129-409: (Redirected from Yellow Star ) Yellow star may refer to: A yellow star in stellar classification A yellow badge , a cloth patch that Jews were ordered to wear on their clothes Any plant of genus Hypoxis in the family Hypoxidaceae "Yellow Star", a song by Donovan from his album Essence to Essence The Yellow Star: The Persecution of
258-730: 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
387-506: 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
516-514: 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
645-611: 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
774-447: 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
903-460: 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;
1032-413: 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,
1161-415: 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
1290-408: 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 –
1419-404: 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
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#17327835914881548-533: A luminosity class of IIIb, while a luminosity class IIIa indicates a star slightly brighter than a typical giant. A sample of extreme V stars with strong absorption in He II λ4686 spectral lines have been given the Vz designation. An example star is HD 93129 B . Additional nomenclature, in the form of lower-case letters, can follow the spectral type to indicate peculiar features of the spectrum. For example, 59 Cygni
1677-477: 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
1806-405: 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
1935-416: 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
2064-668: A nearby observer. The modern classification system is known as the Morgan–Keenan (MK) classification. Each star is assigned a spectral class (from the older Harvard spectral classification, which did not include luminosity ) and a luminosity class using Roman numerals as explained below, forming the star's spectral type. Other modern stellar classification systems , such as the UBV system , are based on color indices —the measured differences in three or more color magnitudes . Those numbers are given labels such as "U−V" or "B−V", which represent
2193-441: 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
2322-399: A particular chemical element or molecule , with the line strength indicating the abundance of that element. The strengths of the different spectral lines vary mainly due to the temperature of the photosphere , although in some cases there are true abundance differences. The spectral class of a star is a short code primarily summarizing the ionization state, giving an objective measure of
2451-531: 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
2580-527: 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
2709-457: A series of twenty-two types numbered from I–XXII. Because the 22 Roman numeral groupings did not account for additional variations in spectra, three additional divisions were made to further specify differences: Lowercase letters were added to differentiate relative line appearance in spectra; the lines were defined as: Antonia Maury published her own stellar classification catalogue in 1897 called "Spectra of Bright Stars Photographed with
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#17327835914882838-598: 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
2967-465: 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
3096-411: 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 a star sheds its outer layers and forms a planetary nebula , it will leave behind a core, which
3225-499: 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
3354-424: 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
3483-458: 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
3612-436: A surface temperature around 5,800 K. The conventional colour description takes into account only the peak of the stellar spectrum. In actuality, however, stars radiate in all parts of the spectrum. Because all spectral colours combined appear white, the actual apparent colours the human eye would observe are far lighter than the conventional colour descriptions would suggest. This characteristic of 'lightness' indicates that
3741-515: 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
3870-439: 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
3999-488: 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
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4128-430: 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
4257-407: Is a synonym for hotter , while "late" is a synonym for cooler . Depending on the context, "early" and "late" may be absolute or relative terms. "Early" as an absolute term would therefore refer to O or B, and possibly A stars. As a relative reference it relates to stars hotter than others, such as "early K" being perhaps K0, K1, K2 and K3. "Late" is used in the same way, with an unqualified use of
4386-520: 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
4515-559: Is based on spectral lines sensitive to stellar temperature and surface gravity , which is related to luminosity (whilst the Harvard classification is based on just surface temperature). Later, in 1953, after some revisions to the list of standard stars and classification criteria, the scheme was named the Morgan–Keenan classification , or MK , which remains in use today. Denser stars with higher surface gravity exhibit greater pressure broadening of spectral lines. The gravity, and hence
4644-467: 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 the Sirius binary star . There are currently thought to be eight white dwarfs among the hundred star systems nearest
4773-469: Is different from Wikidata All article disambiguation pages All disambiguation pages Stellar classification In astronomy , stellar classification is the classification of stars based on their spectral characteristics. Electromagnetic radiation from the star is analyzed by splitting it with a prism or diffraction grating into a spectrum exhibiting the rainbow of colors interspersed with spectral lines . Each line indicates
4902-412: 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
5031-487: 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
5160-438: 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
