Hypergiant
From Freepedia
A hypergiant is a massive star, larger than even a supergiant, with a mass up to 100 times that of our Sun, approaching the theoretical limit of star mass (120 solar masses; any higher, and stellar equilibrium could not be maintained; the star would produce so much energy that it would shear off mass in excess of 120 solar masses). Some hypergiants appear to be more than 100 times the mass of the Sun, and may have initially been 200 to 250 times as massive, challenging current theories of star formation and evolution. Hypergiants are regarded as the most luminous stars - they are thousands to millions of times more luminous than the Sun - and their temperatures are between 3,500 K and 35,000 K. They last approximately 1 to 3 million years, before turning into supernovae, or, in extremely rare cases, into hypernovae. It is theorised that a hypergiant gone supernova or hypernova will leave a black hole.
The knowledge about hypergiants is relatively little, as they are so rare. Until recently, there were only 7 known hypergiants in the Milky Way. They can have different colours: Blue usually indicates that the star is hot, while red indicates it is cool. Yellow hypergiants also exist, but instabilities in the stellar interior at moderate temperatures and high pressures make them much rarer than other hypergiants.
To see what a hypergiant might look like compared to the Sun, refer to this link:
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Known luminous blue variable hypergiants
P Cygni (Is this a hypergiant?), in the northern constellation of Cygnus.
S Doradus, in a nearby galaxy called the Large Magellanic Cloud, in the southern constellation of Dorado. This constellation was also the location of Supernova 1987A.
Epsilon Aurigae, an eclipsing binary in the Milky Way, in the northern constellation of Auriga. Epsilon Aurigae has the unusual characteristic of dimming every 27 years. A clump of dust probably accompanies the as-of-yet undiscovered companion star and causes this dimming.
Eta Carinae, inside the Keyhole Nebula (NGC 3372) in the southern constellation of Carina. 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 Star, near the center of the Milky Way, in the constellation of Sagittarius. The Pistol Star is possibly as much as 150 times more massive than the Sun, and is about 10 million times more luminous.
Several stars in Cluster 1806-20, on the other side of the Milky Way galaxy. One such star, LBV 1806-20, is currently the most massive known star, possibly as much as 130 to 200 times as massive as the Sun, and also the most luminous, from 2 to 40 million times as luminous as the Sun.
Known yellow hypergiants
Rho Cassiopeiae, in the northern constellation of Cassiopeia, is about 500,000 times as luminous as the Sun. (How much more massive??)
HR8752
IRC+10420
Also see stars in Westerlund 1.
