In astronomy, a blue giant is a hot star with a luminosity class of III (giant) or II (bright giant). In the standard Hertzsprung–Russell diagram, these stars lie above and to the right of the main sequence.
The term applies to a variety of stars in different phases of development, all evolved stars that have moved from the main sequence but have little else in common, so blue giant simply refers to stars in a particular region of the HR diagram rather than a specific type of star. They are much rarer than red giants, because they only develop from more massive and less common stars, and because they have short lives in the blue giant stage.
The name blue giant is sometimes misapplied to other high-mass luminous stars, such as main-sequence stars, simply because they are large and hot.
Blue giant is not a strictly defined term and it is applied to a wide variety of different types of stars. What they have in common is: a moderate increase in size and luminosity compared to main-sequence stars of the same mass or temperature, and are hot enough to be called blue, meaning spectral class O, B, and sometimes early A. They have temperatures from around 10,000 K upwards, zero age main sequence (ZAMS) masses greater than about twice the Sun (M☉), and absolute magnitudes around 0 or brighter. These stars are only 5–10 times the radius of the Sun (R☉), compared to red giants which are up to 100 R☉.
The coolest and least luminous stars referred to as blue giants are on the horizontal branch, intermediate-mass stars that have passed through a red giant phase and are now burning helium in their cores. Depending on mass and chemical composition these stars gradually move bluewards until they exhaust the helium in their cores and then they return redwards to the asymptotic giant branch (AGB). The RR Lyrae variable stars, usually with spectral types of A, lie across the middle of the horizontal branch. Horizontal-branch stars hotter than the RR Lyrae gap are generally considered to be blue giants, and sometimes the RR Lyrae stars themselves are called blue giants despite some of them being F class. The hottest stars, blue horizontal branch (BHB) stars, are called extreme horizontal branch (EHB) stars and can be hotter than main-sequence stars of the same luminosity. In these cases they are called blue subdwarf (sdB) stars rather than blue giants, named for their position to the left of the main sequence on the HR diagram rather than for their increased luminosity and temperature compared to when they were themselves main-sequence stars.
There are no strict upper limits for giant stars, but early O types become increasingly difficult to classify separately from main sequence and supergiant stars, have almost identical sizes and temperatures to the main-sequence stars from which they develop, and very short lifetimes. A good example is Plaskett's star, a close binary consisting of two O type giants both over 50 M☉, temperatures over 30,000 K, and more than 100,000 times the luminosity of the Sun (L☉). Astronomers still differ over whether to classify at least one of the stars as a supergiant, based on subtle differences in the spectral lines.
Stars found in the blue giant region of the HR diagram can be in very different stages of their lives, but all are evolved stars that have largely exhausted their core hydrogen supplies.
In the simplest case, a hot luminous star begins to expand as its core hydrogen is exhausted, and first becomes a blue subgiant then a blue giant, becoming both cooler and more luminous. Intermediate-mass stars will continue to expand and cool until they become red giants. Massive stars also continue to expand as hydrogen shell burning progresses, but they do so at approximately constant luminosity and move horizontally across the HR diagram. In this way they can quickly pass through blue giant, bright blue giant, blue supergiant, and yellow supergiant classes, until they become red supergiants. The luminosity class for such stars is determined from spectral lines that are sensitive to the surface gravity of the star, with more expanded and luminous stars being given I (supergiant) classifications while somewhat less expanded and more luminous stars are given luminosity II or III. Because they are massive stars with short lives, many blue giants are found in O-B associations, that are large collections of loosely bound young stars.
BHB stars are more evolved and have helium burning cores, although they still have an extensive hydrogen envelope. They also have moderate masses around 0.5–1.0 M☉ so they are often much older than more massive blue giants. The BHB takes its name from the prominent horizontal grouping of stars seen on colour-magnitude diagrams for older clusters, where core helium burning stars of the same age are found at a variety of temperatures with roughly the same luminosity. These stars also evolve through the core helium burning stage at constant luminosity, first increasing in temperature then decreasing again as they move toward the AGB. However, at the blue end of the horizontal branch, it forms a "blue tail" of stars with lower luminosity, and occasionally a "blue hook" of even hotter stars.
