Daylight is the combination of all direct and indirect sunlight during the daytime . This includes direct sunlight , diffuse sky radiation , and (often) both of these reflected by Earth and terrestrial objects, like landforms and buildings. Sunlight scattered or reflected by astronomical objects is generally not considered daylight. Therefore, daylight excludes moonlight , despite it being reflected indirect sunlight.
161-638: Daylight is present at a particular location, to some degree, whenever the Sun is above the local horizon . This is true for slightly more than 50% of the Earth at any given time, since the Earth's atmosphere refracts some sunlight even when the Sun is below the horizon. Outdoor illuminance varies from 120,000 lux for direct sunlight at noon , which may cause eye pain , to less than 5 lux for thick storm clouds with
322-730: A carbon white dwarf of 0.59 M ☉ with a hydrogen atmosphere. After initially taking approximately 1.5 billion years to cool to a surface temperature of 7140 K, cooling approximately 500 more kelvins to 6590 K takes around 0.3 billion years, but the next two steps of around 500 kelvins (to 6030 K and 5550 K) take first 0.4 and then 1.1 billion years. Most observed white dwarfs have relatively high surface temperatures, between 8000 K and 40 000 K . A white dwarf, though, spends more of its lifetime at cooler temperatures than at hotter temperatures, so we should expect that there are more cool white dwarfs than hot white dwarfs. Once we adjust for
483-506: A band of lowest-available energy states, the Fermi sea . This state of the electrons, called degenerate , meant that a white dwarf could cool to zero temperature and still possess high energy. Compression of a white dwarf will increase the number of electrons in a given volume. Applying the Pauli exclusion principle, this will increase the kinetic energy of the electrons, thereby increasing
644-483: A central subject for astronomical research since antiquity . The Sun orbits the Galactic Center at a distance of 24,000 to 28,000 light-years . From Earth, it is 1 astronomical unit ( 1.496 × 10 km ) or about 8 light-minutes away. Its diameter is about 1,391,400 km ( 864,600 mi ), 109 times that of Earth. Its mass is about 330,000 times that of Earth, making up about 99.86% of
805-514: A companion star or other source, its radiation comes from its stored heat, which is not replenished. White dwarfs have an extremely small surface area to radiate this heat from, so they cool gradually, remaining hot for a long time. As a white dwarf cools, its surface temperature decreases, the radiation that it emits reddens, and its luminosity decreases. Since the white dwarf has no energy sink other than radiation, it follows that its cooling slows with time. The rate of cooling has been estimated for
966-611: A consequence of a physical law he had proposed, which stated that an uncharged, rotating body should generate a magnetic field proportional to its angular momentum . This putative law, sometimes called the Blackett effect , was never generally accepted, and by the 1950s even Blackett felt it had been refuted. In the 1960s, it was proposed that white dwarfs might have magnetic fields due to conservation of total surface magnetic flux that existed in its progenitor star phase. A surface magnetic field of c. 100 gauss (0.01 T) in
1127-447: A density of between 10 and 10 g/cm . White dwarfs are composed of one of the densest forms of matter known, surpassed only by other compact stars such as neutron stars and the hypothetical quark stars . White dwarfs were found to be extremely dense soon after their discovery. If a star is in a binary system, as is the case for Sirius B or 40 Eridani B, it is possible to estimate its mass from observations of
1288-412: A density of over 25 000 times that of the Sun , which was so high that he called it "impossible". As Arthur Eddington put it later, in 1927: We learn about the stars by receiving and interpreting the messages which their light brings to us. The message of the companion of Sirius when it was decoded ran: "I am composed of material 3000 times denser than anything you have ever come across;
1449-515: A distance of one astronomical unit (AU) from the Sun (that is, at or near Earth's orbit). Sunlight on the surface of Earth is attenuated by Earth's atmosphere , so that less power arrives at the surface (closer to 1,000 W/m ) in clear conditions when the Sun is near the zenith . Sunlight at the top of Earth's atmosphere is composed (by total energy) of about 50% infrared light, 40% visible light, and 10% ultraviolet light. The atmosphere filters out over 70% of solar ultraviolet, especially at
1610-403: A fairly small amount of power being generated per cubic metre . Theoretical models of the Sun's interior indicate a maximum power density, or energy production, of approximately 276.5 watts per cubic metre at the center of the core, which, according to Karl Kruszelnicki , is about the same power density inside a compost pile . The fusion rate in the core is in a self-correcting equilibrium:
1771-414: A few millimeters. Re-emission happens in a random direction and usually at slightly lower energy. With this sequence of emissions and absorptions, it takes a long time for radiation to reach the Sun's surface. Estimates of the photon travel time range between 10,000 and 170,000 years. In contrast, it takes only 2.3 seconds for neutrinos , which account for about 2% of the total energy production of
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#17327730805471932-401: A granular appearance called the solar granulation at the smallest scale and supergranulation at larger scales. Turbulent convection in this outer part of the solar interior sustains "small-scale" dynamo action over the near-surface volume of the Sun. The Sun's thermal columns are Bénard cells and take the shape of roughly hexagonal prisms. The visible surface of the Sun, the photosphere,
2093-413: A helium white dwarf may form by mass loss in an interacting binary star system. Because the material in a white dwarf no longer undergoes fusion reactions, it lacks a heat source to support it against gravitational collapse . Instead, it is supported only by electron degeneracy pressure , causing it to be extremely dense. The physics of degeneracy yields a maximum mass for a non-rotating white dwarf,
2254-415: A high color temperature , will lessen and redden with time. Over a very long time, a white dwarf will cool enough that its material will begin to crystallize, starting with the core. The star's low temperature means it will no longer emit significant heat or light, and it will become a cold black dwarf . Because the length of time it takes for a white dwarf to reach this state is calculated to be longer than
2415-408: A hydrogen or mixed hydrogen-helium atmosphere. This makes old white dwarfs with this kind of atmosphere bluer than the main cooling sequence. Hence these white dwarfs are called IR-faint white dwarfs . White dwarfs with hydrogen-poor atmospheres, such as WD J2147–4035, are less affected by CIA and therefore have a yellow to orange color. White dwarf core material is a completely ionized plasma –
2576-404: A limiting mass that no white dwarf can exceed without collapsing to a neutron star is another consequence of being supported by electron degeneracy pressure. Such limiting masses were calculated for cases of an idealized, constant density star in 1929 by Wilhelm Anderson and in 1930 by Edmund C. Stoner . This value was corrected by considering hydrostatic equilibrium for the density profile, and
2737-428: A luminosity from over 100 times that of the Sun to under 1 ⁄ 10 000 that of the Sun. Hot white dwarfs, with surface temperatures in excess of 30 000 K , have been observed to be sources of soft (i.e., lower-energy) X-rays . This enables the composition and structure of their atmospheres to be studied by soft X-ray and extreme ultraviolet observations . White dwarfs also radiate neutrinos through
2898-405: A match for the possible quantum states available to that electron, hence radiative heat transfer within a white dwarf is low; it does, however, have a high thermal conductivity . As a result, the interior of the white dwarf maintains an almost uniform temperature as it cools down, starting at approximately 10 K shortly after the formation of the white dwarf and reaching less than 10 K for
3059-416: A mixture of nuclei and electrons – that is initially in a fluid state. It was theoretically predicted in the 1960s that at a late stage of cooling, it should crystallize into a solid state, starting at its center. The crystal structure is thought to be a body-centered cubic lattice. In 1995 it was suggested that asteroseismological observations of pulsating white dwarfs yielded a potential test of
