Hypergiant
Encyclopedia
Star
A star is a massive, luminous sphere of plasma held together by gravity. At the end of its lifetime, a star can also contain a proportion of degenerate matter. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth...
with a tremendous mass
Mass
Mass can be defined as a quantitive measure of the resistance an object has to change in its velocity.In physics, mass commonly refers to any of the following three properties of matter, which have been shown experimentally to be equivalent:...
and luminosity
Luminosity
Luminosity is a measurement of brightness.-In photometry and color imaging:In photometry, luminosity is sometimes incorrectly used to refer to luminance, which is the density of luminous intensity in a given direction. The SI unit for luminance is candela per square metre.The luminosity function...
, showing signs of a very high rate of mass loss.
Characteristics
The word "hypergiant" is commonly used as a loose term for the most massive stars found, even though there are more precise definitions. In 1956, the astronomers Feast and ThackerayThackeray
Thackeray is the name of:*William Makepeace Thackeray, a novelist*Bal Thackeray, an Indian politician*Edward Talbot Thackeray, a recipient of the Victoria Cross*A David Thackeray, a South African astronomer...
used the term super-supergiant (later changed into hypergiant) for stars with an absolute magnitude
Absolute magnitude
Absolute magnitude is the measure of a celestial object's intrinsic brightness. it is also the apparent magnitude a star would have if it were 32.6 light years away from Earth...
greater than MV = −7. In 1971, Keenan suggested that the term would be used only for supergiant
Supergiant
Supergiants are among the most massive stars. They occupy the top region of the Hertzsprung-Russell diagram. In the Yerkes spectral classification, supergiants are class Ia or Ib . They typically have bolometric absolute magnitudes between -5 and -12...
s showing at least one broad emission component in Hα
H-alpha
H-alpha is a specific red visible spectral line created by hydrogen with a wavelength of 656.28 nm, which occurs when a hydrogen electron falls from its third to second lowest energy level...
, indicating an extended stellar atmosphere or a relatively large mass loss rate. The Keenan criterion is the one most commonly used by scientists today. This means that a hypergiant does not necessarily have to be more massive than a similar supergiant. Still, the most massive stars are considered to be hypergiants, and can have masses ranging up to 100–265 solar masses.
Hypergiants are very luminous stars, up to millions of solar luminosities, and have temperatures varying widely between 3,500 K
Kelvin
The kelvin is a unit of measurement for temperature. It is one of the seven base units in the International System of Units and is assigned the unit symbol K. The Kelvin scale is an absolute, thermodynamic temperature scale using as its null point absolute zero, the temperature at which all...
and 35,000 K
Kelvin
The kelvin is a unit of measurement for temperature. It is one of the seven base units in the International System of Units and is assigned the unit symbol K. The Kelvin scale is an absolute, thermodynamic temperature scale using as its null point absolute zero, the temperature at which all...
. Almost all hypergiants exhibit variations in luminosity over time due to instabilities within their interiors.
Because of their high masses, the lifetime of a hypergiant is very short in astronomical timescales: only a few million years compared to around 10 billion years for stars like the Sun
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...
. Because of this, hypergiants are extremely rare and only a handful are known today.
Hypergiants should not be confused with luminous blue variable
Luminous blue variable
Luminous blue variables, also known as S Doradus variables, are very bright, blue, hypergiant variable stars named after S Doradus, the brightest star of the Large Magellanic Cloud. They exhibit long, slow changes in brightness, punctuated by occasional outbursts in brightness during substantial...
s. A hypergiant is classified as such because of its size and mass loss rate, whereas a luminous blue variable
Luminous blue variable
Luminous blue variables, also known as S Doradus variables, are very bright, blue, hypergiant variable stars named after S Doradus, the brightest star of the Large Magellanic Cloud. They exhibit long, slow changes in brightness, punctuated by occasional outbursts in brightness during substantial...
is thought to be a massive blue supergiant
Blue supergiant
Blue supergiants are supergiant stars of spectral type O or B.They are extremely hot and bright, with surface temperatures of 30,000-50,000 K. They typically have 10 to 50 solar masses on the Hertzsprung-Russell diagram, and can have radii up to about 25 solar radii...
going through an evolutionary phase where it loses a large amount of mass.
