Hypergiants are the largest stars in the universe, even larger than supergiants. The largest known hypergiant was NML Cygni, which is about 1,650 times wider than the Sun. It is one of the extreme luminous supergiant stars. The pulsating red supergiant UY Scuti is probably larger still. The hypergiant with the next largest known diameter is VY Canis Majoris, which is between 1300 and 1540 times wider than the Sun, or roughly the same diameter as the orbit of Jupiter.
Hypergiants are very rare and they have a short lifespan. While the Sun has a lifespan of around 10 billion years, hypergiants will only exist for a few million years.
Most hypergiants are red stars, but there are two special groups: luminous blue variables (LBV), and yellow hypergiants. Both of these types are very rare, with only a few examples in the Milky Way galaxy. Their rareness is probably because each type passes through its stage quite rapidly.
As luminosity of stars increases greatly with mass, the luminosity of hypergiants often lies very close to the Eddington limit. This is the luminosity at which the force of the star's gravity 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 stars ever observed. Its mass is about 130 solar masses and its luminosity four million times that of the Sun. Eta Carinae may occasionally exceed the Eddington limit. The last time might have been outbursts observed in 1840–1860. These reached mass loss rates much higher than stellar winds would normally allow.
Another theory to explain the massive outbursts of 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.
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