3220-441: A non-radiating black dwarf in approximate thermal equilibrium with its surroundings and with the cosmic background radiation . No black dwarfs are thought to exist yet. At very low temperatures (<4000 K) white dwarfs with hydrogen in their atmosphere will be affected by collision induced absoption (CIA) of hydrogen molecules colliding with helium atoms. This affects the optical red and infrared brightness of white dwarfs with
3381-520: A period known as the Maunder minimum . This coincided in time with the era of the Little Ice Age , when Europe experienced unusually cold temperatures. Earlier extended minima have been discovered through analysis of tree rings and appear to have coincided with lower-than-average global temperatures. The temperature of the photosphere is approximately 6,000 K, whereas the temperature of
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#17327730805473542-485: A phenomenon described by Hale's law . During the solar cycle's declining phase, energy shifts from the internal toroidal magnetic field to the external poloidal field, and sunspots diminish in number and size. At solar-cycle minimum, the toroidal field is, correspondingly, at minimum strength, sunspots are relatively rare, and the poloidal field is at its maximum strength. With the rise of the next 11-year sunspot cycle, differential rotation shifts magnetic energy back from
3703-531: A position on the Hertzsprung–Russell diagram between the asymptotic giant branch and the white dwarf region. They may be called pre-white dwarfs . These variables all exhibit small (1%–30%) variations in light output, arising from a superposition of vibrational modes with periods of hundreds to thousands of seconds. Observation of these variations gives asteroseismological evidence about the interiors of white dwarfs. White dwarfs are thought to represent
3864-527: A remnant white dwarf composed chiefly of oxygen , neon, and magnesium , provided that its core does not collapse, and provided that fusion does not proceed so violently as to blow apart the star in a supernova . Although a few white dwarfs have been identified that may be of this type, most evidence for the existence of such comes from the novae called ONeMg or neon novae. The spectra of these novae exhibit abundances of neon, magnesium, and other intermediate-mass elements that appear to be only explicable by
4025-473: A result, the outward-flowing solar wind stretches the interplanetary magnetic field outward, forcing it into a roughly radial structure. For a simple dipolar solar magnetic field, with opposite hemispherical polarities on either side of the solar magnetic equator, a thin current sheet is formed in the solar wind. At great distances, the rotation of the Sun twists the dipolar magnetic field and corresponding current sheet into an Archimedean spiral structure called
4186-598: A shell that then ignites the core. The dominant mechanism by which type Ia supernovae are produced remains unclear. Despite this uncertainty in how type Ia supernovae are produced, type Ia supernovae have very uniform properties and are useful standard candles over intergalactic distances. Some calibrations are required to compensate for the gradual change in properties or different frequencies of abnormal luminosity supernovae at high redshift, and for small variations in brightness identified by light curve shape or spectrum. White dwarfs have low luminosity and therefore occupy
4347-410: A slightly higher rate of fusion would cause the core to heat up more and expand slightly against the weight of the outer layers, reducing the density and hence the fusion rate and correcting the perturbation ; and a slightly lower rate would cause the core to cool and shrink slightly, increasing the density and increasing the fusion rate and again reverting it to its present rate. The radiative zone
4508-465: A spectrum by a symbol that consists of an initial D, a letter describing the primary feature of the spectrum followed by an optional sequence of letters describing secondary features of the spectrum (as shown in the adjacent table), and a temperature index number, computed by dividing 50 400 K by the effective temperature . For example, a white dwarf with only He I lines in its spectrum and an effective temperature of 15 000 K could be given
4669-508: A star sheds its outer layers and forms a planetary nebula , it will leave behind a core, which is the remnant white dwarf. Usually, white dwarfs are composed of carbon and oxygen ( CO white dwarf ). If the mass of the progenitor is between 7 and 9 solar masses ( M ☉ ), the core temperature will be sufficient to fuse carbon but not neon , in which case an oxygen–neon– magnesium ( ONeMg or ONe ) white dwarf may form. Stars of very low mass will be unable to fuse helium; hence,
4830-499: A star will have a carbon–oxygen core that does not undergo fusion reactions, surrounded by an inner helium-burning shell and an outer hydrogen-burning shell. On the Hertzsprung–Russell diagram, it will be found on the asymptotic giant branch. It will then expel most of its outer material, creating a planetary nebula , until only the carbon–oxygen core is left. This process is responsible for the carbon–oxygen white dwarfs that form
4991-424: A star, leading to the commonly quoted value of 1.4 M ☉ . (Near the beginning of the 20th century, there was reason to believe that stars were composed chiefly of heavy elements, so, in his 1931 paper, Chandrasekhar set the average molecular weight per electron, μ e , equal to 2.5, giving a limit of 0.91 M ☉ .) Together with William Alfred Fowler , Chandrasekhar received
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5152-458: A strip at the bottom of the Hertzsprung–Russell diagram , a graph of stellar luminosity versus color or temperature. They should not be confused with low-luminosity objects at the low-mass end of the main sequence, such as the hydrogen-fusing red dwarfs , whose cores are supported in part by thermal pressure, or the even lower-temperature brown dwarfs . The relationship between the mass and radius of low-mass white dwarfs can be estimated using
5313-515: A ton of my material would be a little nugget that you could put in a matchbox." What reply can one make to such a message? The reply which most of us made in 1914 was — "Shut up. Don't talk nonsense." As Eddington pointed out in 1924, densities of this order implied that, according to the theory of general relativity , the light from Sirius B should be gravitationally redshifted . This was confirmed when Adams measured this redshift in 1925. Such densities are possible because white dwarf material
5474-406: A transition layer, the tachocline . This is a region where the sharp regime change between the uniform rotation of the radiative zone and the differential rotation of the convection zone results in a large shear between the two—a condition where successive horizontal layers slide past one another. Presently, it is hypothesized that a magnetic dynamo, or solar dynamo , within this layer generates
5635-439: A white dwarf, which is For a more accurate computation of the mass-radius relationship and limiting mass of a white dwarf, one must compute the equation of state that describes the relationship between density and pressure in the white dwarf material. If the density and pressure are both set equal to functions of the radius from the center of the star, the system of equations consisting of the hydrostatic equation together with
5796-488: A wide color range, from the whitish-blue color of an O-, B- or A-type main sequence star to the yellow-orange of a late K- or early M-type star. White dwarf effective surface temperatures extend from over 150 000 K to barely under 4000 K. In accordance with the Stefan–Boltzmann law , luminosity increases with increasing surface temperature (proportional to T ); this surface temperature range corresponds to
5957-430: Is DA have hydrogen-dominated atmospheres. They make up the majority, approximately 80%, of all observed white dwarfs. The next class in number is of DBs, approximately 16%. The hot, above 15 000 K , DQ class (roughly 0.1%) have carbon-dominated atmospheres. Those classified as DB, DC, DO, DZ, and cool DQ have helium-dominated atmospheres. Assuming that carbon and metals are not present, which spectral classification
6118-413: Is a stellar core remnant composed mostly of electron-degenerate matter . A white dwarf is very dense : its mass is comparable to the Sun 's, while its volume is comparable to Earth 's. No nuclear fusion takes place in a white dwarf. Instead, the light it radiates comes from the residual heat stored in it. The nearest known white dwarf is Sirius B , at 8.6 light years, the smaller component of