Stability
As luminosity of stars increases greatly with mass, the luminosity of hypergiants often lies very close to the Eddington limit, which is the luminosity at which the force of the star's gravity equals the radiation pressure outward. This means that the radiative fluxRadiative flux
Radiative flux, or radiative flux density, is the amount of power radiated through a given area, in the form of photons or other elementary particles, typically measured in W/m2. It is used in astronomy to determine the magnitude and spectral class of a star...
passing through the photosphere
Photosphere
The photosphere of an astronomical object is the region from which externally received light originates. The term itself is derived from Ancient Greek roots, φῶς, φωτός/phos, photos meaning "light" and σφαῖρα/sphaira meaning "sphere", in reference to the fact that it is a spheric surface perceived...
of a hypergiant may be nearly strong enough to lift off the photosphere. Above the Eddington limit, the star would generate so much radiation that parts of its outer layers would be thrown off in massive outbursts; this would effectively restrict the star from shining at higher luminosities for longer periods.
A good candidate for hosting a continuum-driven wind is Eta Carinae, one of the most massive and luminous stars ever observed. With an estimated mass of around 130 solar masses and a luminosity four million times that of the Sun
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...
, astrophysicists speculate that Eta Carinae may occasionally exceed the Eddington limit. The last time might have been a series of outbursts in 1840–1860, reaching mass loss rates much higher than our current understanding of what stellar winds would allow.
As opposed to line-driven stellar wind
Stellar wind
A stellar wind is a flow of neutral or charged gas ejected from the upper atmosphere of a star. It is distinguished from the bipolar outflows characteristic of young stars by being less collimated, although stellar winds are not generally spherically symmetric.Different types of stars have...
s (that is, ones driven by absorbing light from the star in huge numbers of narrow spectral lines), continuum driving does not require the presence of "metallic"
Metallicity
In astronomy and physical cosmology, the metallicity of an object is the proportion of its matter made up of chemical elements other than hydrogen and helium...
atoms — atoms other than hydrogen
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...
and helium
Helium
Helium is the chemical element with atomic number 2 and an atomic weight of 4.002602, which is represented by the symbol He. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas that heads the noble gas group in the periodic table...
, which have few such lines — in the photosphere
Photosphere
The photosphere of an astronomical object is the region from which externally received light originates. The term itself is derived from Ancient Greek roots, φῶς, φωτός/phos, photos meaning "light" and σφαῖρα/sphaira meaning "sphere", in reference to the fact that it is a spheric surface perceived...
. This is important, since most massive stars also are very metal-poor, which means that the effect must work independently of the metallicity
Metallicity
In astronomy and physical cosmology, the metallicity of an object is the proportion of its matter made up of chemical elements other than hydrogen and helium...
. In the same line of reasoning, the continuum driving may also contribute to an upper mass limit even for the first generation of stars right after the Big Bang
Big Bang
The Big Bang theory is the prevailing cosmological model that explains the early development of the Universe. According to the Big Bang theory, the Universe was once in an extremely hot and dense state which expanded rapidly. This rapid expansion caused the young Universe to cool and resulted in...
, which did not contain any metals at all.
Another theory to explain the massive outbursts of, for example, Eta Carinae is the idea of a deeply situated hydrodynamic explosion, blasting off parts of the star’s outer layers. The idea is that the star, even at luminosities below the Eddington limit, would have insufficient heat convection in the inner layers, resulting in a density inversion potentially leading to a massive explosion. The theory has however not been explored very much, and it is uncertain whether this really can happen.
Known hypergiants
Hypergiants are difficult to study due to their rarity. For reasons that are currently unknown, there appears to be an upper luminosity limit for the coolest hypergiants (those colored yellow and red): none of them are brighter than around bolometric magnitude –9.5, which corresponds to roughly 500,000 times solar luminosity.Luminous blue variables
Most luminous blue variableLuminous blue variable
Luminous blue variables, also known as S Doradus variables, are very bright, blue, hypergiant variable stars named after S Doradus, the brightest star of the Large Magellanic Cloud. They exhibit long, slow changes in brightness, punctuated by occasional outbursts in brightness during substantial...
s are classified as hypergiants, and indeed they are the most luminous stars known:
- P CygniP CygniP Cygni is a variable star in the constellation Cygnus. The designation "P" was originally assigned by Johann Bayer in Uranometria as a nova....
