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89-492: The stellar classification for this object is K7Ve, matching a K-type main-sequence star that displays emission features. It is an eruptive variable of the T Tauri type with an estimated age of 2.4 million years. The object has 76% of the mass of the Sun , 181% of the Sun's radius , and is spinning with a projected rotational velocity of 13 km/s. AA Tauri is radiating 80% of

178-647: A different category of star from supergiants, although in all important respects they are just a more luminous category of supergiant. They are evolved, expanded, massive and luminous stars like supergiants, but at the most massive and luminous extreme, and with particular additional properties of undergoing high mass-loss due to their extreme luminosities and instability. Generally only the more evolved supergiants show hypergiant properties, since their instability increases after high mass-loss and some increase in luminosity. Some B[e] stars are supergiants although other B[e] stars are clearly not. Some researchers distinguish

267-880: A different stage of development (helium shell burning), and their lives ending in a different way ( planetary nebula and white dwarf rather than supernova), astrophysicists prefer to keep them separate. The dividing line becomes blurred at around 7–10  M ☉ (or as high as 12  M ☉ in some models ) where stars start to undergo limited fusion of elements heavier than helium. Specialists studying these stars often refer to them as super AGB stars, since they have many properties in common with AGB such as thermal pulsing. Others describe them as low-mass supergiants since they start to burn elements heavier than helium and can explode as supernovae. Many post-AGB stars receive spectral types with supergiant luminosity classes. For example, RV Tauri has an Ia ( bright supergiant ) luminosity class despite being less massive than

356-451: A few hundred times the luminosity of the sun. These are not massive stars, though; instead, they are stars of intermediate mass that have particularly low surface gravities, often due to instability such as Cepheid pulsations. These intermediate mass stars' being classified as supergiants during a relatively long-lasting phase of their evolution account for the large number of low luminosity yellow supergiants. The most luminous yellow stars,

445-526: 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

534-513: A massive star with increased size and luminosity due to fusion products building up, but still with some hydrogen remaining. The first stars in the universe are thought to have been considerably brighter and more massive than the stars in the modern universe. Part of the theorized population III of stars, their existence is necessary to explain observations of elements other than hydrogen and helium in quasars . Possibly larger and more luminous than any supergiant known today, their structure

623-405: A million times the Sun ( L ☉ ). They vary greatly in radius , usually from 30 to 500, or even in excess of 1,000 solar radii ( R ☉ ). They are massive enough to begin helium-core burning gently before the core becomes degenerate, without a flash and without the strong dredge-ups that lower-mass stars experience. They go on to successively ignite heavier elements, usually all

712-706: A million  L ☉ and are often unstable such as α Cygni variables and luminous blue variables . The very hottest supergiants with early O spectral types occur in an extremely narrow range of luminosities above the highly luminous early O main sequence and giant stars. They are not classified separately into normal (Ib) and luminous (Ia) supergiants, although they commonly have other spectral type modifiers such as "f" for nitrogen and helium emission (e.g. O2 If for HD 93129A ). Yellow supergiants can be considerably fainter than absolute magnitude −5, with some examples around −2 (e.g. 14 Persei ). With bolometric corrections around zero, they may only be

801-665: 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

890-399: 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

979-474: A period of hydrogen and helium shell burning; instead, they continue to burn heavier elements in their cores until they collapse. They cannot lose enough mass to form a white dwarf, so they will leave behind a neutron star or black hole remnant, usually after a core collapse supernova explosion. Stars more massive than about 40  M ☉ cannot expand into a red supergiant. Because they burn too quickly and lose their outer layers too quickly, they reach

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1068-451: A planet, instead finding multiple rings with accretion streams between them. 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

1157-506: A separate category without being referred to as supergiants or given a supergiant spectral type. Often their spectral type will be given just as "LBV" because they have peculiar and highly variable spectral features, with temperatures varying from about 8,000 K in outburst up to 20,000 K or more when "quiescent." The abundance of various elements at the surface of supergiants is different from less luminous stars. Supergiants are evolved stars and may have undergone convection of fusion products to

1246-457: 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

1335-546: A star, does not have a single concrete definition. The term giant star was first coined by Hertzsprung when it became apparent that the majority of stars fell into two distinct regions of the Hertzsprung–Russell diagram . One region contained larger and more luminous stars of spectral types A to M and received the name giant . Subsequently, as they lacked any measurable parallax, it became apparent that some of these stars were significantly larger and more luminous than

