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 a particular chemical element or molecule, with the line strength indicating the abundance of that element. The strengths of the different spectral lines vary 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 summarizing the ionization state, giving an objective measure of the photosphere's temperature. Most stars are classified under the Morgan-Keenan system using the letters O, B, A, F, G, K, M, a sequence from the hottest to the coolest; each letter class is subdivided using a numeric digit with 0 being hottest and 9 being coolest. The sequence has been expanded with classes for other stars and star-like objects that do not fit in the classical system, such as class D for white dwarfs and classes S and C for carbon stars.
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+ is used for hypergiants, class I for supergiants, class II for bright giants, class III for regular giants, class IV for sub-giants, class V for main-sequence stars, class sd for sub-dwarfs, class D for white dwarfs; the full spectral class for the Sun is G2V, indicating a main-sequence star with a temperature around 5,800 K. The conventional color description takes into account only the peak of the stellar spectrum. In actuality, stars radiate in all parts of the spectrum; because all spectral colors combined appear white, the actual apparent colors the human eye would observe are far lighter than the conventional color descriptions would suggest. This characteristic of'lightness' indicates that the simplified assignment of colors within the spectrum can be misleading.
Excluding color-contrast illusions in dim light, there are indigo, or violet stars. Red dwarfs are a deep shade of orange, brown dwarfs do not appear brown, but hypothetically would appear dim grey to a nearby observer; the modern classification system is known as the Morgan–Keenan classification. Each star is assigned a spectral class from the older Harvard spectral classification 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 indexes—the measured differences in three or more color magnitudes; those numbers are given labels such as "U-V" or "B-V", which represent the colors passed by two standard filters. The Harvard system is a one-dimensional classification scheme by astronomer Annie Jump Cannon, who re-ordered and simplified a prior alphabetical system. 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 2,000 to 50,000 K, whereas more-evolved stars can have temperatures above 100,000 K. Physically, the classes indicate the temperature of the star's atmosphere and are listed from hottest to coldest; the spectral classes O through M, as well as other more specialized classes discussed are subdivided by Arabic numerals, where 0 denotes the hottest stars of a given class. For example, A0 denotes A9 denotes the coolest ones. Fractional numbers are allowed; the Sun is classified as G2. Conventional color descriptions are traditional in astronomy, represent colors relative to the mean color of an A class star, considered to be white; the apparent color descriptions are what the observer would see if trying to describe the stars under a dark sky without aid to the eye, or with binoculars. However, most stars in the sky, except the brightest ones, appear white or bluish white to the unaided eye because they are too dim for color vision to work. Red supergiants are cooler and redder than dwarfs of the same spectral type, stars with particular spectral features such as carbon stars may be far redder than any black body.
The fact that the Harvard classification of a star indicated its surface or photospheric temperature was not understood until after its development, though by the time the first Hertzsprung–Russell diagram was formulated, this was 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 the solar chromosphere to stellar spectra. Harvard astronomer Cecilia Payne demonstrated that the O-B-A-F-G-K-M spectral sequence is 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 estimates of 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 called the MKK system from the authors' initial
Carina is a constellation in the southern sky. Its name is Latin for the keel of a ship, it was part of the larger constellation of Argo Navis until that constellation was divided into three pieces, the other two being Puppis, Vela. Carina was once a part of Argo Navis, the great ship of Jason and the Argonauts who searched for the Golden Fleece; the constellation of Argo was introduced in ancient Greece. However, due to the massive size of Argo Navis and the sheer number of stars that required separate designation, Nicolas Louis de Lacaille divided Argo into three sections in 1763, including Carina. In the 19th century, these three became established as separate constellations, were formally included in the list of 88 modern IAU constellations in 1930. Lacaille kept a single set of Greek letters for the whole of Argo, separate sets of Latin letter designations for each of the three sections. Therefore, Carina has the α, β and ε, Vela has γ and δ, Puppis has ζ, so on. Carina contains Canopus, a white-hued supergiant, the second brightest star in the night sky at magnitude −0.72, 313 light-years from Earth.