6279-520: Is able to reveal the presence of a magnetic field of 1 megagauss or more. Thus the basic identification process also sometimes results in discovery of magnetic fields. It has been estimated that at least 10% of white dwarfs have fields in excess of 1 million gauss (100 T). The magnetic fields in a white dwarf may allow for the existence of a new type of chemical bond , perpendicular paramagnetic bonding , in addition to ionic and covalent bonds , though detecting molecules bonded in this way
6440-591: Is by far the brightest object in the Earth's sky , with an apparent magnitude of −26.74. This is about 13 billion times brighter than the next brightest star, Sirius , which has an apparent magnitude of −1.46. One astronomical unit (about 150 million kilometres; 93 million miles) is defined as the mean distance between the centres of the Sun and the Earth. The instantaneous distance varies by about ± 2.5 million km or 1.55 million miles as Earth moves from perihelion on ~ January 3rd to aphelion on ~ July 4th. At its average distance, light travels from
6601-436: Is defined to begin at the distance where the flow of the solar wind becomes superalfvénic —that is, where the flow becomes faster than the speed of Alfvén waves, at approximately 20 solar radii ( 0.1 AU ). Turbulence and dynamic forces in the heliosphere cannot affect the shape of the solar corona within, because the information can only travel at the speed of Alfvén waves. The solar wind travels outward continuously through
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6762-412: Is expected to be difficult. The highly magnetized white dwarf in the binary system AR Scorpii was identified in 2016 as the first pulsar in which the compact object is a white dwarf instead of a neutron star. A second white dwarf pulsar was discovered in 2023. Early calculations suggested that there might be white dwarfs whose luminosity varied with a period of around 10 seconds, but searches in
6923-402: Is facilitated by the full ionization of helium in the transition region, which significantly reduces radiative cooling of the plasma. The transition region does not occur at a well-defined altitude, but forms a kind of nimbus around chromospheric features such as spicules and filaments , and is in constant, chaotic motion. The transition region is not easily visible from Earth's surface, but
7084-487: Is just these exceptions that lead to an advance in our knowledge", and so the white dwarfs entered the realm of study! The spectral type of 40 Eridani B was officially described in 1914 by Walter Adams . The white dwarf companion of Sirius, Sirius B, was next to be discovered. During the nineteenth century, positional measurements of some stars became precise enough to measure small changes in their location. Friedrich Bessel used position measurements to determine that
7245-438: Is kept from cooling very quickly only by its outer layers' opacity to radiation. The first attempt to classify white dwarf spectra appears to have been by G. P. Kuiper in 1941, and various classification schemes have been proposed and used since then. The system currently in use was introduced by Edward M. Sion , Jesse L. Greenstein and their coauthors in 1983 and has been subsequently revised several times. It classifies
7406-412: Is measured in standard solar radii and mass in standard solar masses. These computations all assume that the white dwarf is non-rotating. If the white dwarf is rotating, the equation of hydrostatic equilibrium must be modified to take into account the centrifugal pseudo-force arising from working in a rotating frame . For a uniformly rotating white dwarf, the limiting mass increases only slightly. If
7567-419: Is no real property of mass. The existence of numberless visible stars can prove nothing against the existence of numberless invisible ones. Bessel roughly estimated the period of the companion of Sirius to be about half a century; C.A.F. Peters computed an orbit for it in 1851. It was not until 31 January 1862 that Alvan Graham Clark observed a previously unseen star close to Sirius, later identified as
7728-455: Is not composed of atoms joined by chemical bonds , but rather consists of a plasma of unbound nuclei and electrons . There is therefore no obstacle to placing nuclei closer than normally allowed by electron orbitals limited by normal matter. Eddington wondered what would happen when this plasma cooled and the energy to keep the atoms ionized was no longer sufficient. This paradox was resolved by R. H. Fowler in 1926 by an application of
7889-409: Is only 84% of what it was in the protostellar phase (before nuclear fusion in the core started). In the future, helium will continue to accumulate in the core, and in about 5 billion years this gradual build-up will eventually cause the Sun to exit the main sequence and become a red giant . The chemical composition of the photosphere is normally considered representative of the composition of
8050-441: Is readily observable from space by instruments sensitive to extreme ultraviolet . The corona is the next layer of the Sun. The low corona, near the surface of the Sun, has a particle density around 10 m to 10 m . The average temperature of the corona and solar wind is about 1,000,000–2,000,000 K; however, in the hottest regions it is 8,000,000–20,000,000 K. Although no complete theory yet exists to account for
8211-431: Is seen depends on the effective temperature. Between approximately 100 000 K to 45 000 K , the spectrum will be classified DO, dominated by singly ionized helium. From 30 000 K to 12 000 K , the spectrum will be DB, showing neutral helium lines, and below about 12 000 K , the spectrum will be featureless and classified DC. Molecular hydrogen ( H 2 ) has been detected in spectra of
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#17327730805478372-410: Is strongly attenuated by Earth's ozone layer , so that the amount of UV varies greatly with latitude and has been partially responsible for many biological adaptations, including variations in human skin color . High-energy gamma ray photons initially released with fusion reactions in the core are almost immediately absorbed by the solar plasma of the radiative zone, usually after traveling only
8533-483: Is suggested by a high abundance of heavy elements in the Solar System, such as gold and uranium , relative to the abundances of these elements in so-called Population II , heavy-element-poor, stars. The heavy elements could most plausibly have been produced by endothermic nuclear reactions during a supernova, or by transmutation through neutron absorption within a massive second-generation star. The Sun
8694-470: Is tens to hundreds of kilometers thick, and is slightly less opaque than air on Earth. Because the upper part of the photosphere is cooler than the lower part, an image of the Sun appears brighter in the center than on the edge or limb of the solar disk, in a phenomenon known as limb darkening . The spectrum of sunlight has approximately the spectrum of a black-body radiating at 5,772 K (9,930 °F), interspersed with atomic absorption lines from
8855-458: Is that the Universe's age is finite; there has not been enough time for white dwarfs to cool below this temperature. The white dwarf luminosity function can therefore be used to find the time when stars started to form in a region; an estimate for the age of our galactic disk found in this way is 8 billion years. A white dwarf will eventually, in many trillions of years, cool and become
9016-437: Is the layer below which the Sun becomes opaque to visible light. Photons produced in this layer escape the Sun through the transparent solar atmosphere above it and become solar radiation, sunlight. The change in opacity is due to the decreasing amount of H ions , which absorb visible light easily. Conversely, the visible light perceived is produced as electrons react with hydrogen atoms to produce H ions. The photosphere
9177-424: Is the most prominent variation in which the number and size of sunspots waxes and wanes. The solar magnetic field extends well beyond the Sun itself. The electrically conducting solar wind plasma carries the Sun's magnetic field into space, forming what is called the interplanetary magnetic field . In an approximation known as ideal magnetohydrodynamics , plasma particles only move along magnetic field lines. As
9338-531: Is the only region of the Sun that produces an appreciable amount of thermal energy through fusion; 99% of the Sun's power is generated in the innermost 24% of its radius, and almost no fusion occurs beyond 30% of the radius. The rest of the Sun is heated by this energy as it is transferred outward through many successive layers, finally to the solar photosphere where it escapes into space through radiation (photons) or advection (massive particles). The proton–proton chain occurs around 9.2 × 10 times each second in