There are other highly evolved hot stars not generally referred to as blue giants: Wolf–Rayet stars, highly luminous and distinguished by their extreme temperatures and prominent helium and nitrogen emission lines; post-AGB stars forming planetary nebulae, similar to Wolf–Rayet stars but smaller and less massive; blue stragglers, uncommon luminous blue stars observed apparently on the main sequence in clusters where main-sequence stars of their luminosity should have evolved into giants or supergiants; and the true blue supergiants, the most massive stars evolved beyond blue giants and identified by the effects of greater expansion on their spectra.
A purely theoretical group of stars could be formed when red dwarfs finally exhaust their core hydrogen trillions of years into the future. These stars are convective through their depth and are expected to very slowly increase both their temperature and luminosity as they accumulate more and more helium until eventually they cannot sustain fusion and they quickly collapse to white dwarfs. Although these stars can become hotter than the Sun they will never become more luminous, so are hardly blue giants as we see them today. The name blue dwarf has been coined although that name could easily be confusing.
32 Pegasi (32 Peg) is a class B9III (blue giant) star in the constellation Pegasus. Its apparent magnitude is 4.81 and it is approximately 700 light years away based on parallax.It is a multiple star; the closest companion is Ab, at separation 0.42" and magnitude 8.86. Other companions include B, at 70.7" separation from the primary and magnitude 10.73, and C, at separation from B of 3.2" and magnitude 12.4, as well as D (separation from A of 42.8" and magnitude 11.9) and E (separation from A of 58.3" and magnitude 11.9).35 Aquarii
35 Aquarii is a single star located roughly 2,200 light years away from the Sun in the zodiac constellation of Aquarius. 35 Aquarii is its Flamsteed designation. It is visible to the naked eye as a dim, blue-white hued star with an apparent visual magnitude of 5.80. This object is moving closer to the Earth with a heliocentric radial velocity of −7 km/s, and is a suspected runaway star that may have been ejected from an open cluster as the result of a binary–binary interaction.This is a blue giant star with a stellar classification of B2 III;, a massive star that has evolved off the main sequence. It is around 22.5 million years old with a relatively low projected rotational velocity of 10 km/s. The star has 10 times the mass of the Sun and is radiating 1,622 times the Sun's luminosity from its photosphere at an effective temperature of 17,400 K.3 Monocerotis
3 Monocerotis is a binary star system in the equatorial constellation of Monoceros, located approximately 780 light years away from the Sun based on parallax. It is visible to the naked eye as a faint, blue-white hued star with a combined apparent visual magnitude of 4.92. The system is moving further from the Earth with a heliocentric radial velocity of +39 km/s.The magnitude 4.98 primary, designated component A, has a stellar classification of B5 III, matching an evolved blue giant star. It has 5.85 times the mass of the Sun and is radiating 1,105 times the Sun's luminosity from its photosphere at an effective temperature of 15,000 K. The companion, component B, is magnitude 7.96 with an angular separation of 1.9″ from the primary.68 Cygni
68 Cygni is the Flamsteed designation for a star in the constellation Cygnus. Located approximately 1,400 parsecs (4,600 ly) distant, the star is a hot blue giant of spectral type O7.5IIIn((f)), a massive star that is likely currently expanding to become a supergiant. The star is surrounded by a ring-shaped nebula (likely a Strömgren sphere) named S 119.