1424-533: A supergiant classification (e.g. W Virginis itself). The faintest red supergiants are around absolute magnitude −3. While most supergiants such as Alpha Cygni variables , semiregular variables , and irregular variables show some degree of photometric variability, certain types of variables amongst the supergiants are well defined. The instability strip crosses the region of supergiants, and specifically many yellow supergiants are Classical Cepheid variables . The same region of instability extends to include

1513-805: A supergiant luminosity class on account of their low surface gravity, and they are amongst the most luminous of the AGB and post-AGB stars, having masses similar to the sun; likewise, the even rarer PV Tel variables are often classified as supergiants, but have lower luminosities than supergiants and peculiar B[e] spectra extremely deficient in hydrogen. Possibly they are also post-AGB objects or "born-again" AGB stars. The LBVs are variable with multiple semi-regular periods and less predictable eruptions and giant outbursts. They are usually supergiants or hypergiants, occasionally with Wolf-Rayet spectra—extremely luminous, massive, evolved stars with expanded outer layers, but they are so distinctive and unusual that they are often treated as

1602-436: 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

1691-769: A third of the atmosphere. O type main-sequence stars and the most massive of the B type blue-white stars become supergiants. Due to their extreme masses, they have short lifespans, between 30 million years and a few hundred thousand years. They are mainly observed in young galactic structures such as open clusters , the arms of spiral galaxies , and in irregular galaxies . They are less abundant in spiral galaxy bulges and are rarely observed in elliptical galaxies , or globular clusters , which are composed mainly of old stars. Supergiants develop when massive main-sequence stars run out of hydrogen in their cores, at which point they start to expand, just like lower-mass stars. Unlike lower-mass stars, however, they begin to fuse helium in

1780-583: Is a clue to their nature as stars even more evolved than supergiants. Just as the AGB stars occur in almost the same region of the HR diagram as red supergiants, Wolf–Rayet stars can occur in the same region of the HR diagram as the hottest blue supergiants and main-sequence stars. The most massive and luminous main-sequence stars are almost indistinguishable from the supergiants they quickly evolve into. They have almost identical temperatures and very similar luminosities, and only

1869-407: 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

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1958-456: Is an indicator of the speed of stellar evolution and is used as a powerful test of models of the evolution of massive stars. The supergiants lie more or less on a horizontal band occupying the entire upper portion of the HR diagram, but there are some variations at different spectral types. These variations are due partly to different methods for assigning luminosity classes at different spectral types, and partly to actual physical differences in

2047-554: 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

2136-464: Is expected theoretically since they would be catastrophically unstable; however, there are potential exceptions among extreme stars such as VX Sagittarii . Although supergiants exist in every class from O to M, the majority are spectral type B (blue supergiants), more than at all other spectral classes combined. A much smaller grouping consists of very low-luminosity G-type supergiants, intermediate mass stars burning helium in their cores before reaching

2225-507: 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

2314-638: Is much more of a continuum than well defined bands for these classifications, and classifications such as Iab are used for intermediate luminosity supergiants. Supergiant spectra are frequently annotated to indicate spectral peculiarities , for example B2 Iae or F5 Ipec . Supergiants can also be defined as a specific phase in the evolutionary history of certain stars. Stars with initial masses above 8-10  M ☉ quickly and smoothly initiate helium core fusion after they have exhausted their hydrogen, and continue fusing heavier elements after helium exhaustion until they develop an iron core, at which point

2403-505: Is still used today, with refinements based on the increased resolution of modern spectra. Supergiants occur in every spectral class from young blue class O supergiants to highly evolved red class M supergiants. Because they are enlarged compared to main-sequence and giant stars of the same spectral type, they have lower surface gravities, and changes can be observed in their line profiles. Supergiants are also evolved stars with higher levels of heavy elements than main-sequence stars. This

2492-596: Is the basis of the MK luminosity system which assigns stars to luminosity classes purely from observing their spectra. In addition to the line changes due to low surface gravity and fusion products, the most luminous stars have high mass-loss rates and resulting clouds of expelled circumstellar materials which can produce emission lines , P Cygni profiles , or forbidden lines . The MK system assigns stars to luminosity classes: Ib for supergiants; Ia for luminous supergiants; and 0 (zero) or Ia for hypergiants. In reality there