9499-420: Is the thickest layer of the Sun, at 0.45 solar radii. From the core out to about 0.7 solar radii , thermal radiation is the primary means of energy transfer. The temperature drops from approximately 7 million to 2 million kelvins with increasing distance from the core. This temperature gradient is less than the value of the adiabatic lapse rate and hence cannot drive convection, which explains why
9660-440: Is thought that, over a lifespan that considerably exceeds the age of the universe ( c. 13.8 billion years), such a star will eventually burn all its hydrogen, for a while becoming a blue dwarf , and end its evolution as a helium white dwarf composed chiefly of helium-4 nuclei. Due to the very long time this process takes, it is not thought to be the origin of the observed helium white dwarfs. Rather, they are thought to be
9821-403: Is thought to have a surface field of approximately 300 million gauss (30 kT). Since 1970, magnetic fields have been discovered in well over 200 white dwarfs, ranging from 2 × 10 to 10 gauss (0.2 T to 100 kT). The large number of presently known magnetic white dwarfs is due to the fact that most white dwarfs are identified by low-resolution spectroscopy, which
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#17327730805479982-538: Is ultimately related to the word for sun in other branches of the Indo-European language family, though in most cases a nominative stem with an l is found, rather than the genitive stem in n , as for example in Latin sōl , ancient Greek ἥλιος ( hēlios ), Welsh haul and Czech slunce , as well as (with *l > r ) Sanskrit स्वर् ( svár ) and Persian خور ( xvar ). Indeed,
10143-405: Is usually at least 1000 times more abundant than all other elements. As explained by Schatzman in the 1940s, the high surface gravity is thought to cause this purity by gravitationally separating the atmosphere so that heavy elements are below and the lighter above. This atmosphere, the only part of the white dwarf visible to us, is thought to be the top of an envelope that is a residue of
10304-402: Is wave heating, in which sound, gravitational or magnetohydrodynamic waves are produced by turbulence in the convection zone. These waves travel upward and dissipate in the corona, depositing their energy in the ambient matter in the form of heat. The other is magnetic heating, in which magnetic energy is continuously built up by photospheric motion and released through magnetic reconnection in
10465-547: The Alfvén surface , the boundary separating the corona from the solar wind, defined as where the coronal plasma's Alfvén speed and the large-scale solar wind speed are equal. During the flyby, Parker Solar Probe passed into and out of the corona several times. This proved the predictions that the Alfvén critical surface is not shaped like a smooth ball, but has spikes and valleys that wrinkle its surface. The Sun emits light across
10626-580: The Chandrasekhar limit — approximately 1.44 times M ☉ — beyond which electron degeneracy pressure cannot support it. A carbon–oxygen white dwarf that approaches this mass limit, typically by mass transfer from a companion star, may explode as a Type Ia supernova via a process known as carbon detonation ; SN 1006 is a likely example. A white dwarf is very hot when it forms, and it will gradually cool as it radiates its energy away. This means that its radiation, which initially has
10787-473: The DAV , or ZZ Ceti , stars, including HL Tau 76, with hydrogen-dominated atmospheres and the spectral type DA; DBV , or V777 Her , stars, with helium-dominated atmospheres and the spectral type DB; and GW Vir stars , sometimes subdivided into DOV and PNNV stars, with atmospheres dominated by helium, carbon, and oxygen. GW Vir stars are not, strictly speaking, white dwarfs, but are stars that are in
10948-556: The Nobel Prize for this and other work in 1983. The limiting mass is now called the Chandrasekhar limit . If a carbon-oxygen white dwarf accreted enough matter to reach the Chandrasekhar limit of about 1.44 solar masses (for a non-rotating star), it would no longer be able to support the bulk of its mass through electron degeneracy pressure and, in the absence of nuclear reactions, would begin to collapse. However,
11109-524: The Parker spiral . Sunspots are visible as dark patches on the Sun's photosphere and correspond to concentrations of magnetic field where convective transport of heat is inhibited from the solar interior to the surface. As a result, sunspots are slightly cooler than the surrounding photosphere, so they appear dark. At a typical solar minimum , few sunspots are visible, and occasionally none can be seen at all. Those that do appear are at high solar latitudes. As
11270-574: The Solar System , see sunlight . Sun The Sun is the star at the center of the Solar System . It is a massive, nearly perfect sphere of hot plasma , heated to incandescence by nuclear fusion reactions in its core, radiating the energy from its surface mainly as visible light and infrared radiation with 10% at ultraviolet energies. It is by far the most important source of energy for life on Earth . The Sun has been an object of veneration in many cultures. It has been
11431-473: The Urca process . This process has more effect on hotter and younger white dwarfs. Because neutrinos can pass easily through stellar plasma, they can drain energy directly from the dwarf's interior; this mechanism is the dominant contribution to cooling for approximately the first 20 million years of a white dwarf's existence. As was explained by Leon Mestel in 1952, unless the white dwarf accretes matter from
11592-410: The corona , and the heliosphere . The coolest layer of the Sun is a temperature minimum region extending to about 500 km above the photosphere, and has a temperature of about 4,100 K . This part of the Sun is cool enough to allow for the existence of simple molecules such as carbon monoxide and water. The chromosphere, transition region, and corona are much hotter than the surface of
11753-531: The gravitational collapse of matter within a region of a large molecular cloud . Most of this matter gathered in the center, whereas the rest flattened into an orbiting disk that became the Solar System . The central mass became so hot and dense that it eventually initiated nuclear fusion in its core . Every second, the Sun's core fuses about 600 billion kilograms (kg) of hydrogen into helium and converts 4 billion kg of matter into energy . About 4 to 7 billion years from now, when hydrogen fusion in
11914-614: The l -stem survived in Proto-Germanic as well, as * sōwelan , which gave rise to Gothic sauil (alongside sunnō ) and Old Norse prosaic sól (alongside poetic sunna ), and through it the words for sun in the modern Scandinavian languages: Swedish and Danish sol , Icelandic sól , etc. The principal adjectives for the Sun in English are sunny for sunlight and, in technical contexts, solar ( / ˈ s oʊ l ər / ), from Latin sol . From
12075-428: The photosphere . For the purpose of measurement, the Sun's radius is considered to be the distance from its center to the edge of the photosphere, the apparent visible surface of the Sun. By this measure, the Sun is a near-perfect sphere with an oblateness estimated at 9 millionths, which means that its polar diameter differs from its equatorial diameter by only 10 kilometers (6.2 mi). The tidal effect of
12236-484: The radius of the Sun ; this is comparable to the Earth's radius of approximately 0.9% solar radius. A white dwarf, then, packs mass comparable to the Sun's into a volume that is typically a million times smaller than the Sun's; the average density of matter in a white dwarf must therefore be, very roughly, 1 000 000 times greater than the average density of the Sun, or approximately 10 g/cm , or 1 tonne per cubic centimetre. A typical white dwarf has
12397-454: The selection effect that hotter, more luminous white dwarfs are easier to observe, we do find that decreasing the temperature range examined results in finding more white dwarfs. This trend stops when we reach extremely cool white dwarfs; few white dwarfs are observed with surface temperatures below 4000 K , and one of the coolest so far observed, WD J2147–4035 , has a surface temperature of approximately 3050 K. The reason for this
12558-444: The visible spectrum , so its color is white , with a CIE color-space index near (0.3, 0.3), when viewed from space or when the Sun is high in the sky. The Solar radiance per wavelength peaks in the green portion of the spectrum when viewed from space. When the Sun is very low in the sky, atmospheric scattering renders the Sun yellow, red, orange, or magenta, and in rare occasions even green or blue . Some cultures mentally picture
12719-597: The 1940s. By 1950, over a hundred were known, and by 1999, over 2000 were known. Since then the Sloan Digital Sky Survey has found over 9000 white dwarfs, mostly new. Although white dwarfs are known with estimated masses as low as 0.17 M ☉ and as high as 1.33 M ☉ , the mass distribution is strongly peaked at 0.6 M ☉ , and the majority lie between 0.5 and 0.7 M ☉ . The estimated radii of observed white dwarfs are typically 0.8–2%