Alpha Carinae, as Canopus is formally designated, is a variable star that varies by 0.1 magnitudes. Its traditional name comes from the mythological Canopus, a navigator for Menelaus, king of Sparta. There are several other stars above magnitude 3 in Carina. Beta Carinae, traditionally called Miaplacidus, is a blue-white hued star of magnitude 1.7, 111 light-years from Earth. Epsilon Carinae is an orange-hued giant star bright to Miaplacidus at magnitude 1.9. Another bright star is the blue-white hued Theta Carinae. Theta Carinae is the most prominent member of the cluster IC 2602. Iota Carinae is a white-hued supergiant star of 690 light-years from Earth. Eta Carinae is the most prominent variable star in Carina, it was first discovered to be unusual in 1677, when its magnitude rose to 4, attracting the attention of Edmond Halley. Eta Carinae is inside NGC 3372 called the Carina Nebula, it had a long outburst in 1827, when it brightened to magnitude 1, only fading to magnitude 1.5 in 1828. Its most prominent outburst made Eta Carinae the equal of Sirius.
However, since 1843, Eta Carinae has remained placid, having a magnitude between 6.5 and 7.9. However, in 1998, it brightened though only to magnitude 5.0, a far less drastic outburst. Eta Carinae is a binary star, with a companion. There are several less prominent variable stars in Carina. L Carinae is a Cepheid variable noted for its brightness, it is a yellow-hued supergiant star with a minimum magnitude of 4.2 and a maximum magnitude of 3.3. Two bright Mira variable stars are in Carina: S Carinae. R Carinae has a minimum magnitude of 10.0 and a maximum magnitude of 4.0. Its period is 309 days and it is 416 light-years from Earth. S Carinae is similar, with a minimum magnitude of 10.0 and a maximum magnitude of 5.0. However, S Carinae has a shorter period – 150 days, though it is much more distant at 1300 light-years from Earth. Carina is home to binary stars. Upsilon Carinae is a binary star with two blue-white hued giant components, 1600 light-years from Earth; the primary is of magnitude 3.0 and the secondary is of magnitude 6.0.
Two asterisms are prominent in Carina. One is known as the'Diamond Cross', larger than the Southern Cross, from the perspective of the southern hemisphere viewer, upside down, the long axes of the two crosses being close to parallel. Another asterism in the constellation is the False Cross mistaken for the Southern Cross, an asterism in Crux; the False Cross consists of two stars in Carina, Iota Carinae and Epsilon Carinae, two stars in Vela, Kappa Velorum and Delta Velorum. Carina is known for its namesake nebula, NGC 3372, discovered by French astronomer Nicolas Louis de Lacaille in 1751, which contains several nebulae; the Carina Nebula overall is an extended emission nebula 8,000 light-years away and 300 light-years wide that includes vast star-forming regions. It has an apparent diameter of over 2 degrees, its central region is called the Keyhole Nebula. This was described in 1847 by John Herschel, likened to a keyhole by Emma Converse in 1873; the Keyhole is about seven light-years wide and is composed of ionized hydrogen, with two major star-forming regions.
