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61-528: (Redirected from Blue Sun ) Blue sun may refer to: A star with a suitable spectral type O Blue Sun (album) , a 1982 album by Ralph Towner Blue Sun (album), a 2017 album by Jack Nunn on Atlantic Jaxx records Blue Sun Corporation, a fictional corporation in the Firefly television series Topics referred to by the same term [REDACTED] This disambiguation page lists articles associated with

122-506: A distinct evolutionary track towards true giants. Examples: Bright giants are stars of luminosity class II in the Yerkes spectral classification . These are stars which straddle the boundary between ordinary giants and supergiants , based on the appearance of their spectra. The bright giant luminosity class was first defined in 1943. Well known stars which are classified as bright giants include: Within any giant luminosity class,

183-482: A few million years the core reaches the Schönberg–Chandrasekhar limit , rapidly collapses, and may become degenerate. This causes the outer layers to expand even further and generates a strong convective zone that brings heavy elements to the surface in a process called the first dredge-up . This strong convection also increases the transport of energy to the surface, the luminosity increases dramatically, and

244-418: A giant, but any main-sequence star is properly called a dwarf, regardless of how large and luminous it is. A star becomes a giant after all the hydrogen available for fusion at its core has been depleted and, as a result, leaves the main sequence . The behaviour of a post-main-sequence star depends largely on its mass. For a star with a mass above about 0.25 solar masses ( M ☉ ), once

305-533: 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

366-668: 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

427-484: A number of important classes of variable stars. High-luminosity yellow stars are generally unstable, leading to the instability strip on the HR diagram where the majority of stars are pulsating variables. The instability strip reaches from the main sequence up to hypergiant luminosities, but at the luminosities of giants there are several classes of pulsating variable stars: Yellow giants may be moderate-mass stars evolving for

488-493: 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 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

549-457: 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

610-537: A supernova. Stars in the 8~12  M ☉ range have somewhat intermediate properties and have been called super-AGB stars. They largely follow the tracks of lighter stars through RGB, HB, and AGB phases, but are massive enough to initiate core carbon burning and even some neon burning. They form oxygen–magnesium–neon cores, which may collapse in an electron-capture supernova, or they may leave behind an oxygen–neon white dwarf. O class main sequence stars are already highly luminous. The giant phase for such stars

671-419: A very heterogeneous grouping, ranging from high-mass, high-luminosity stars just leaving the main sequence to low-mass, horizontal-branch stars . Higher-mass stars leave the main sequence to become blue giants, then bright blue giants, and then blue supergiants, before expanding into red supergiants, although at the very highest masses the giant stage is so brief and narrow that it can hardly be distinguished from

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732-520: Is a brief phase of slightly increased size and luminosity before developing a supergiant spectral luminosity class. Type O giants may be more than a hundred thousand times as luminous as the sun, brighter than many supergiants. Classification is complex and difficult with small differences between luminosity classes and a continuous range of intermediate forms. The most massive stars develop giant or supergiant spectral features while still burning hydrogen in their cores, due to mixing of heavy elements to

793-407: 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

854-559: 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

915-529: Is expected to become a helium white dwarf , although the universe is too young for any such star to exist yet, so no star with that history has ever been observed. There are a wide range of giant-class stars and several subdivisions are commonly used to identify smaller groups of stars. Subgiants are an entirely separate spectroscopic luminosity class (IV) from giants, but share many features with them. Although some subgiants are simply over-luminous main-sequence stars due to chemical variation or age, others are

976-512: 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

1037-538: Is the asymptotic giant branch (AGB) analogous to the red-giant branch but more luminous, with a hydrogen-burning shell contributing most of the energy. Stars only remain on the AGB for around a million years, becoming increasingly unstable until they exhaust their fuel, go through a planetary nebula phase, and then become a carbon–oxygen white dwarf. Main-sequence stars with masses above about 12  M ☉ are already very luminous and they move horizontally across

1098-423: 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 a particular chemical element or molecule , with the line strength indicating the abundance of that element. The strengths of

1159-589: 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,

1220-591: 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

1281-505: 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

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1342-496: The Sun is then G2V, indicating a main-sequence star with 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

1403-639: 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

1464-453: 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

1525-477: The HR diagram when they leave the main sequence, briefly becoming blue giants before they expand further into blue supergiants. They start core-helium burning before the core becomes degenerate and develop smoothly into red supergiants without a strong increase in luminosity. At this stage they have comparable luminosities to bright AGB stars although they have much higher masses, but will further increase in luminosity as they burn heavier elements and eventually become

1586-689: 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+ 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

1647-519: 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

1708-560: 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. Giant star Giant stars have radii up to a few hundred times the Sun and luminosities between 10 and a few thousand times that of the Sun . Stars still more luminous than giants are referred to as supergiants and hypergiants . A hot, luminous main-sequence star may also be referred to as