2581-406: 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

2670-576: 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,

2759-543: 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

AA Tauri - Misplaced Pages Continue

2848-501: 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

2937-412: The asymptotic giant branch . A distinct grouping is made up of high-luminosity supergiants at early B (B0-2) and very late O (O9.5), more common even than main sequence stars of those spectral types. The number of post-main sequence blue supergiants is greater than those expected from theoretical models, leading to the "blue supergiant problem". The relative numbers of blue, yellow, and red supergiants

3026-401: The blue supergiant stage, or perhaps yellow hypergiant, before returning to become hotter stars. The most massive stars, above about 100  M ☉ , hardly move at all from their position as O main-sequence stars. These convect so efficiently that they mix hydrogen from the surface right down to the core. They continue to fuse hydrogen until it is almost entirely depleted throughout

3115-430: The luminosity of the Sun at an effective temperature of 4,060 K. AA Tauri shows brightness variations of one to two magnitudes over an 8.2-day period. The brightness has been described as "roughly constant, interrupted by quasi-cyclic fading episodes". The periodic variations are ascribed to eclipses of the star by a warped dust disk around it. In 2011, AA Tauri faded by about two magnitudes and has remained at

3204-412: The yellow hypergiants , are amongst the visually brightest stars, with absolute magnitudes around −9, although still less than a million  L ☉ . There is a strong upper limit to the luminosity of red supergiants at around half a million  L ☉ . Stars that would be brighter than this shed their outer layers so rapidly that they remain hot supergiants after they leave

3293-630: 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

3382-450: 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

3471-526: The B[e] objects as separate from supergiants, while researchers prefer to define massive evolved B[e] stars as a subgroup of supergiants. The latter has become more common with the understanding that the B[e] phenomenon arises separately in a number of distinct types of stars, including some that are clearly just a phase in the life of supergiants. Supergiants have masses from 8 to 12 times the Sun ( M ☉ ) upwards, and luminosities from about 1,000 to over

3560-515: 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

3649-515: The basis of their spectra, with distinctive lines sensitive to high luminosity and low surface gravity . In 1897, Antonia C. Maury had divided stars based on the widths of their spectral lines, with her class "c" identifying stars with the narrowest lines. Although it was not known at the time, these were the most luminous stars. In 1943, Morgan and Keenan formalised the definition of spectral luminosity classes, with class I referring to supergiant stars. The same system of MK luminosity classes

AA Tauri - Misplaced Pages Continue

3738-613: 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. Supergiant Supergiants are among the most massive and most luminous stars . Supergiant stars occupy the top region of the Hertzsprung–Russell diagram with absolute visual magnitudes between about −3 and −8. The temperature range of supergiant stars spans from about 3,400 K to over 20,000 K. The title supergiant, as applied to

3827-465: The brightest stars in the sky. Rigel , the brightest star in the constellation Orion is a typical blue-white supergiant; the three stars of Orion's Belt are all blue supergiants; Deneb is the brightest star in Cygnus , another blue supergiant; and Delta Cephei (itself the prototype) and Polaris are Cepheid variables and yellow supergiants. Antares and VV Cephei A are red supergiants . μ Cephei

3916-789: The bulk, and the term super-giant arose, quickly adopted as supergiant . Supergiants with spectral classes of O to A are typically referred to as blue supergiants , supergiants with spectral classes F and G are referred to as yellow supergiants , while those of spectral classes K to M are red supergiants . Another convention uses temperature: supergiants with effective temperatures below 4800 K are deemed red supergiants; those with temperatures between 4800 and 7500 K are yellow supergiants, and those with temperatures exceeding 7500 K are blue supergiants. These correspond approximately to spectral types M and K for red supergiants, G, F, and late A for yellow supergiants, and early A, B, and O for blue supergiants. Supergiant stars can be identified on

4005-463: The central star. In their 2003 paper, Bouvier et al. invoked the possible presence of a substellar object to explain peculiar and periodic eclipses occurring to the young star every 8.3 days, though they considered it unlikely that such a companion could be responsible for said variability. They inferred a mass of 20 times that of Jupiter for the perturbing object and an orbital separation of 0.08 Astronomical Units . Later studies find no evidence for

4094-520: 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

4183-511: 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+

4272-637: 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,

4361-418: The core collapses to produce a Type II supernova . Once these massive stars leave the main sequence, their atmospheres inflate, and they are described as supergiants. Stars initially under 10  M ☉ will never form an iron core and in evolutionary terms do not become supergiants, although they can reach luminosities thousands of times the sun's. They cannot fuse carbon and heavier elements after