The Homunculus Nebula is a planetary nebula visible to the naked eye, being ejected by the erratic luminous blue variable star Eta Carinae, the most massive visible star known. Eta Carinae is so massive that it has reached the theoretical upper limit for the mass of a star and is therefore unstable, it is known for its outbursts. Because of this instability and history of outbursts, Eta Carinae is considered a prime supernova candidate for the next several hundred thousand years because it has reached the end of its estimated million-year life span. NGC 2516 is an open cluster, both quite large (ap
A-type main-sequence star
An A-type main-sequence star or A dwarf star is a main-sequence star of spectral type A and luminosity class V. These stars have spectra, they have masses from 1.4 to 2.1 times the mass of the Sun and surface temperatures between 7600 and 10,000 K. Bright and nearby examples are Altair, Sirius A, Vega. A-type stars don't have a convective zone and thus aren't expected to harbor a magnetic dynamo; as a consequence, because they don't have strong stellar winds they lack a means to generate X-ray emission. The revised Yerkes Atlas system listed a dense grid of A-type dwarf spectral standard stars, but not all of these have survived to this day as standards; the "anchor points" and "dagger standards" of the MK spectral classification system among the A-type main-sequence dwarf stars, i.e. those standard stars that have remain unchanged over years and can be considered to define the system, are Vega, Gamma Ursae Majoris, Fomalhaut. The seminal review of MK classification by Morgan & Keenan didn't provide any dagger standards between types A3 V and F2 V. HD 23886 was suggested as an A5 V standard in 1978.
Richard Gray & Robert Garrison provided the most recent contributions to the A dwarf spectral sequence in a pair of papers in 1987 and 1989. They list an assortment of fast- and slow-rotating A-type dwarf spectral standards, including HD 45320, HD 88955, 2 Hydri, 21 Leonis Minoris, 44 Ceti. Besides the MK standards provided in Morgan's papers and the Gray & Garrison papers, one occasionally sees Delta Leonis listed as a standard. There are no published A6 A8 V standard stars. A-type stars are young and many emit infrared radiation beyond what would be expected from the star alone; this IR excess is attributable to dust emission from a debris disk. Surveys indicate massive planets form around A-type stars although these planets are difficult to detect using the Doppler spectroscopy method; this is because A-type stars rotate quickly, which makes it difficult to measure the small Doppler shifts induced by orbiting planets since the spectral lines are broad. However, this type of massive star evolves into a cooler red giant which rotates more and thus can be measured using the radial velocity method.
As of early 2011 about 30 Jupiter class planets have been found around evolved K-giant stars including Pollux, Gamma Cephei and Iota Draconis. Doppler surveys around a wide variety of stars indicate about 1 in 6 stars having twice the mass of the Sun are orbited by one or more Jupiter-sized planets, compared to about 1 in 16 for Sun-like stars. A-type star systems known to feature planets include Fomalhaut, HD 15082, Beta Pictoris and HD 95086 b. Star count, survey of stars B-type main-sequence star
Subdwarf B star
A B-type subdwarf is a kind of subdwarf star with spectral type B. They differ from the typical subdwarf by being much brighter, they are situated at the "extreme horizontal branch" of the Hertzsprung–Russell diagram. Masses of these stars are around 0.5 solar masses, they contain only about 1% hydrogen, with the rest being helium. Their radius is from 0.15 to 0.25 solar radii, their temperature is from 20,000 to 40,000K. These stars represent a late stage in the evolution of some stars, caused when a red giant star loses its outer hydrogen layers before the core begins to fuse helium; the reasons why this premature mass loss occurs are unclear, but the interaction of stars in a binary star system is thought to be one of the main mechanisms. Single subdwarfs may be the result of a merger of two white dwarfs; the sdB stars are expected to become white dwarfs without going through any more giant stages. Subdwarf B stars, being more luminous than white dwarfs, are a significant component in the hot star population of old stellar systems, such as globular clusters, spiral galaxy bulges and elliptical galaxies.
They are prominent on ultraviolet images. The hot subdwarfs are proposed to be the cause of the UV upturn in the light output of elliptical galaxies. Subdwarf B stars were discovered by Zwicky and Humason around 1947 when they found subluminous blue stars around the north galactic pole. In the Palomar-Green survey they were discovered to be the commonest kind of faint blue star with a magnitude over 18. During the 1960s spectroscopy discovered that many of the sdB stars are deficient in hydrogen, with abundances below that predicted by the big bang theory. In the early 1970s Greenstein and Sargent measured temperatures and gravity strengths and were able to plot their correct position on the Hertzsprung–Russell diagram. There are three kinds of variable stars in this category: Firstly there are the sdBV with periods from 90 to 600 seconds, they are called EC14026 or V361 Hya stars. A proposed new nomenclature is sdBVr, with r standing for rapid; the Charpinet theory of the oscillations of these stars is that the variations in brightness are due to acoustic mode oscillations with low degree and low order.
