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Hypergiant star is a giant star in the universe.
Hypergiant star caracteristics.
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 Thackeray used the term super-supergiant (later changed into hypergiant) for stars with an absolute magnitude greater than MV = –7. In 1971, Keenan suggested that the term would only be used for supergiants showing at least one broad emission component in Ha, 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 doesn’t 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 100–150 solar masses.
Hypergiants are very luminous stars, up to millions of solar luminosities, and have temperatures varying widely between 3,500 K and 35,000 K. 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. Because of this, hypergiants are extremely rare and only a handful are known today.
Hypergiants should not be confused with luminous blue variables. A hypergiant is classified as such because of its size and mass loss rate, whereas a luminous blue variable is thought to be a massive blue supergiant going through an evolutionary phase where it loses a large amount of mass.
The stability of hypergiant stars.
As luminosity of stars increases greatly with mass, the luminosity of hypergiants often lies very close to the Eddington limit which, somewhat simply put, is the luminosity at which the gravitational pressure inward equals the radiation pressure outward. This means that the radiative flux passing through the photosphere 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. However, even with a mass of around 130 solar masses and a luminosity four million times higher than the Sun, Eta Carinae is thought to reach luminosities above the Eddington limit only occasionally. The last time might have been a series of outbursts in 1840-1860, reaching mass loss rates much higher than any of the more well known stellar winds can explain.
As opposed to line-driven stellar winds (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" atoms - atoms other than hydrogen and helium, which have few such lines - in the photosphere. This is important, since most massive stars also are very metal-poor, which means that the effect must work independently of the metallicity. 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, which did not contain any metals at all.
Another theory to explain the massive outbursts of, for example, 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 hypergiant stars.
Hypergiants are difficult to study due to their rarity. 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. The reasons for that are currently unknown.
Hypergiant stars: Luminous blue variables
Most luminous blue variables are classified as hypergiants, and indeed they are the most luminous stars known:
Blue hypergiant stars.
White hypergiant stars.
Yellow hypergiants form an extremely rare class of stars, with only seven being known in our galaxy:
Red hypergiant stars.
References to hypergiant stars.
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