12880-436: The 1960s failed to observe this. The first variable white dwarf found was HL Tau 76 ; in 1965 and 1966, and was observed to vary with a period of approximately 12.5 minutes. The reason for this period being longer than predicted is that the variability of HL Tau 76, like that of the other pulsating variable white dwarfs known, arises from non-radial gravity wave pulsations. Known types of pulsating white dwarf include
13041-478: The CNO cycle may keep these white dwarfs hot on a long timescale. In addition, they remain in a bloated proto-white dwarf stage for up to 2 Gyr before they reach the cooling track. Although most white dwarfs are thought to be composed of carbon and oxygen, spectroscopy typically shows that their emitted light comes from an atmosphere that is observed to be either hydrogen or helium dominated. The dominant element
13202-437: The Earth, and hence white dwarfs. Willem Luyten appears to have been the first to use the term white dwarf when he examined this class of stars in 1922; the term was later popularized by Arthur Eddington . Despite these suspicions, the first non-classical white dwarf was not definitely identified until the 1930s. 18 white dwarfs had been discovered by 1939. Luyten and others continued to search for white dwarfs in
13363-465: The Greek helios comes the rare adjective heliac ( / ˈ h iː l i æ k / ). In English, the Greek and Latin words occur in poetry as personifications of the Sun, Helios ( / ˈ h iː l i ə s / ) and Sol ( / ˈ s ɒ l / ), while in science fiction Sol may be used to distinguish the Sun from other stars. The term sol with a lowercase s is used by planetary astronomers for
13524-493: The Sirius binary star . There are currently thought to be eight white dwarfs among the hundred star systems nearest the Sun. The unusual faintness of white dwarfs was first recognized in 1910. The name white dwarf was coined by Willem Jacob Luyten in 1922. White dwarfs are thought to be the final evolutionary state of stars whose mass is not high enough to become a neutron star or black hole . This includes over 97% of
13685-446: The Solar System . Long-term secular change in sunspot number is thought, by some scientists, to be correlated with long-term change in solar irradiance, which, in turn, might influence Earth's long-term climate. The solar cycle influences space weather conditions, including those surrounding Earth. For example, in the 17th century, the solar cycle appeared to have stopped entirely for several decades; few sunspots were observed during
13846-443: The Sun as yellow and some even red; the cultural reasons for this are debated. The Sun is classed as a G2 star, meaning it is a G-type star , with 2 indicating its surface temperature is in the second range of the G class. The solar constant is the amount of power that the Sun deposits per unit area that is directly exposed to sunlight. The solar constant is equal to approximately 1,368 W/m (watts per square meter) at
14007-425: The Sun at the horizon (even <1 lux for the most extreme case), which may make shadows from distant street lights visible. It may be darker under unusual circumstances like a solar eclipse or very high levels of atmospheric particulates , which include smoke (see New England's Dark Day ), dust, and volcanic ash. For comparison, nighttime illuminance levels are: For a table of approximate daylight intensity in
14168-424: The Sun extends from the center to about 20–25% of the solar radius. It has a density of up to 150 g/cm (about 150 times the density of water) and a temperature of close to 15.7 million kelvin (K). By contrast, the Sun's surface temperature is about 5800 K . Recent analysis of SOHO mission data favors the idea that the core is rotating faster than the radiative zone outside it. Through most of
14329-688: The Sun will shed its outer layers and become a dense type of cooling star (a white dwarf ), and no longer produce energy by fusion, but will still glow and give off heat from its previous fusion for perhaps trillions of years. After that, it is theorized to become a super dense black dwarf , giving off negligible energy. The English word sun developed from Old English sunne . Cognates appear in other Germanic languages , including West Frisian sinne , Dutch zon , Low German Sünn , Standard German Sonne , Bavarian Sunna , Old Norse sunna , and Gothic sunnō . All these words stem from Proto-Germanic * sunnōn . This
14490-413: The Sun's magnetic field . The Sun's convection zone extends from 0.7 solar radii (500,000 km) to near the surface. In this layer, the solar plasma is not dense or hot enough to transfer the heat energy of the interior outward via radiation. Instead, the density of the plasma is low enough to allow convective currents to develop and move the Sun's energy outward towards its surface. Material heated at
14651-398: The Sun's core by radiation rather than by convection (see Radiative zone below), so the fusion products are not lifted outward by heat; they remain in the core, and gradually an inner core of helium has begun to form that cannot be fused because presently the Sun's core is not hot or dense enough to fuse helium. In the current photosphere, the helium fraction is reduced, and the metallicity
14812-426: The Sun's core diminishes to the point where the Sun is no longer in hydrostatic equilibrium , its core will undergo a marked increase in density and temperature which will cause its outer layers to expand, eventually transforming the Sun into a red giant . This process will make the Sun large enough to render Earth uninhabitable approximately five billion years from the present. After the red giant phase, models suggest
14973-403: The Sun's horizon to Earth's horizon in about 8 minutes and 20 seconds, while light from the closest points of the Sun and Earth takes about two seconds less. The energy of this sunlight supports almost all life on Earth by photosynthesis , and drives Earth's climate and weather. The Sun does not have a definite boundary, but its density decreases exponentially with increasing height above
15134-450: The Sun's life, energy has been produced by nuclear fusion in the core region through the proton–proton chain ; this process converts hydrogen into helium. Currently, 0.8% of the energy generated in the Sun comes from another sequence of fusion reactions called the CNO cycle ; the proportion coming from the CNO cycle is expected to increase as the Sun becomes older and more luminous. The core
15295-551: The Sun's life, they account for 74.9% and 23.8%, respectively, of the mass of the Sun in the photosphere. All heavier elements, called metals in astronomy, account for less than 2% of the mass, with oxygen (roughly 1% of the Sun's mass), carbon (0.3%), neon (0.2%), and iron (0.2%) being the most abundant. The Sun's original chemical composition was inherited from the interstellar medium out of which it formed. Originally it would have been about 71.1% hydrogen, 27.4% helium, and 1.5% heavier elements. The hydrogen and most of
15456-438: The Sun, to reach the surface. Because energy transport in the Sun is a process that involves photons in thermodynamic equilibrium with matter , the time scale of energy transport in the Sun is longer, on the order of 30,000,000 years. This is the time it would take the Sun to return to a stable state if the rate of energy generation in its core were suddenly changed. Electron neutrinos are released by fusion reactions in
15617-402: The Sun. The reason is not well understood, but evidence suggests that Alfvén waves may have enough energy to heat the corona. Above the temperature minimum layer is a layer about 2,000 km thick, dominated by a spectrum of emission and absorption lines. It is called the chromosphere from the Greek root chroma , meaning color, because the chromosphere is visible as a colored flash at
15778-451: The absolute luminosity and distance, the star's surface area and its radius can be calculated. Reasoning of this sort led to the realization, puzzling to astronomers at the time, that due to their relatively high temperature and relatively low absolute luminosity, Sirius B and 40 Eridani B must be very dense. When Ernst Öpik estimated the density of a number of visual binary stars in 1916, he found that 40 Eridani B had
15939-442: The accretion of material onto an oxygen–neon–magnesium white dwarf. Type Iax supernovae , that involve helium accretion by a white dwarf, have been proposed to be a channel for transformation of this type of stellar remnant. In this scenario, the carbon detonation produced in a Type Ia supernova is too weak to destroy the white dwarf, expelling just a small part of its mass as ejecta, but produces an asymmetric explosion that kicks