, in the northern constellation of CygnusCygnus (constellation)Cygnus is a northern constellation lying on the plane of the Milky Way. Its name is the Latinized Hellenic word for swan. One of the most recognizable constellations of the northern summer and autumn, it features a prominent asterism known as the Northern Cross...
. - S DoradusS DoradusS Doradus is the brightest star in the Large Magellanic Cloud, a satellite of the Milky Way. A hypergiant, it is one of the most luminous stars known , but so far away that it is invisible to the naked eye.This star belongs to its own eponymous S Doradus class of variable stars S Doradus is the...
, in a nearby galaxyGalaxyA galaxy is a massive, gravitationally bound system that consists of stars and stellar remnants, an interstellar medium of gas and dust, and an important but poorly understood component tentatively dubbed dark matter. The word galaxy is derived from the Greek galaxias , literally "milky", a...
called the Large Magellanic CloudLarge Magellanic CloudThe Large Magellanic Cloud is a nearby irregular galaxy, and is a satellite of the Milky Way. At a distance of slightly less than 50 kiloparsecs , the LMC is the third closest galaxy to the Milky Way, with the Sagittarius Dwarf Spheroidal and Canis Major Dwarf Galaxy lying closer to the center...
, in the southern constellation of Dorado. This galaxy was also the location of Supernova 1987A, itself a hypergiant. - Eta Carinae, inside the Keyhole Nebula (NGC 3372) in the southern constellation of CarinaCarina (constellation)Carina is a constellation in the southern sky. Its name is Latin for the keel of a ship, and it was formerly part of the larger constellation of Argo Navis until that constellation was divided in three.-Stars:...
. Eta Carinae is extremely massive, possibly as much as 120 to 150 times the mass of the Sun, and is four to five million times as luminous. - The Pistol StarPistol StarThe Pistol Star is a blue hypergiant and is one of the most luminous known stars in the Milky Way Galaxy.It is one of many massive young stars in the Quintuplet cluster in the Galactic Center region....
, near the center of the Milky Way, in the constellation of SagittariusSagittarius (constellation)Sagittarius is a constellation of the zodiac, the one containing the galactic center. Its name is Latin for the archer, and its symbol is , a stylized arrow. Sagittarius is commonly represented as a centaur drawing a bow...
. The Pistol Star is possibly as much as 150 times more massive than the Sun, and is about 1.7 million times more luminous. - Several stars in the cluster Cl* 1806-20, on the other side of the Milky Way galaxy. One such star, LBV 1806-20, is the most luminous star known, from 2 to 40 million times as luminous as the Sun, and also one of the most massive.
Blue hypergiants
- Zeta¹ ScorpiiZeta¹ ScorpiiZeta1 Scorpii is a B-type hypergiant star in the constellation of Scorpius. It has an apparent visual magnitude which varies between 4.66 and 4.86...
, the brightest star of the OB association Scorpius OB1 and a LBVLuminous blue variableLuminous blue variables, also known as S Doradus variables, are very bright, blue, hypergiant variable stars named after S Doradus, the brightest star of the Large Magellanic Cloud. They exhibit long, slow changes in brightness, punctuated by occasional outbursts in brightness during substantial...
candidate. - MWC 314, in the constellation of AquilaAquila (constellation)Aquila is a stellar constellation. Its name is Latin for 'eagle' and it is commonly represented as such. In mythology, Aquila was owned by the Roman god Jupiter and performed many tasks for him....
, another LBV candidate. - HD 169454, in Scutum
- BD -14° 5037 near the latter.
- Cygnus OB2-12Cygnus OB2-12Cygnus OB2-12 is an extremely bright blue hypergiant with an absolute bolometric magnitude of -12.2, which approaches the upper limit believed possible for normal single stars....
, which some authors consider an LBVLuminous blue variableLuminous blue variables, also known as S Doradus variables, are very bright, blue, hypergiant variable stars named after S Doradus, the brightest star of the Large Magellanic Cloud. They exhibit long, slow changes in brightness, punctuated by occasional outbursts in brightness during substantial...
. - R136a1R136a1R136a1 is a blue hypergiant star and the most massive star known. It is an estimated 265 solar masses. The star is also the most luminous at 8,700,000 times the luminosity of the Sun....
the most massive star reported so far, with an estimated mass of 265 suns.