4450-413: The core smoothly and not long after exhausting their hydrogen. This means that they do not increase their luminosity as dramatically as lower-mass stars, and they progress nearly horizontally across the HR diagram to become red supergiants. Also unlike lower-mass stars, red supergiants are massive enough to fuse elements heavier than helium, so they do not puff off their atmospheres as planetary nebulae after

4539-525: 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

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4628-478: The even more luminous yellow hypergiants , an extremely rare and short-lived class of luminous supergiant. Many R Coronae Borealis variables , although not all, are yellow supergiants , but this variability is due to their unusual chemical composition rather than a physical instability. Further types of variable stars such as RV Tauri variables and PV Telescopii variables are often described as supergiants. RV Tau stars are frequently assigned spectral types with

4717-716: 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

4806-420: The fainter level since then. The star also became significantly more reddened . The eight-day variations continue, with a maximum brightness now around magnitude 14 and magnitude 16.5 at its faintest. It is theorised that the root cause of this dimness is a warp in the accretion disk , located at a distance of 7.7 AU or more from the centre, that was brought into the line of sight by its elliptical motion around

4895-924: The helium is exhausted, so they eventually just lose their outer layers, leaving the core of a white dwarf . The phase where these stars have both hydrogen and helium burning shells is referred to as the asymptotic giant branch (AGB), as stars gradually become more and more luminous class M stars. Stars of 8-10  M ☉ may fuse sufficient carbon on the AGB to produce an oxygen-neon core and an electron-capture supernova , but astrophysicists categorise these as super-AGB stars rather than supergiants. There are several categories of evolved stars that are not supergiants in evolutionary terms but may show supergiant spectral features or have luminosities comparable to supergiants. Asymptotic-giant-branch (AGB) and post-AGB stars are highly evolved lower-mass red giants with luminosities that can be comparable to more massive red supergiants, but because of their low mass, being in

4984-614: 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

5073-404: 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 )

5162-408: 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

5251-479: 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

5340-457: The main sequence due to rotation or because some blue supergiants are newly evolved from the main sequence while others have previously been through a red supergiant phase. Post-red supergiant stars have a generally higher level of nitrogen relative to carbon due to convection of CNO-processed material to the surface and the complete loss of the outer layers. Surface enhancement of helium is also stronger in post-red supergiants, representing more than

5429-419: 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

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5518-617: The main sequence. The majority of red supergiants were 10-15  M ☉ main sequence stars and now have luminosities below 100,000  L ☉ , and there are very few bright supergiant (Ia) M class stars. The least luminous stars classified as red supergiants are some of the brightest AGB and post-AGB stars, highly expanded and unstable low mass stars such as the RV Tauri variables . The majority of AGB stars are given giant or bright giant luminosity classes, but particularly unstable stars such as W Virginis variables may be given

5607-485: 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

5696-401: The most detailed analyses can distinguish the spectral features that show they have evolved away from the narrow early O-type main-sequence to the nearby area of early O-type supergiants. Such early O-type supergiants share many features with WNLh Wolf–Rayet stars and are sometimes designated as slash stars , intermediates between the two types. Luminous blue variables (LBVs) stars occur in

5785-454: The most luminous (and unstable) stars having log(g) around zero. Hotter supergiants, even the most luminous, have surface gravities around one, due to their higher masses and smaller radii. There are supergiant stars at all of the main spectral classes and across the whole range of temperatures from mid-M class stars at around 3,400 K to the hottest O class stars over 40,000 K. Supergiants are generally not found cooler than mid-M class. This

5874-639: The most luminous and massive will actually go on to develop an iron core. The majority of them are intermediate mass stars fusing helium in their cores and will eventually transition to the asymptotic giant branch. δ Cephei itself is an example with a luminosity of 2,000  L ☉ and a mass of 4.5  M ☉ . Wolf–Rayet stars are also high-mass luminous evolved stars, hotter than most supergiants and smaller, visually less bright but often more luminous because of their high temperatures. They have spectra dominated by helium and other heavier elements, usually showing little or no hydrogen, which

5963-692: The most massive hot supergiants to around a million L ☉ ( M bol around −10). Stars near and occasionally beyond these limits become unstable, pulsate, and experience rapid mass loss. The supergiant luminosity class is assigned on the basis of spectral features that are largely a measure of surface gravity, although such stars are also affected by other properties such as microturbulence . Supergiants typically have surface gravities of around log(g) 2.0 cgs and lower, although bright giants (luminosity class II) have statistically very similar surface gravities to normal Ib supergiants. Cool luminous supergiants have lower surface gravities, with