16100-433: The atmospheres of some white dwarfs. Around 25–33% of white dwarfs have metal lines in their spectra, which is notable because any heavy elements in a white dwarf should sink into the star's interior in just a small fraction of the star's lifetime. The prevailing explanation for metal-rich white dwarfs is that they have recently accreted rocky planetesimals . The bulk composition of the accreted object can be measured from
16261-486: The beginning and end of total solar eclipses. The temperature of the chromosphere increases gradually with altitude, ranging up to around 20,000 K near the top. In the upper part of the chromosphere helium becomes partially ionized . Above the chromosphere, in a thin (about 200 km ) transition region, the temperature rises rapidly from around 20,000 K in the upper chromosphere to coronal temperatures closer to 1,000,000 K . The temperature increase
16422-457: The binary orbit. This was done for Sirius B by 1910, yielding a mass estimate of 0.94 M ☉ , which compares well with a more modern estimate of 1.00 M ☉ . Since hotter bodies radiate more energy than colder ones, a star's surface brightness can be estimated from its effective surface temperature , and that from its spectrum . If the star's distance is known, its absolute luminosity can also be estimated. From
16583-461: The classification of DB3, or, if warranted by the precision of the temperature measurement, DB3.5. Likewise, a white dwarf with a polarized magnetic field , an effective temperature of 17 000 K , and a spectrum dominated by He I lines that also had hydrogen features could be given the classification of DBAP3. The symbols "?" and ":" may also be used if the correct classification is uncertain. White dwarfs whose primary spectral classification
16744-424: The collapse. If a white dwarf star accumulates sufficient material from a stellar companion to raise its core temperature enough to ignite carbon fusion , it will undergo runaway nuclear fusion, completely disrupting it. There are three avenues by which this detonation is theorised to happen: stable accretion of material from a companion, the collision of two white dwarfs, or accretion that causes ignition in
16905-405: The coolest known white dwarfs. An outer shell of non-degenerate matter sits on top of the degenerate core. The outermost layers, which are cooler than the interior, radiate roughly as a black body . A white dwarf remains visible for a long time, as its tenuous outer atmosphere slowly radiates the thermal content of the degenerate interior. The visible radiation emitted by white dwarfs varies over
17066-407: The core of the star will collapse and it will explode in a core-collapse supernova that will leave behind a remnant neutron star, black hole , or possibly a more exotic form of compact star . Some main-sequence stars, of perhaps 8 to 10 M ☉ , although sufficiently massive to fuse carbon to neon and magnesium , may be insufficiently massive to fuse neon . Such a star may leave
17227-460: The core, but, unlike photons, they rarely interact with matter, so almost all are able to escape the Sun immediately. However, measurements of the number of these neutrinos produced in the Sun are lower than theories predict by a factor of 3. In 2001, the discovery of neutrino oscillation resolved the discrepancy: the Sun emits the number of electron neutrinos predicted by the theory, but neutrino detectors were missing 2 ⁄ 3 of them because
17388-501: The core, converting about 3.7 × 10 protons into alpha particles (helium nuclei) every second (out of a total of ~8.9 × 10 free protons in the Sun), or about 6.2 × 10 kg/s . However, each proton (on average) takes around 9 billion years to fuse with another using the PP chain. Fusing four free protons (hydrogen nuclei) into a single alpha particle (helium nucleus) releases around 0.7% of
17549-401: The corona reaches 1,000,000–2,000,000 K . The high temperature of the corona shows that it is heated by something other than direct heat conduction from the photosphere. It is thought that the energy necessary to heat the corona is provided by turbulent motion in the convection zone below the photosphere, and two main mechanisms have been proposed to explain coronal heating. The first
17710-411: The crystallization theory, and in 2004, observations were made that suggested approximately 90% of the mass of BPM 37093 had crystallized. Other work gives a crystallized mass fraction of between 32% and 82%. As a white dwarf core undergoes crystallization into a solid phase, latent heat is released, which provides a source of thermal energy that delays its cooling. Another possible mechanism that
17871-404: The current age of the known universe (approximately 13.8 billion years), it is thought that no black dwarfs yet exist. The oldest known white dwarfs still radiate at temperatures of a few thousand kelvins , which establishes an observational limit on the maximum possible age of the universe . The first white dwarf discovered was in the triple star system of 40 Eridani , which contains
18032-430: The current view is that this limit is not normally attained; increasing temperature and density inside the core ignite carbon fusion as the star approaches the limit (to within about 1%) before collapse is initiated. In contrast, for a core primarily composed of oxygen, neon and magnesium, the collapsing white dwarf will typically form a neutron star . In this case, only a fraction of the star's mass will be ejected during
18193-479: The discovery that all the stars of very faint absolute magnitude were of spectral class M. In conversation on this subject (as I recall it), I asked Pickering about certain other faint stars, not on my list, mentioning in particular 40 Eridani B. Characteristically, he sent a note to the Observatory office and before long the answer came (I think from Mrs. Fleming) that the spectrum of this star
18354-400: The duration of a solar day on another planet such as Mars . The astronomical symbol for the Sun is a circle with a center dot, [REDACTED] . It is used for such units as M ☉ ( Solar mass ), R ☉ ( Solar radius ) and L ☉ ( Solar luminosity ). The scientific study of the Sun is called heliology . The Sun is a G-type main-sequence star that makes up about 99.86% of
18515-533: The end point of stellar evolution for main-sequence stars with masses from about 0.07 to 10 M ☉ . The composition of the white dwarf produced will depend on the initial mass of the star. Current galactic models suggest the Milky Way galaxy currently contains about ten billion white dwarfs. If the mass of a main-sequence star is lower than approximately half a solar mass , it will never become hot enough to ignite and fuse helium in its core. It
18676-417: The equation of state can then be solved to find the structure of the white dwarf at equilibrium. In the non-relativistic case, we will still find that the radius is inversely proportional to the cube root of the mass. Relativistic corrections will alter the result so that the radius becomes zero at a finite value of the mass. This is the limiting value of the mass – called the Chandrasekhar limit – at which
18837-563: The external poloidal dipolar magnetic field is near its dynamo-cycle minimum strength; but an internal toroidal quadrupolar field, generated through differential rotation within the tachocline, is near its maximum strength. At this point in the dynamo cycle, buoyant upwelling within the convective zone forces emergence of the toroidal magnetic field through the photosphere, giving rise to pairs of sunspots, roughly aligned east–west and having footprints with opposite magnetic polarities. The magnetic polarity of sunspot pairs alternates every solar cycle,
18998-445: The form of large solar flares and myriad similar but smaller events— nanoflares . Currently, it is unclear whether waves are an efficient heating mechanism. All waves except Alfvén waves have been found to dissipate or refract before reaching the corona. In addition, Alfvén waves do not easily dissipate in the corona. Current research focus has therefore shifted towards flare heating mechanisms. White dwarf A white dwarf
19159-404: The fused mass as energy, so the Sun releases energy at the mass–energy conversion rate of 4.26 billion kg/s (which requires 600 billion kg of hydrogen ), for 384.6 yottawatts ( 3.846 × 10 W ), or 9.192 × 10 megatons of TNT per second. The large power output of the Sun is mainly due to the huge size and density of its core (compared to Earth and objects on Earth), with only
19320-482: The heliosphere, forming the solar magnetic field into a spiral shape, until it impacts the heliopause more than 50 AU from the Sun. In December 2004, the Voyager 1 probe passed through a shock front that is thought to be part of the heliopause. In late 2012, Voyager 1 recorded a marked increase in cosmic ray collisions and a sharp drop in lower energy particles from the solar wind, which suggested that