5289-512: Is listed as spectral type B1.5Vnne, indicating a spectrum with the general classification B1.5V, as well as very broad absorption lines and certain emission lines. The reason for the odd arrangement of letters in the Harvard classification is historical, having evolved from the earlier Secchi classes and been progressively modified as understanding improved. During the 1860s and 1870s, pioneering stellar spectroscopist Angelo Secchi created
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5418-412: 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
5547-419: 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
5676-455: 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
5805-431: 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
5934-458: 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
6063-411: 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,
6192-440: 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
6321-403: 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
6450-406: Is used for hypergiants , class I for supergiants , class II for bright giants , class III for regular giants , class IV for subgiants , class V for main-sequence stars , class sd (or VI ) for subdwarfs , and class D (or VII ) for white dwarfs . The full spectral class for the Sun is then G2V, indicating a main-sequence star with
6579-405: 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
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#17327835914886708-580: 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
6837-473: 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
6966-589: The He II λ4541 disappears. However, with modern equipment, the line is still apparent in the early B-type stars. Today for main-sequence stars, the B class is instead defined by the intensity of the He ;I violet spectrum, with the maximum intensity corresponding to class B2. For supergiants, lines of silicon are used instead; the Si ;IV λ4089 and Si III λ4552 lines are indicative of early B. At mid-B,
7095-543: The Kelvin–Helmholtz mechanism , which is now known to not apply to main-sequence stars . If that were true, then stars would start their lives as very hot "early-type" stars and then gradually cool down into "late-type" stars. This mechanism provided ages of the Sun that were much smaller than what is observed in the geologic record , and was rendered obsolete by the discovery that stars are powered by nuclear fusion . The terms "early" and "late" were carried over, beyond
7224-556: 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,
7353-505: The Secchi classes in order to classify observed spectra. By 1866, he had developed three classes of stellar spectra, shown in the table below. In the late 1890s, this classification began to be superseded by the Harvard classification, which is discussed in the remainder of this article. The Roman numerals used for Secchi classes should not be confused with the completely unrelated Roman numerals used for Yerkes luminosity classes and
7482-473: 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
7611-536: 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
7740-454: 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
7869-639: The 11 inch Draper Telescope as Part of the Henry Draper Memorial", which included 4,800 photographs and Maury's analyses of 681 bright northern stars. This was the first instance in which a woman was credited for an observatory publication. In 1901, Annie Jump Cannon returned to the lettered types, but dropped all letters except O, B, A, F, G, K, M, and N used in that order, as well as P for planetary nebulae and Q for some peculiar spectra. She also used types such as B5A for stars halfway between types B and A, F2G for stars one fifth of
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#17327835914887998-597: 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%
8127-436: 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
8256-453: The B2 subclass, and moderate hydrogen lines. As O- and B-type stars are so energetic, they only live for a relatively short time. Thus, due to the low probability of kinematic interaction during their lifetime, they are unable to stray far from the area in which they formed, apart from runaway stars . The transition from class O to class B was originally defined to be the point at which
8385-478: 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
8514-437: 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
8643-651: The Jews in Europe 1933-45 , a 1980 documentary film Yellow Star (novel) , a 2006 children's novel describing life in the Lodz Ghetto during World War II Bora Kim, also known as YellOwStaR , French professional League of Legends player Yellow Star (film) , a 1922 German silent drama film See also [ edit ] Gold Star Honorary Order of the Yellow Star Topics referred to by
8772-514: 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 the stars in the Milky Way . After the hydrogen - fusing period of a main-sequence star of low or intermediate mass ends, such
8901-451: 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
9030-442: 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
9159-519: The alphabet. This classification system was later modified by Annie Jump Cannon and Antonia Maury to produce the Harvard spectral classification scheme. In 1897, another astronomer at Harvard, Antonia Maury , placed the Orion subtype of Secchi class I ahead of the remainder of Secchi class I, thus placing the modern type B ahead of the modern type A. She was the first to do so, although she did not use lettered spectral types, but rather
9288-433: 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
9417-457: 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
9546-409: The brighter stars of the constellation Orion . About 1 in 800 (0.125%) of the main-sequence stars in the solar neighborhood are B-type main-sequence stars . B-type stars are relatively uncommon and the closest is Regulus, at around 80 light years. White dwarf A white dwarf is a stellar core remnant composed mostly of electron-degenerate matter . A white dwarf is very dense : its mass
9675-520: The classes indicate the temperature of the star's atmosphere and are normally listed from hottest to coldest. A common mnemonic for remembering the order of the spectral type letters, from hottest to coolest, is " O h, B e A F ine G uy/ G irl: K iss M e!", or another one is " O ur B right A stronomers F requently G enerate K iller M nemonics!" . The spectral classes O through M, as well as other more specialized classes discussed later, are subdivided by Arabic numerals (0–9), where 0 denotes