They are driven by ionisation of iron group atoms causing opacity. The velocity curve is 90 degrees out of phase with the brightness curve, while the effective temperature and surface gravity acceleration curves appear to be in phase with the flux variations. In plots of temperature against surface gravity, the short-period pulsators cluster together in the so-called empirical instability strip defined by T=28000–35000 K and log g=5.2–6.0. Only 10% of sdBs falling in the empirical strip are observed to pulsate. Secondly there are the long period variables with periods from 45 to 180 minutes. A proposed new nomenclature is sdBVs, with s standing for slow; these only have a small variation of 0.1%. They have been called PG1716 or V1093 Her or abbreviated as LPsdBV; the long-period pulsating sdB stars are cooler than their rapid counterparts, with T~23000–30000K. Stars that oscillate in both period regimes are'hybrids', with a standard nomenclature of sdBVrs. A prototype is DW Lyn identified as HS 0702+6043.
*eclipsing binary star At least two sdB stars are known to have planets. V391 Pegasi was the first known sdB planet-host, Kepler-70 has a system of close-orbiting planets that may be the remnants of a giant planet, engulfed by the red giant progenitor
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 3,450 K to over 20,000 K. The title supergiant, as applied to 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 larger and more luminous than the bulk, the term super-giant arose adopted as supergiant. Supergiant stars can be identified on 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 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 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, changes can be observed in their line profiles. Supergiants are evolved stars with higher levels of heavy elements than main-sequence stars; this 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. In reality there is much more of a continuum than well defined bands for these classifications, classifications such as Iab are used for intermediate luminosity supergiants. Supergiant spectra are annotated to indicate spectral peculiarities, for example B2 Iae or F5 Ipec. Supergiants can be defined as a specific phase in the evolutionary history of certain stars. Stars with initial masses above 8-10 M☉ and smoothly initiate helium core fusion after they have exhausted their hydrogen, continue fusing heavier elements after helium exhaustion until they develop an iron core, at which point the core collapses to produce a Type 2 supernova. Once these massive stars leave the main sequence, their atmospheres inflate, they are described as supergiants. Stars 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 the helium is exhausted, so they 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, as stars 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 which are not supergiants in evolutionary terms but may show supergiant spectral features or have luminosities comparable to supergiants. Asymptotic-giant-branch and post-AGB stars are evolved lower-mass red giants with luminosities that can be comparable to more massive red supergiants, but because of their low mass, being in a different stage of development, their lives ending in a different way, astrophysicists prefer to keep them separate; the dividing line becomes blurred at around 7–10 M☉ where stars start to undergo limited fusion of elements heavier than helium. Specialists studying these stars 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 luminosity class despite being less massive than the sun; some AGB stars 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 small number of Mira variables and other late AGB stars have supergiant luminosity classes, for example α Herculis. Classical Cepheid variables have supergiant luminosity classes, although only the most luminous and massive will go on to develop an iron core; the majority of them are intermediate mass stars fusing helium in their cores and will 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 high-mass luminous evolved stars, hotter than most supergiants and smaller, visually less
Stellar rotation is the angular motion of a star about its axis. The rate of rotation can be measured from the spectrum of the star, or by timing the movements of active features on the surface; the rotation of a star produces an equatorial bulge due to centrifugal force. As stars are not solid bodies, they can undergo differential rotation, thus the equator of the star can rotate at a different angular velocity than the higher latitudes. These differences in the rate of rotation within a star may have a significant role in the generation of a stellar magnetic field; the magnetic field of a star interacts with the stellar wind. As the wind moves away from the star its rate of angular velocity slows; the magnetic field of the star interacts with the wind, which applies a drag to the stellar rotation. As a result, angular momentum is transferred from the star to the wind, over time this slows the star's rate of rotation. Unless a star is being observed from the direction of its pole, sections of the surface have some amount of movement toward or away from the observer.