68 Cygni is currently classified as a rotating ellipsoidal variable, varying between apparent magnitudes 4.98 and 5.09, although the classification as a rotating ellipsoidal variable is subject to controversy. Barely visible to the naked eye, the star is likely to have a mass of around 26 solar masses and a temperature of approximately 34,000 kelvins, although many of the star's physical parameters are subject to uncertainties due to the unclear nature of the system.Agastache foeniculum
Agastache foeniculum (blue giant hyssop; syn. Agastache anethiodora (Nutt.) Britton), commonly called anise hyssop, blue giant hyssop, Fragrant giant hyssop, or the lavender giant hyssop, is a species of perennial plant in the mint family, (Lamiaceae). This plant is native to much of north-central and northern North America, notably the Great Plains and other prairies, and can be found in areas of Canada. It is tolerant of deer and drought, and also attracts hummingbirds and butterflies making it an attractive selection for gardeners.Anise hyssop is in the same family as hyssop (the mint family Lamiaceae), but they are not closely related. Hyssop (Hyssopus) is a genus of about 10-12 species of herbaceous or semi-woody plants native from the east Mediterranean to central Asia.Blue Giant (band)
Blue Giant is an American rock band from Portland, Oregon. An eclectic mix of country, indie rock, and psychedelic styles, Blue Giant have been called a Portland supergroup. The band was originally composed of the songwriters and musicians from Viva Voce, Kevin Leigh Robinson and Anita Lee Elliott, Chris Funk of The Decemberists, Evan Railton of Swords, and Seth Lorinczi of The Golden Bears, Circus Lupus & The Quails. Musically, Blue Giant could be characterized as rural psychedelic rock, with one music critic calling them a perfect country rock band.Gamma Lupi
Gamma Lupi (γ Lupi, γ Lup) is a 3rd-magnitude, B-type blue giant star in the constellation of Lupus. It is also known in ancient Chinese astronomy as 騎官一 or "the 1st (star) of the Cavalry Officer". With a telescope, Gamma Lupi can be resolved into a binary star system in close orbit. This is known as the Gamma Lupi AB system, often abbreviated as γ Lupi AB or γ Lup AB. Gamma Lupi A is itself a spectroscopic binary with a period of 2.8081 days.
This star is a proper motion member of the Upper-Centaurus Lupus sub-group in the
Scorpius-Centaurus OB association,
the nearest such co-moving association of massive stars to the Sun.HD 34880
HD 34880 is a blue giant star of magnitude 6.41 in the constellation of Orion. It is 679 light years from the solar system.HD 57197
M Puppis (M Pup, HR 2789, HD 57197) is a blue giant or bright giant star (spectral type B8II/III) in the constellation Puppis. Its apparent magnitude is 5.84, which means it is visible with the naked eye under optimal conditions. It is approximately 590 light years away based on parallax.HD 63922
HD 63922 is a class B0III (blue giant) star in the constellation Puppis. Its apparent magnitude is 4.11 and it is approximately 1600 light years away based on parallax.It is a multiple star; the primary has one close companion, Ab, at 0.3" separation and magnitude 7.19, and a more distant one, B, at 59.1" and 8.79 magnitude.Lepidochrysops quassi
Lepidochrysops quassi, the tailed blue giant Cupid, is a butterfly in the family Lycaenidae. It is found in Ivory Coast, Ghana, Togo, Nigeria and Cameroon. Its habitat consists of open areas in the forest zone.
Adults have been recorded on wing in August and September and from June to November.
The larvae feed on Solenostemon ocymoides and probably also on Ocimum species. They are associated with the ant species Camponotus maculatus.Lepidochrysops synchrematiza
Lepidochrysops synchrematiza, the untailed blue giant Cupid, is a butterfly in the family Lycaenidae. It is found in Senegal, the Gambia, Guinea, Sierra Leone, Liberia, Ivory Coast, Ghana and Togo. The habitat consists of the forest/savanna transition zone.Maia (star)
Maia , designated 20 Tauri (abbreviated 20 Tau), is a star in the constellation of Taurus. It is the fourth-brightest star in the Pleiades open star cluster (M45), after Alcyone, Atlas and Electra, in that order. Maia is a blue giant of spectral type B8 III, and a mercury-manganese star.