9804-514: The classical system: W , S and C . Some non-stellar objects have also been assigned letters: D for white dwarfs and L , T and Y for Brown dwarfs . In the MK system, a luminosity class is added to the spectral class using Roman numerals . This is based on the width of certain absorption lines in the star's spectrum, which vary with the density of the atmosphere and so distinguish giant stars from dwarfs. Luminosity class 0 or Ia+
9933-461: 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
10062-424: 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
10191-637: The colors passed by two standard filters (e.g. U ltraviolet, B lue and V isual). The Harvard system is a one-dimensional classification scheme by astronomer Annie Jump Cannon , who re-ordered and simplified the prior alphabetical system by Draper (see History ). Stars are grouped according to their spectral characteristics by single letters of the alphabet, optionally with numeric subdivisions. Main-sequence stars vary in surface temperature from approximately 2,000 to 50,000 K , whereas more-evolved stars – in particular, newly-formed white dwarfs – can have surface temperatures above 100,000 K. Physically,
10320-405: 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
10449-407: 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
10578-411: 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
10707-404: 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
10836-430: 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
10965-525: The demise of the model they were based on. O-type stars are very hot and extremely luminous, with most of their radiated output in the ultraviolet range. These are the rarest of all main-sequence stars. About 1 in 3,000,000 (0.00003%) of the main-sequence stars in the solar neighborhood are O-type stars. Some of the most massive stars lie within this spectral class. O-type stars frequently have complicated surroundings that make measurement of their spectra difficult. O-type spectra formerly were defined by
11094-479: 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
11223-533: 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
11352-417: 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
11481-719: The extreme velocity of their stellar wind , which may reach 2,000 km/s. Because they are so massive, O-type stars have very hot cores and burn through their hydrogen fuel very quickly, so they are the first stars to leave the main sequence . When the MKK classification scheme was first described in 1943, the only subtypes of class O used were O5 to O9.5. The MKK scheme was extended to O9.7 in 1971 and O4 in 1978, and new classification schemes that add types O2, O3, and O3.5 have subsequently been introduced. Spectral standards: B-type stars are very luminous and blue. Their spectra have neutral helium lines, which are most prominent at
11610-627: The help of the Harvard computers , especially Williamina Fleming , the first iteration of the Henry Draper catalogue was devised to replace the Roman-numeral scheme established by Angelo Secchi. The catalogue used a scheme in which the previously used Secchi classes (I to V) were subdivided into more specific classes, given letters from A to P. Also, the letter Q was used for stars not fitting into any other class. Fleming worked with Pickering to differentiate 17 different classes based on
11739-404: The hottest stars of a given class. For example, A0 denotes the hottest stars in class A and A9 denotes the coolest ones. Fractional numbers are allowed; for example, the star Mu Normae is classified as O9.7. The Sun is classified as G2. The fact that the Harvard classification of a star indicated its surface or photospheric temperature (or more precisely, its effective temperature )
11868-408: The intensity of hydrogen spectral lines, which causes variation in the wavelengths emanated from stars and results in variation in color appearance. The spectra in class A tended to produce the strongest hydrogen absorption lines while spectra in class O produced virtually no visible lines. The lettering system displayed the gradual decrease in hydrogen absorption in the spectral classes when moving down
11997-428: The intensity of the latter relative to that of Si II λλ4128-30 is the defining characteristic, while for late B, it is the intensity of Mg II λ4481 relative to that of He I λ4471. These stars tend to be found in their originating OB associations , which are associated with giant molecular clouds . The Orion OB1 association occupies a large portion of a spiral arm of the Milky Way and contains many of
12126-419: The main sequence). Nominal luminosity class VII (and sometimes higher numerals) is now rarely used for white dwarf or "hot sub-dwarf" classes, since the temperature-letters of the main sequence and giant stars no longer apply to white dwarfs. Occasionally, letters a and b are applied to luminosity classes other than supergiants; for example, a giant star slightly less luminous than typical may be given
12255-485: The modern definition uses the ratio of the nitrogen line N IV λ4058 to N III λλ4634-40-42. O-type stars have dominant lines of absorption and sometimes emission for He II lines, prominent ionized ( Si IV, O III, N III, and C III) and neutral helium lines, strengthening from O5 to O9, and prominent hydrogen Balmer lines , although not as strong as in later types. Higher-mass O-type stars do not retain extensive atmospheres due to
12384-524: 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
12513-406: 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
12642-399: 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
12771-538: The photosphere's temperature. Most stars are currently classified under the Morgan–Keenan (MK) system using the letters O , B , A , F , G , K , and M , a sequence from the hottest ( O type) to the coolest ( M type). Each letter class is then subdivided using a numeric digit with 0 being hottest and 9 being coolest (e.g., A8, A9, F0, and F1 form a sequence from hotter to cooler). The sequence has been expanded with three classes for other stars that do not fit in
12900-477: 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
13029-526: 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