The component of movement, in the direction of the observer is called the radial velocity. For the portion of the surface with a radial velocity component toward the observer, the radiation is shifted to a higher frequency because of Doppler shift; the region that has a component moving away from the observer is shifted to a lower frequency. When the absorption lines of a star are observed, this shift at each end of the spectrum causes the line to broaden. However, this broadening must be separated from other effects that can increase the line width; the component of the radial velocity observed through line broadening depends on the inclination of the star's pole to the line of sight. The derived value is given as v e ⋅ sin i, where ve is the rotational velocity at the equator and i is the inclination. However, i is not always known, so the result gives a minimum value for the star's rotational velocity; that is, if i is not a right angle the actual velocity is greater than v e ⋅ sin i. This is sometimes referred to as the projected rotational velocity.
In fast rotating stars polarimetry offers a method of recovering the actual velocity rather than just the rotational velocity. For giant stars, the atmospheric microturbulence can result in line broadening, much larger than effects of rotational drowning out the signal. However, an alternate approach can be employed; these occur when a massive object passes in front of the more distant star and functions like a lens magnifying the image. The more detailed information gathered by this means allows the effects of microturbulence to be distinguished from rotation. If a star displays magnetic surface activity such as starspots these features can be tracked to estimate the rotation rate. However, such features can form at locations other than equator and can migrate across latitudes over the course of their life span, so differential rotation of a star can produce varying measurements. Stellar magnetic activity is associated with rapid rotation, so this technique can be used for measurement of such stars.
Observation of starspots has shown that these features can vary the rotation rate of a star, as the magnetic fields modify the flow of gases in the star. Gravity tends to contract celestial bodies into a perfect sphere, the shape where all the mass is as close to the center of gravity as possible, but a rotating star is not spherical in shape, it has an equatorial bulge. As a rotating proto-stellar disk contracts to form a star its shape becomes more and more spherical, but the contraction doesn't proceed all the way to a perfect sphere. At the poles all of the gravity acts to increase the contraction, but at the equator the effective gravity is diminished by the centrifugal force; the final shape of the star after star formation is an equilibrium shape, in the sense that the effective gravity in the equatorial region cannot pull the star to a more spherical shape. The rotation gives rise to gravity darkening at the equator, as described by the von Zeipel theorem. An extreme example of an equatorial bulge is found on the star Regulus A.
The equator of this star has a measured rotational velocity of 317 ± 3 km/s. This corresponds to a rotation period of 15.9 hours, 86% of the velocity at which the star would break apart. The equatorial radius of this star is 32% larger than polar radius. Other rotating stars include Alpha Arae, Pleione and Achernar; the break-up velocity of a star is an expression, used to describe the case where the centrifugal force at the equator is equal to the gravitational force. For a star to be stable the rotational velocity must be below this value. Surface differential rotation is observed on stars such as the Sun when the angular velocity varies with latitude; the angular velocity decreases with increasing latitude. However the reverse has been observed, such as on the star designated HD 31993; the first such star, other than the Sun, to have its differential rotation mapped in detail is AB Doradus. The underlying mechanism that causes differential rotation is turbulent convection inside a star. Convective motion carries energy toward the surface through the mass movement of plasma.