Maia's visual magnitude is 3.871, requiring darker skies to be seen. Its total bolometric luminosity is 660 times solar, mostly in the ultraviolet, thus suggesting a radius that is 5.5 times that of the Sun and a mass that is slightly more than 4 times solar. It was thought to be a variable star by astronomer Otto Struve. A class of stars known as Maia variables was proposed, which included Gamma Ursae Minoris, but Maia and some others in the class have since been found to be stable.Maia is surrounded by the Maia Nebula (also known as NGC 1432), one of the brightest patches of nebulosity within the Pleiades star cluster.Messier 21
Messier 21 or M21, also designated NGC 6531, is an open cluster of stars in the constellation of Sagittarius. It was discovered and catalogued by Charles Messier on June 5, 1764. This cluster is relatively young and tightly packed. A few blue giant stars have been identified in the cluster, but Messier 21 is composed mainly of small dim stars. With a magnitude of 6.5, M21 is not visible to the naked eye; however, with the smallest binoculars it can be easily spotted on a dark night. The cluster is positioned near the Trifid nebula (NGC 6514), but is not associated with that nebulosity. It forms part of the Sagittarius OB1 association.This cluster is located 1,205 pc away from Earth with an extinction of 0.87. Messier 21 is around 6.6 million years old with a mass of 783.4 M☉. It has a tidal radius of 11.7 pc, with a nucleus radius of 1.6±0.1 pc and a coronal radius of 3.6±0.2 pc. There are at least 105±11 members within the coronal radius down to visual magnitude 15.5, including many early B-type stars. An estimated 40–60 of the observed low-mass members are expected to be pre-main-sequence stars, with 26 candidates identified based upon hydrogen alpha emission and the presence of lithium in the spectrum. The stars in the cluster do not show a significant spread in ages, suggesting that the star formation was triggered all at once.Mu Ophiuchi
Mu Ophiuchi (μ Oph) is a class B8II-III (blue giant) star in the constellation Ophiuchus. Its apparent magnitude is 4.62 and it is approximately 760 light years away based on parallax.In 2006, a new nearby star cluster, Mamajek 2 (), was discovered associated with Mu Ophiuchi.N Scorpii
N Scorpii (N Sco) is a blue giant star in the constellation Scorpius. Its apparent magnitude is 4.23. It was initially given the Bayer designation Alpha Normae by Lacaille but later moved from Norma to Scorpius.PU Puppis
PU Puppis (PU Pup) is a class B8III (blue giant) star in the constellation Puppis. Its apparent magnitude is 4.69 and it is approximately 620 light years away based on parallax.It is a β Lyrae variable, ranging from 4.75 to 4.69 magnitude with a period of 2.58 days. The secondary is estimated at about 5.6 magnitude, although recent observations have failed to confirm it.V810 Centauri
V810 Centauri is a double star consisting of a yellow supergiant primary (V810 Cen A) and blue giant secondary (V810 Cen B). It is a small amplitude variable star, entirely due to the supergiant primary which is visually over three magnitudes brighter than the secondary.V810 Cen A shows semi-regular variations with several component periods. The dominant mode is around 156 days and corresponds to Cepheid fundamental mode radial pulsation. Without the other pulsation modes it would be considered a Classical Cepheid variable. Other pulsation modes have been detected at 89 to 234 days, with the strongest being a possible non-radial p-mode at 107 days and a possible non-radial g-mode at 185 days.The blue giant secondary has a similar mass and luminosity to the supergiant primary, but is visually much fainter. The primary is expected to have lost around 5 M☉ since it was on the main sequence, and has expanded and cooled so it lies at the blue edge of the Cepheid instability strip. It is expected to get no cooler and may perform a blue loop while slowly increasing in luminosity.V810 Cen was once thought to be a member of the Stock 14 open cluster at 2.6 kpc, but now appears to be more distant. The distance derived from spectrophotometric study is larger than the mean Hipparcos parallax value but within the margin of error.WR 86
WR 86 is a visual binary in the constellation Scorpius consisting of a Wolf-Rayet star and a β Cephei variable. It lies 2° west of NGC 6357 on the edge of the Great Rift in the Milky Way in the tail of the Scorpion.
WR 86 is a binary with two components of equal visual brightness 0.3" apart. One has the emission-line spectrum of a WC7 Wolf-Rayet star, while the other is a B0 giant. The blue giant varies slightly in brightness every 3.5 hours. The WR star may also be slightly variable.The pulsations of the B-type giant are characteristic of a β Cephei variable. Analysis of its pulsations and comparison to the expected properties of a WC7 star suggest that both stars could have evolved without mass exchange. The WR and B stars would have had initial masses of 40 M☉ and 20 M☉ respectively four million years ago.
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