13158-401: The pressure, on the surface of a giant star is much lower than for a dwarf star because the radius of the giant is much greater than a dwarf of similar mass. Therefore, differences in the spectrum can be interpreted as luminosity effects and a luminosity class can be assigned purely from examination of the spectrum. A number of different luminosity classes are distinguished, as listed in
13287-412: 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
13416-438: 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
13545-456: 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
13674-638: The proposed neutron star classes. In the 1880s, the astronomer Edward C. Pickering began to make a survey of stellar spectra at the Harvard College Observatory , using the objective-prism method. A first result of this work was the Draper Catalogue of Stellar Spectra , published in 1890. Williamina Fleming classified most of the spectra in this catalogue and was credited with classifying over 10,000 featured stars and discovering 10 novae and more than 200 variable stars. With
13803-428: The ratio of the strength of the He II λ4541 relative to that of He I λ4471, where λ is the radiation wavelength . Spectral type O7 was defined to be the point at which the two intensities are equal, with the He I line weakening towards earlier types. Type O3 was, by definition, the point at which said line disappears altogether, although it can be seen very faintly with modern technology. Due to this,
13932-429: 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
14061-402: 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
14190-419: The same term [REDACTED] This disambiguation page lists articles associated with the title Yellow star . If an internal link led you here, you may wish to change the link to point directly to the intended article. Retrieved from " https://en.wikipedia.org/w/index.php?title=Yellow_star&oldid=1021303527 " Category : Disambiguation pages Hidden categories: Short description
14319-410: The simplified assignment of colours within the spectrum can be misleading. Excluding colour-contrast effects in dim light, in typical viewing conditions there are no green, cyan, indigo, or violet stars. "Yellow" dwarfs such as the Sun are white, "red" dwarfs are a deep shade of yellow/orange, and "brown" dwarfs do not literally appear brown, but hypothetically would appear dim red or grey/black to
14448-462: The solar chromosphere, then to stellar spectra. Harvard astronomer Cecilia Payne then demonstrated that the O-B-A-F-G-K-M spectral sequence is actually a sequence in temperature. Because the classification sequence predates our understanding that it is a temperature sequence, the placement of a spectrum into a given subtype, such as B3 or A7, depends upon (largely subjective) estimates of
14577-417: 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
14706-492: 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
14835-404: 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
14964-630: The strengths of absorption features in stellar spectra. As a result, these subtypes are not evenly divided into any sort of mathematically representable intervals. The Yerkes spectral classification , also called the MK, or Morgan-Keenan (alternatively referred to as the MKK, or Morgan-Keenan-Kellman) system from the authors' initials, is a system of stellar spectral classification introduced in 1943 by William Wilson Morgan , Philip C. Keenan , and Edith Kellman from Yerkes Observatory . This two-dimensional ( temperature and luminosity ) classification scheme
15093-450: 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
15222-463: The table below. Marginal cases are allowed; for example, a star may be either a supergiant or a bright giant, or may be in between the subgiant and main-sequence classifications. In these cases, two special symbols are used: For example, a star classified as A3-4III/IV would be in between spectral types A3 and A4, while being either a giant star or a subgiant. Sub-dwarf classes have also been used: VI for sub-dwarfs (stars slightly less luminous than
15351-483: The term indicating stars with spectral types such as K and M, but it can also be used for stars that are cool relative to other stars, as in using "late G" to refer to G7, G8, and G9. In the relative sense, "early" means a lower Arabic numeral following the class letter, and "late" means a higher number. This obscure terminology is a hold-over from a late nineteenth century model of stellar evolution , which supposed that stars were powered by gravitational contraction via
15480-420: 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
15609-426: 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
15738-482: The way from F to G, and so on. Finally, by 1912, Cannon had changed the types B, A, B5A, F2G, etc. to B0, A0, B5, F2, etc. This is essentially the modern form of the Harvard classification system. This system was developed through the analysis of spectra on photographic plates, which could convert light emanated from stars into a readable spectrum. A luminosity classification known as the Mount Wilson system
15867-436: 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
15996-407: 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
16125-455: 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
16254-461: Was not fully understood until after its development, though by the time the first Hertzsprung–Russell diagram was formulated (by 1914), this was generally suspected to be true. In the 1920s, the Indian physicist Meghnad Saha derived a theory of ionization by extending well-known ideas in physical chemistry pertaining to the dissociation of molecules to the ionization of atoms. First he applied it to
16383-486: 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
16512-435: 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
16641-488: Was used to distinguish between stars of different luminosities. This notation system is still sometimes seen on modern spectra. The stellar classification system is taxonomic , based on type specimens , similar to classification of species in biology : The categories are defined by one or more standard stars for each category and sub-category, with an associated description of the distinguishing features. Stars are often referred to as early or late types. "Early"
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