19481-432: The helium in the Sun would have been produced by Big Bang nucleosynthesis in the first 20 minutes of the universe, and the heavier elements were produced by previous generations of stars before the Sun was formed, and spread into the interstellar medium during the final stages of stellar life and by events such as supernovae . Since the Sun formed, the main fusion process has involved fusing hydrogen into helium. Over
19642-505: The mass of the Solar System. It has an absolute magnitude of +4.83, estimated to be brighter than about 85% of the stars in the Milky Way , most of which are red dwarfs . It is more massive than 95% of the stars within 7 pc (23 ly). The Sun is a Population I , or heavy-element-rich, star. Its formation approximately 4.6 billion years ago may have been triggered by shockwaves from one or more nearby supernovae . This
19803-444: The neutrinos had changed flavor by the time they were detected. The Sun has a stellar magnetic field that varies across its surface. Its polar field is 1–2 gauss (0.0001–0.0002 T ), whereas the field is typically 3,000 gauss (0.3 T) in features on the Sun called sunspots and 10–100 gauss (0.001–0.01 T) in solar prominences . The magnetic field varies in time and location. The quasi-periodic 11-year solar cycle
19964-524: The newly devised quantum mechanics . Since electrons obey the Pauli exclusion principle , no two electrons can occupy the same state , and they must obey Fermi–Dirac statistics , also introduced in 1926 to determine the statistical distribution of particles that satisfy the Pauli exclusion principle. At zero temperature, therefore, electrons can not all occupy the lowest-energy, or ground , state; some of them would have to occupy higher-energy states, forming
20125-406: The non-relativistic formula T = p / 2 m for the kinetic energy, it is non-relativistic. When the electron velocity in a white dwarf is close to the speed of light , the kinetic energy formula approaches T = pc where c is the speed of light, and it can be shown that there is no stable equilibrium in the ultrarelativistic limit . In particular, this analysis yields the maximum mass of
20286-399: The nonrelativistic Fermi gas equation of state, which gives where R is the radius, M is the total mass of the star, N is the number of electrons per unit mass (dependent only on composition), m e is the electron mass , ℏ {\displaystyle \hbar } is the reduced Planck constant , and G is the gravitational constant . Since this analysis uses
20447-419: The past 4.6 billion years, the amount of helium and its location within the Sun has gradually changed. The proportion of helium within the core has increased from about 24% to about 60% due to fusion, and some of the helium and heavy elements have settled from the photosphere toward the center of the Sun because of gravity . The proportions of heavier elements are unchanged. Heat is transferred outward from
20608-414: The photospheric surface. Both coronal mass ejections and high-speed streams of solar wind carry plasma and the interplanetary magnetic field outward into the Solar System. The effects of solar activity on Earth include auroras at moderate to high latitudes and the disruption of radio communications and electric power . Solar activity is thought to have played a large role in the formation and evolution of
20769-455: The planets is weak and does not significantly affect the shape of the Sun. The Sun rotates faster at its equator than at its poles . This differential rotation is caused by convective motion due to heat transport and the Coriolis force due to the Sun's rotation. In a frame of reference defined by the stars, the rotational period is approximately 25.6 days at the equator and 33.5 days at
20930-473: The poles. Viewed from Earth as it orbits the Sun, the apparent rotational period of the Sun at its equator is about 28 days. Viewed from a vantage point above its north pole, the Sun rotates counterclockwise around its axis of spin. A survey of solar analogs suggest the early Sun was rotating up to ten times faster than it does today. This would have made the surface much more active, with greater X-ray and UV emission. Sun spots would have covered 5–30% of
21091-557: The poloidal to the toroidal field, but with a polarity that is opposite to the previous cycle. The process carries on continuously, and in an idealized, simplified scenario, each 11-year sunspot cycle corresponds to a change, then, in the overall polarity of the Sun's large-scale magnetic field. The Sun's magnetic field leads to many effects that are collectively called solar activity . Solar flares and coronal mass ejections tend to occur at sunspot groups. Slowly changing high-speed streams of solar wind are emitted from coronal holes at
21252-477: The predicted companion. Walter Adams announced in 1915 that he had found the spectrum of Sirius B to be similar to that of Sirius. In 1917, Adriaan van Maanen discovered van Maanen's Star , an isolated white dwarf. These three white dwarfs, the first discovered, are the so-called classical white dwarfs . Eventually, many faint white stars that had high proper motion were found, indicating that they could be suspected to be low-luminosity stars close to
21413-526: The presently known value of the limit was first published in 1931 by Subrahmanyan Chandrasekhar in his paper "The Maximum Mass of Ideal White Dwarfs". For a non-rotating white dwarf, it is equal to approximately 5.7 M ☉ / μ e , where μ e is the average molecular weight per electron of the star. As the carbon-12 and oxygen-16 that predominantly compose a carbon–oxygen white dwarf both have atomic numbers equal to half their atomic weight , one should take μ e equal to 2 for such
21574-412: The pressure. This electron degeneracy pressure supports a white dwarf against gravitational collapse. The pressure depends only on density and not on temperature. Degenerate matter is relatively compressible; this means that the density of a high-mass white dwarf is much greater than that of a low-mass white dwarf and that the radius of a white dwarf decreases as its mass increases. The existence of
21735-448: The primordial Solar System. Typically, the solar heavy-element abundances described above are measured both by using spectroscopy of the Sun's photosphere and by measuring abundances in meteorites that have never been heated to melting temperatures. These meteorites are thought to retain the composition of the protostellar Sun and are thus not affected by the settling of heavy elements. The two methods generally agree well. The core of
21896-470: The probe had passed through the heliopause and entered the interstellar medium , and indeed did so on August 25, 2012, at approximately 122 astronomical units (18 Tm) from the Sun. The heliosphere has a heliotail which stretches out behind it due to the Sun's peculiar motion through the galaxy. On April 28, 2021, NASA's Parker Solar Probe encountered the specific magnetic and particle conditions at 18.8 solar radii that indicated that it penetrated
22057-438: The product of mass loss in binary systems or mass loss due to a large planetary companion. If the mass of a main-sequence star is between 0.5 and 8 M ☉ , its core will become sufficiently hot to fuse helium into carbon and oxygen via the triple-alpha process , but it will never become sufficiently hot to fuse carbon into neon . Near the end of the period in which it undergoes fusion reactions, such
22218-456: The progenitor star would thus become a surface magnetic field of c. 100 × 100 = 1 million gauss (100 T) once the star's radius had shrunk by a factor of 100. The first magnetic white dwarf to be discovered was GJ 742 (also known as GRW +70 8247 ), which was identified by James Kemp, John Swedlund, John Landstreet and Roger Angel in 1970 to host a magnetic field by its emission of circularly polarized light. It
22379-429: The relatively bright main sequence star 40 Eridani A , orbited at a distance by the closer binary system of the white dwarf 40 Eridani B and the main sequence red dwarf 40 Eridani C . The pair 40 Eridani B/C was discovered by William Herschel on 31 January 1783. In 1910, Henry Norris Russell , Edward Charles Pickering and Williamina Fleming discovered that, despite being a dim star, 40 Eridani B
22540-402: The rigorous mathematical literature. The fine structure of the free boundary of white dwarfs has also been analysed mathematically rigorously. The degenerate matter that makes up the bulk of a white dwarf has a very low opacity , because any absorption of a photon requires that an electron must transition to a higher empty state, which may not be possible as the energy of the photon may not be
