Hypergiant

A hypergiant (luminosity class 0) is a star with an enormous mass and luminosity, It shows signs of a very high rate of mass loss. The exact definition is not yet settled.

Hypergiants are the largest stars in the universe, usually larger and brighter than supergiants.However, note the definition is for huge luminosity and rapid mass loss, not simply size.  

Another large hypergiant is VY Canis Majoris, about 1,420 times wider than the Sun. It is one of the extreme luminous supergiant stars. 

Just as supergiants evolve from massive stars that were once main sequence stars (between 8 to 22 solar masses, one solar mass being the mass of the Sun), hypergiants were once main-sequence stars that were more massive.

Hypergiants are very hard to find and they have a short lifespan because of their size. While the Sun has a lifespan of around 10 billion years, hypergiants will only exist for a few million years.

 Spectrum 

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 this stage quite rapidly.

 Stability 

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 away 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.

 Examples 
 Blue hypergiants 
 Zeta1 Scorpii
 BP Crucis
 Pistol Star
 Eta Carinae

 Luminous blue variables (LBVs) 
 P Cygni
 S Doradus
 LBV 1806-20
 AG Carinae

 Yellow hypergiants 
 Rho Cassiopeiae
 V509 Cassiopeiae
 HR 5171
 V382 Carinae

 Red hypergiants 
 Mu Cephei (the Garnet Star)
 NML Cygni
 VV Cephei A
 VY Canis Majoris
 WOH G64
 RW Cephei
 VX Sagitarii
 Stephenson 2-18

 Related pages 
List of largest stars

 References