This mass of plasma carries a portion of the angular velocity of the star. When turbulence occurs through shear and rotation, the angular momentum can become redistributed to different latitudes thro
Blue supergiant star
Blue supergiant stars are hot luminous stars, referred to scientifically as OB supergiants. They have earlier. Blue supergiants are found towards the top left of the Hertzsprung–Russell diagram to the right of the main sequence, they are larger than the Sun but smaller than a red supergiant, with surface temperatures of 10,000–50,000 K and luminosities from about 10,000 to a million times the Sun. Supergiants are evolved high-mass stars and more luminous than main-sequence stars. O class and early B class stars with initial masses around 10-300 M☉ evolve away from the main sequence in just a few million years as their hydrogen is consumed and heavy elements start to appear near the surface of the star; these stars become blue supergiants, although it is possible that some of them evolve directly to Wolf–Rayet stars. Expansion into the supergiant stage occurs when hydrogen in the core of the star is depleted and hydrogen shell burning starts, but it may be caused as heavy elements are dredged up to the surface by convection and mass loss due to radiation pressure increase.
Blue supergiants are newly evolved from the main sequence, have high luminosities, high mass loss rates, are unstable. Many of them become luminous blue variables with episodes of extreme mass loss. Lower mass blue supergiants continue to expand. In the process they must spend some time as yellow supergiants or yellow hypergiants, but this expansion occurs in just a few thousand years and so these stars are rare. Higher mass red supergiants blow away their outer atmospheres and evolve back to blue supergiants, onwards to Wolf–Rayet stars. Depending on the exact mass and composition of a red supergiant, it can execute a number of blue loops before either exploding as a type II supernova or dumping enough of its outer layers to become a blue supergiant again, less luminous than the first time but more unstable. If such a star can pass through the yellow evolutionary void it is expected that it becomes one of the lower luminosity LBVs; the most massive blue supergiants are too luminous to retain an extensive atmosphere and they never expand into a red supergiant.
The dividing line is 40 M☉, although the coolest and largest red supergiants develop from stars with initial masses of 15-25 M☉. It is not clear whether more massive blue supergiants can lose enough mass to evolve safely into old age as a Wolf Rayet star and a white dwarf, or they reach the Wolf Rayet stage and explode as supernovae, or they explode as supernovae while blue supergiants. Supernova progenitors are most red supergiants and it was believed that only red supergiants could explode as supernovae. SN 1987A, forced astronomers to re-examine this theory, as its progenitor, Sanduleak -69° 202, was a B3 blue supergiant. Now it is known from observation that any class of evolved high-mass star, including blue and yellow supergiants, can explode as a supernova although theory still struggles to explain how in detail. While most supernovae are of the homogeneous type II-P and are produced by red supergiants, blue supergiants are observed to produce supernovae with a wide range of luminosities and spectral types, sometimes sub-luminous like SN 1987A, sometimes super-luminous such as many type IIn supernovae.
Because of their extreme masses they have short lifespans and are observed in young cosmic structures such as open clusters, the arms of spiral galaxies, in irregular galaxies. They are observed in spiral galaxy cores, elliptical galaxies, or globular clusters, most of which are believed to be composed of older stars, although the core of the Milky Way has been found to be home to several massive open clusters and associated young hot stars; the best known example is the brightest star in the constellation of Orion. Its mass is about 20 times that of the Sun, its luminosity is around 117,000 times greater. Despite their rarity and their short lives they are represented among the stars visible to the naked eye. Blue supergiants have fast stellar winds and the most luminous, called hypergiants, have spectra dominated by emission lines that indicate strong continuum driven mass loss. Blue supergiants show varying quantities of heavy elements in their spectra, depending on their age and the efficiency with which the products of nucleosynthesis in the core are convected up to the surface.
Rotating supergiants can be mixed and show high proportions of helium and heavier elements while still burning hydrogen at the core. While the stellar wind from a red supergiant is dense and slow, the wind from a blue supergiant is fast but sparse; when a red supergiant becomes a blue supergiant, the faster wind it produces impacts the emitted slow wind and causes the outflowing material to condense into a thin shell. In some cases several concentric faint shells can be seen from successive episodes of mass loss, either previous blue loops from the red supergiant stage, or eruptions such as LBV outbursts. MACS J1149 Lensed Star 1 – most distant individual star detected Rigel, a blue-white supergiant UW Canis Majoris, a blue supergiant Zeta Puppis, a blue supergiant