6052-538: 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

6141-401: 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

6230-619: 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

6319-428: 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,

6408-498: The same region of the HR diagram as blue supergiants but are generally classified separately. They are evolved, expanded, massive, and luminous stars, often hypergiants, but they have very specific spectral variability, which defies the assignment of a standard spectral type. LBVs observed only at a particular time or over a period of time when they are stable, may simply be designated as hot supergiants or as candidate LBVs due to their luminosity. Hypergiants are frequently treated as

6497-432: The same temperature. This means that hot supergiants lie on a relatively narrow band above bright main sequence stars. A B0 main sequence star has an absolute magnitude of about −5, meaning that all B0 supergiants are significantly brighter than absolute magnitude −5. Bolometric luminosities for even the faintest blue supergiants are tens of thousands of times the sun ( L ☉ ). The brightest can be over

6586-475: The same time, carbon and oxygen abundances are reduced. Red supergiants can be distinguished from luminous but less massive AGB stars by unusual chemicals at the surface, enhancement of carbon from deep third dredge-ups, as well as carbon-13, lithium and s-process elements. Late-phase AGB stars can become highly oxygen-enriched, producing OH masers . Hotter supergiants show differing levels of nitrogen enrichment. This may be due to different levels of mixing on

6675-410: 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

6764-455: 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

6853-409: The star, then rapidly evolve through a series of stages of similarly hot and luminous stars: supergiants, slash stars, WNh-, WN-, and possibly WC- or WO-type stars. They are expected to explode as supernovae, but it is not clear how far they evolve before this happens. The existence of these supergiants still burning hydrogen in their cores may necessitate a slightly more complex definition of supergiant:

6942-538: The stars. The bolometric luminosity of a star reflects its total output of electromagnetic radiation at all wavelengths. For very hot and very cool stars, the bolometric luminosity is dramatically higher than the visual luminosity, sometimes several magnitudes or a factor of five or more. This bolometric correction is approximately one magnitude for mid B, late K, and early M stars, increasing to three magnitudes (a factor of 15) for O and mid M stars. All supergiants are larger and more luminous than main sequence stars of

7031-583: 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

7120-422: The sun. Some AGB stars also receive a supergiant luminosity class, most notably W Virginis variables such as W Virginis itself, stars that are executing a blue loop triggered by thermal pulsing . A very small number of Mira variables and other late AGB stars have supergiant luminosity classes, for example α Herculis . Classical Cepheid variables typically have supergiant luminosity classes, although only

7209-423: The surface. Cool supergiants show enhanced helium and nitrogen at the surface due to convection of these fusion products to the surface during the main sequence of very massive stars, to dredge-ups during shell burning, and to the loss of the outer layers of the star. Helium is formed in the core and shell by fusion of hydrogen and nitrogen which accumulates relative to carbon and oxygen during CNO cycle fusion. At

7298-463: 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

7387-483: 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

7476-493: The unusual type II Supernova 1987A was a blue supergiant , thought to have already passed through the red supergiant phase of its life, and this is now known to be far from an exceptional situation. Much research is now focused on how blue supergiants can explode as a supernova and when red supergiants can survive to become hotter supergiants again. Supergiants are rare and short-lived stars, but their high luminosity means that there are many naked-eye examples, including some of

7565-476: 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

7654-443: The way to iron. Also because of their high masses, they are destined to explode as supernovae . The Stefan–Boltzmann law dictates that the relatively cool surfaces of red supergiants radiate much less energy per unit area than those of blue supergiants ; thus, for a given luminosity, red supergiants are larger than their blue counterparts. Radiation pressure limits the largest cool supergiants to around 1,500 R ☉ and

7743-457: 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

7832-882: Was quite different, with reduced convection and less mass loss. Their very short lives are likely to have ended in violent photodisintegration or pair instability supernovae. Most type II supernova progenitors are thought to be red supergiants, while the less common type Ib/c supernovae are produced by hotter Wolf–Rayet stars that have completely lost more of their hydrogen atmosphere. Almost by definition, supergiants are destined to end their lives violently. Stars large enough to start fusing elements heavier than helium do not seem to have any way to lose enough mass to avoid catastrophic core collapse, although some may collapse, almost without trace, into their own central black holes. The simple "onion" models showing red supergiants inevitably developing to an iron core and then exploding have been shown, however, to be too simplistic. The progenitor for

7921-488: 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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