Yellow hypergiants
Yellow hypergiantYellow hypergiant
Generally speaking, a yellow hypergiant is a massive star with an extended atmosphere, which can be classified as spectral class from late A to K, with a mass of as much as 20-50 solar masses...
s form an extremely rare class of stars, with only seven being known in our galaxy
Galaxy
A galaxy is a massive, gravitationally bound system that consists of stars and stellar remnants, an interstellar medium of gas and dust, and an important but poorly understood component tentatively dubbed dark matter. The word galaxy is derived from the Greek galaxias , literally "milky", a...
:
- Rho CassiopeiaeRho CassiopeiaeRho Cassiopeiae is a yellow hypergiant star in the constellation Cassiopeia. It is about away from Earth, yet can still be seen by the naked eye , as it is 550,000 times as luminous as the Sun. On average, it has an absolute magnitude of −7.5, making it one of the most luminous stars known...
, in the northern constellation of CassiopeiaCassiopeia (constellation)Cassiopeia is a constellation in the northern sky, named after the vain queen Cassiopeia in Greek mythology, who boasted about her unrivalled beauty. Cassiopea was one of the 48 constellations listed by the 2nd century Greek astronomer Ptolemy, and it remains one of the 88 modern constellations today...
, is about 500,000 times as luminous as the Sun. - V509 CassiopeiaeV509 CassiopeiaeV509 Cassiopeiae is a star in the constellation Cassiopeia.V509 Cassiopeiae is a yellow-white G-type hypergiant with a mean apparent magnitude of +5.10. It is at least 7800 light years from Earth. It is classified as a semiregular variable star and its brightness varies from magnitude +4.75 to...
- IRC+10420
- IRAS 17163-3907IRAS 17163-3907IRAS 17163-3907 is a yellow hypergiant star located 13,000 light years from Earth in the constellation of Scorpius, making it the closest-known yellow hypergiant to Earth. The star is embedded in thick shells of expelled gases and dust, and owing to its appearance has been nicknamed by astronomers...
. - V382 CarinaeV382 CarinaeV382 Carinae, also known as x Carinae , is a star in the constellation Carina.V382 Carinae is a yellow G-type hypergiant with a mean apparent magnitude of +3.93. It is 5930.90 light years from Earth. It is classified as a Cepheid variable star and its brightness varies from magnitude +3.84 to...
- Also see stars in Westerlund 1Westerlund 1Westerlund 1 is a compact young star cluster in the Milky Way galaxy, about 3.5–5 kpc away from Earth. In fact, it is the most massive compact young star cluster known in the entire Local Group of galaxies...
.
Red hypergiants
- RW CepheiRW CepheiRW Cephei is an M-class red hypergiant star in the constellation Cepheus. One of the largest stars known, RW Cephei is estimated at 1650 solar radii. RW Cephei, while nearly as large as the orbit of Jupiter, is not as large as other stars in the constellation of Cephus, namely V354 Cephei and VV...
- NML Cygni
- VX SagittariiVX SagittariiVX Sagittarii is a late-type red supergiant star located more than 1.7 kiloparsec away from Sun in the constellation of Sagittarius.-Stellar characteristics:...
- VV CepheiVV CepheiVV Cephei, also known as HD 208816, is an eclipsing binary star system located in the constellation Cepheus, approximately 2,400 light years from Earth.Size, mass and luminosity estimates are all considerably uncertain due to insufficient knowledge of the Cephei star system: Professor Kaler writes...
- S PerseiS PerseiS Persei is a red supergiant or hypergiant located in the Double Cluster in Perseus, north of the cluster NGC 869. It is a representative of the semiregular variables, whose periods of variation are subject to more irregularities than those of Mira variable red supergiants.Many of the visually...
- VY Canis MajorisVY Canis MajorisVY Canis Majoris is the largest known star and also one of the most luminous. Located in the constellation Canis Major, it is a red hypergiant, between 1800 and 2100 solar radii, 8.4–9.8 astronomical units in radius, about 3.0 billion km or 1.9 billion miles in diameter, and about 1.5 kiloparsecs ...
has the largest diameter of any known star, 1,800–2,100 times that of the Sun; this gives it a diameter comparable to the orbit of Saturn.