22701-437: The shorter wavelengths. Solar ultraviolet radiation ionizes Earth's dayside upper atmosphere, creating the electrically conducting ionosphere . Ultraviolet light from the Sun has antiseptic properties and can be used to sanitize tools and water. This radiation causes sunburn , and has other biological effects such as the production of vitamin D and sun tanning . It is the main cause of skin cancer . Ultraviolet light
22862-425: The solar cycle progresses toward its maximum , sunspots tend to form closer to the solar equator, a phenomenon known as Spörer's law . The largest sunspots can be tens of thousands of kilometers across. An 11-year sunspot cycle is half of a 22-year Babcock –Leighton dynamo cycle, which corresponds to an oscillatory exchange of energy between toroidal and poloidal solar magnetic fields. At solar-cycle maximum,
23023-417: The star is allowed to rotate nonuniformly, and viscosity is neglected, then, as was pointed out by Fred Hoyle in 1947, there is no limit to the mass for which it is possible for a model white dwarf to be in static equilibrium. Not all of these model stars will be dynamically stable. Rotating white dwarfs and the estimates of their diameter in terms of the angular velocity of rotation has been treated in
23184-492: The star's envelope in the AGB phase and may also contain material accreted from the interstellar medium . The envelope is believed to consist of a helium-rich layer with mass no more than 1 ⁄ 100 of the star's total mass, which, if the atmosphere is hydrogen-dominated, is overlain by a hydrogen-rich layer with mass approximately 1 ⁄ 10 000 of the star's total mass. Although thin, these outer layers determine
23345-404: The stars Sirius (α Canis Majoris) and Procyon (α Canis Minoris) were changing their positions periodically. In 1844 he predicted that both stars had unseen companions: If we were to regard Sirius and Procyon as double stars, the change of their motions would not surprise us; we should acknowledge them as necessary, and have only to investigate their amount by observation. But light
23506-450: The stars in the Milky Way . After the hydrogen - fusing period of a main-sequence star of low or intermediate mass ends, such a star will expand to a red giant and fuse helium to carbon and oxygen in its core by the triple-alpha process . If a red giant has insufficient mass to generate the core temperatures required to fuse carbon (around 10 K ), an inert mass of carbon and oxygen will build up at its center. After such
23667-450: The strengths of the metal lines. For example, a 2015 study of the white dwarf Ton 345 concluded that its metal abundances were consistent with those of a differentiated , rocky planet whose mantle had been eroded by the host star's wind during its asymptotic giant branch phase. Magnetic fields in white dwarfs with a strength at the surface of c. 1 million gauss (100 teslas ) were predicted by P. M. S. Blackett in 1947 as
23828-417: The surface. The rotation rate was gradually slowed by magnetic braking , as the Sun's magnetic field interacted with the outflowing solar wind. A vestige of this rapid primordial rotation still survives at the Sun's core, which has been found to be rotating at a rate of once per week; four times the mean surface rotation rate. The Sun consists mainly of the elements hydrogen and helium . At this time in
23989-431: The tachocline picks up heat and expands, thereby reducing its density and allowing it to rise. As a result, an orderly motion of the mass develops into thermal cells that carry most of the heat outward to the Sun's photosphere above. Once the material diffusively and radiatively cools just beneath the photospheric surface, its density increases, and it sinks to the base of the convection zone, where it again picks up heat from
24150-424: The temperature of the corona, at least some of its heat is known to be from magnetic reconnection . The corona is the extended atmosphere of the Sun, which has a volume much larger than the volume enclosed by the Sun's photosphere. A flow of plasma outward from the Sun into interplanetary space is the solar wind . The heliosphere, the tenuous outermost atmosphere of the Sun, is filled with solar wind plasma and
24311-422: The tenuous layers above the photosphere. The photosphere has a particle density of ~10 m (about 0.37% of the particle number per volume of Earth's atmosphere at sea level). The photosphere is not fully ionized—the extent of ionization is about 3%, leaving almost all of the hydrogen in atomic form. The Sun's atmosphere is composed of five layers: the photosphere, the chromosphere , the transition region ,
24472-420: The thermal evolution of the white dwarf. The degenerate electrons in the bulk of a white dwarf conduct heat well. Most of a white dwarf's mass is therefore at almost the same temperature ( isothermal ), and it is also hot: a white dwarf with surface temperature between 8000 K and 16 000 K will have a core temperature between approximately 5 000 000 K and 20 000 000 K . The white dwarf
24633-404: The top of the radiative zone and the convective cycle continues. At the photosphere, the temperature has dropped 350-fold to 5,700 K (9,800 °F) and the density to only 0.2 g/m (about 1/10,000 the density of air at sea level, and 1 millionth that of the inner layer of the convective zone). The thermal columns of the convection zone form an imprint on the surface of the Sun giving it
24794-424: The total mass of the Solar System. Roughly three-quarters of the Sun's mass consists of hydrogen (~73%); the rest is mostly helium (~25%), with much smaller quantities of heavier elements, including oxygen , carbon , neon , and iron . The Sun is a G-type main-sequence star (G2V), informally called a yellow dwarf , though its light is actually white. It formed approximately 4.6 billion years ago from
24955-418: The transfer of energy through this zone is by radiation instead of thermal convection. Ions of hydrogen and helium emit photons, which travel only a brief distance before being reabsorbed by other ions. The density drops a hundredfold (from 20 000 kg/m to 200 kg/m ) between 0.25 solar radii and 0.7 radii, the top of the radiative zone. The radiative zone and the convective zone are separated by
25116-426: The vast majority of observed white dwarfs. If a star is massive enough, its core will eventually become sufficiently hot to fuse carbon to neon, and then to fuse neon to iron. Such a star will not become a white dwarf, because the mass of its central, non-fusing core, initially supported by electron degeneracy pressure, will eventually exceed the largest possible mass supportable by degeneracy pressure. At this point
25277-436: The white dwarf can no longer be supported by electron degeneracy pressure. The graph on the right shows the result of such a computation. It shows how radius varies with mass for non-relativistic (blue curve) and relativistic (green curve) models of a white dwarf. Both models treat the white dwarf as a cold Fermi gas in hydrostatic equilibrium. The average molecular weight per electron, μ e , has been set equal to 2. Radius
25438-407: Was A. I knew enough about it, even in these paleozoic days, to realize at once that there was an extreme inconsistency between what we would then have called "possible" values of the surface brightness and density. I must have shown that I was not only puzzled but crestfallen, at this exception to what looked like a very pretty rule of stellar characteristics; but Pickering smiled upon me, and said: "It
25599-455: Was first confirmed in 2019 after the identification of a pile up in the cooling sequence of more than 15 000 white dwarfs observed with the Gaia satellite. Low-mass helium white dwarfs (mass < 0.20 M ☉ ), often referred to as extremely low-mass white dwarfs (ELM WDs), are formed in binary systems. As a result of their hydrogen-rich envelopes, residual hydrogen burning via
25760-486: Was of spectral type A, or white. In 1939, Russell looked back on the discovery: I was visiting my friend and generous benefactor, Prof. Edward C. Pickering. With characteristic kindness, he had volunteered to have the spectra observed for all the stars – including comparison stars – which had been observed in the observations for stellar parallax which Hinks and I made at Cambridge, and I discussed. This piece of apparently routine work proved very fruitful – it led to
25921-435: Was suggested to explain the seeming delay in the cooling of some types of white dwarves is a solid–liquid distillation process: the crystals formed in the core are buoyant and float up, thereby displacing heavier liquid downward, thus causing a net release of gravitational energy. Chemical fractionation between the ionic species in the plasma mixture can release a similar or even greater amount of energy. This energy release
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