1769-532: The central temperatures necessary to fuse helium . It will therefore remain a hydrogen-fusing red giant until it runs out of hydrogen, at which point it will become a helium white dwarf . According to stellar evolution theory, no star of such low mass can have evolved to that stage within the age of the Universe. In stars above about 0.4  M ☉ the core temperature eventually reaches 10 K and helium will begin to fuse to carbon and oxygen in

1830-520: 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

1891-637: 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,

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1952-500: The cooler stars of spectral class K, M, S, and C, (and sometimes some G-type stars ) are called red giants. Red giants include stars in a number of distinct evolutionary phases of their lives: a main red-giant branch (RGB); a red horizontal branch or red clump ; the asymptotic giant branch (AGB), although AGB stars are often large enough and luminous enough to get classified as supergiants; and sometimes other large cool stars such as immediate post-AGB stars . The RGB stars are by far

2013-426: The core by the triple-alpha process . When the core is degenerate helium fusion begins explosively , but most of the energy goes into lifting the degeneracy and the core becomes convective. The energy generated by helium fusion reduces the pressure in the surrounding hydrogen-burning shell, which reduces its energy-generation rate. The overall luminosity of the star decreases, its outer envelope contracts again, and

2074-561: The core is depleted of hydrogen it contracts and heats up so that hydrogen starts to fuse in a shell around the core. The portion of the star outside the shell expands and cools, but with only a small increase in luminosity, and the star becomes a subgiant . The inert helium core continues to grow and increase in temperature as it accretes helium from the shell, but in stars up to about 10-12  M ☉ it does not become hot enough to start helium burning (higher-mass stars are supergiants and evolve differently). Instead, after just

2135-508: The current age of the Universe . They steadily become hotter and more luminous throughout this time. Eventually they do develop a radiative core, subsequently exhausting hydrogen in the core and burning hydrogen in a shell surrounding the core. (Stars with a mass in excess of 0.16  M ☉ may expand at this point, but will never become very large.) Shortly thereafter, the star's supply of hydrogen will be completely exhausted and it

2196-525: 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

2257-488: 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 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 ,

2318-719: 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

2379-527: The first time towards the red-giant branch, or they may be more evolved stars on the horizontal branch. Evolution towards the red-giant branch for the first time is very rapid, whereas stars can spend much longer on the horizontal branch. Horizontal-branch stars, with more heavy elements and lower mass, are more unstable. Examples: The hottest giants, of spectral classes O, B, and sometimes early A, are called blue giants . Sometimes A- and late-B-type stars may be referred to as white giants. The blue giants are

2440-627: 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

2501-404: 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 )

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2562-408: 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

2623-484: 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

2684-419: 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

2745-485: 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

2806-592: The most common type of giant star due to their moderate mass, relatively long stable lives, and luminosity. They are the most obvious grouping of stars after the main sequence on most HR diagrams, although white dwarfs are more numerous but far less luminous. Examples: Giant stars with intermediate temperatures (spectral class G, F, and at least some A) are called yellow giants. They are far less numerous than red giants, partly because they only form from stars with somewhat higher masses, and partly because they spend less time in that phase of their lives. However, they include

2867-401: 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

2928-638: 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

2989-428: 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,

3050-410: 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

3111-462: 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

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3172-426: The star moves from the red-giant branch to the horizontal branch . When the core helium is exhausted, a star with up to about 8  M ☉ has a carbon–oxygen core that becomes degenerate and starts helium burning in a shell. As with the earlier collapse of the helium core, this starts convection in the outer layers, triggers a second dredge-up, and causes a dramatic increase in size and luminosity. This

3233-417: The star moves onto the red-giant branch where it will stably burn hydrogen in a shell for a substantial fraction of its entire life (roughly 10% for a Sun-like star). The core continues to gain mass, contract, and increase in temperature, whereas there is some mass loss in the outer layers. If the star's mass, when on the main sequence, was below approximately 0.4  M ☉ , it will never reach

3294-630: 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

3355-422: The surface and high luminosity which produces a powerful stellar wind and causes the star's atmosphere to expand. A star whose initial mass is less than approximately 0.25  M ☉ will not become a giant star at all. For most of their lifetimes, such stars have their interior thoroughly mixed by convection and so they can continue fusing hydrogen for a time in excess of 10 years, much longer than

3416-463: 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

3477-483: 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

3538-497: The title Blue sun . 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=Blue_sun&oldid=1085885187 " Category : Disambiguation pages Hidden categories: Short description is different from Wikidata All article disambiguation pages All disambiguation pages Stellar classification In astronomy , stellar classification

3599-482: 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

3660-461: 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

3721-488: 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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