which they are formed will disperse through intergalactic space. Well, it is thought that white dwarfs will survive this fact,
The lifespan of a white dwarf is believed to be similar to the proton’s half-life, which is estimated to be 1032 to 1049 years according to some grand unification theories. If these theories were wrong, the proton would have to decay through complex nuclear processes, or forming virtual black holes through quantum gravity processes, and in this case the proton’s half-life would be around 10,200 years. If we take as true that protons disintegrate, the mass of the white dwarf would decrease very slowly due to the disintegration of its atomic nuclei, until it reached the point where it would become a piece of non-degenerate matter, and finally it would disappear. completely.
Radiation and cooling
White dwarfs emit a broad spectrum of visible radiation, ranging from an intense blue corresponding to main sequence O-type stars to M-type red dwarfs. The surface temperature of white dwarfs, that is, their effective temperature, It ranges from 150,000 K to temperatures below 4,000 K. According to the Stefan-Boltzmann law, higher luminosity implies higher surface temperature, so this range of surface temperatures corresponds to a luminosity of 100 times that of the Sun. , up to one ten thousandth of it (1 / 10,000).
The hottest white dwarfs, whose surface temperature exceeds 30,000 K, are sources of soft X-rays (longer wavelengths, closer to the ultraviolet band), that is, lower energy. This allows, by observing ultraviolet rays and X-rays, to obtain information about the composition and structure of the atmospheres of white dwarfs, and thus be able to be studied in depth.
The radiation from a white dwarf comes from stored thermal energy, unless it accretes mass from a companion or some other source. Because they have such a small surface area, heat radiates very slowly, so they stay warm for a long period. As a white dwarf cools, the surface temperature drops, the spectrum of radiation shifts towards a reddish color, and the luminosity decreases, and since it has no other type of energy sink than radiation, it follows that with time is cooling more slowly.
For example, Bergeron, Ruiz, and Leggett estimated that a 0.59 solar mass carbon white dwarf with a hydrogen atmosphere had cooled to a surface temperature of 7,140 K in about 1.5 billion years. However, they calculated that for it to cool about 500 more kelvin (up to 6,590 K), it would take 0.3 billion years, but if we repeat the process two more times (up to 6,030 K and 5,550 K), it would take 0.4 and 1.1 billion years respectively. [56] Most of the observed white dwarfs have a relatively high surface temperature, between 8,000 K and 40,000 K. As they cool down more slowly, they spend most of their lives in cold temperatures, so when observing the universe, the logical thing would be that we find more cold white dwarfs than hot ones. This seems to be the case, but this trend slows down when temperatures are extremely cold.
Only a few white dwarfs have been observed below 4,000 K, and one of the coldest observed is WD 0346 + 246, with an approximate surface temperature of 3,900 K. This is explained by the fact that the age of the universe is finite, and they have not had time to cool down below those temperatures. A practical consequence of this is that the white dwarf luminosity function can be used to calculate the age of stars in a given region of space.
Over time, white dwarfs will cool to such an extent that they will stop radiating and turn into black dwarfs, approaching the temperature of their surroundings and matching the microwave background radiation. However, at present, and due to the young age of the universe, there are no indications of the existence of black dwarfs.
Atmosphere
Although most white dwarfs are composed of oxygen and carbon, spectroscopy of the emitted light reveals that their atmosphere is composed almost entirely of either hydrogen or helium, and this dominant element is about 1,000 times more abundant. in the atmosphere than others. The explanation for this fact was provided by Évry Schatzman in the 1940s, who stated that the high surface gravity separated the elements, attracting heavy elements more strongly towards their center, leaving the lighter ones on the surface.
The atmosphere, the only part of the white dwarfs that we can observe, is the upper part of a residue of the giant asymptotic branch phase, and may contain material obtained from the interstellar medium. It has been calculated that an atmosphere rich in helium has a mass of approximately 1% of the total mass of the star, and an atmosphere composed of hydrogen, 0.01% of the total.
Despite the fraction it represents, this outer layer determines the thermal evolution of the white dwarf; degenerate electrons conduct heat well, so the mass of the white dwarf is almost isothermal: a surface temperature between 8,000 K and 16,000 K corresponds to a core temperature between 5,000,000 K and 20,000,000 K. The opacity at radiation from the outer layers is a measure of white dwarfs that allows them to cool more slowly.
White dwarfs of the DA type, which are characterized by having hydrogen-rich atmospheres, make up 80% of the white dwarfs analyzed spectroscopically. The vast majority of the other types (DB, DC, DO, DZ) have helium-rich atmospheres. Only a small fraction of white dwarfs, about 0.1%, have atmospheres in which the main element is carbon (type DQ).
Assuming no carbon or metals, the spectral type depends exclusively on the effective temperature. Between approximately 45,000 K and 100,000 K the most abundant spectrum would be the DO, characterized by ionized helium. Between 12,000 K and 30,000 K, the helium lines would stand out, and would be classified as DB. Below 12,000 K, the spectrum is continuous and is classified as DC. It is not clear why DB white dwarfs are scarce, with effective temperatures between 30,000 K and 45,000 K. One hypothesis suggests that it is due to atmospheric evolutionary processes, such as gravitational separation and convective mixing.
Magnetic field
In 1947, PMS Blackett predicted that white dwarfs should possess magnetic fields with a surface strength of approximately 1 million gauss (100 teslas), as a consequence of a physical law that he himself proposed, which stated that a rotating body and no charge should generate a magnetic field proportional to its angular momentum. This theory is called gravitational magnetism, also known as the Blackett effect, which has never been observed or generally accepted by the scientific community. A few years later, in the 1950s, the Blackett effect was disproved.
In the 1960s, another theory was proposed that white dwarfs possess such magnetic fields because the surface magnetic flux had to be conserved during the evolution from a non-degenerative star to a white dwarf. A magnetic field on the surface of the parent star of 100 gauss (0.01 tesla) would thus become a field of 100 · 1002 = 1 million gauss (100 T) if the radius is reduced by 100 times in size.
The first white dwarf whose magnetic field is known is GJ 742, in 1970 it was detected that the star had a magnetic field from the emission of circularly polarized light. The magnetic field strength on its surface is estimated to be 300 million gauss (30 kT). [65] Since then, magnetic fields have been discovered in more than 100 white dwarfs, the lowest value being 2 × 103 gauss (0.2 T), and the highest 109 (100 kT). Only the magnetic field of a small number of white dwarfs has been calculated, and it is estimated that at least 10% of white dwarfs have fields greater than 1 million gauss (100T).