A term used to describe some of the most luminous of stars. There is no strict formal definition of a 'supergiant', but typically the term is used to describe stars with an absolute magnitude brighter than about -3. Stars even more luminous than this, brighter than about absolute magnitude -8, are classified as the rare hypergiants. To put these numbers into perspective, if a supergiant lay ten parsecs from the Sun (the standard for calculating absolute magnitude) it would shine at least as brightly as Venus in the skies of Earth.
There are in fact no supergiants even remotely this close to the Solar System, but some are close enough to be easily visible to the naked eye. The nearest is Betelgeuse in Orion which, even at a distance of about five hundred light years, is nonetheless among the brightest stars in the sky. Other prominent examples include Rigel (also in Orion), as well as Deneb in Cygnus and Antares in Scorpius.
On a spectral classification, a supergiant is denoted by the suffix 'I' (that is, the Roman numeral for 1). This classification is further subdivided into 'Ia' for more luminous supergiants, and 'Ib' for less luminous examples (with 'Iab' being used for those that fall between the two major classifications).
Origins and Lifespan
A supergiant starts life as a hot blue star on the main sequence, with a mass greater than about eight times that of the Sun. When such a star reaches the point where the hydrogren in its core has been consumed, it possesses sufficient mass for more complex forms of fusion to take place. Its outer layers expand as a complex array of different fusion processes are initiated. The result is a star with a diameter hundreds of times that of the Sun, and a luminosity that can be as much as a million times greater. The expanding shells of the star, however, mean that supergiants are far less dense than main sequence stars, so despite their immense size and energy, their mass rarely exceeds about ten times that of the Sun.
Supergiants can occur across the spectrum, from hot blue supergiants to cooler red forms. Yellow supergiants occur in the region of the Hertzsprung–Russell diagram known as the 'instability strip', and stars of this kind are commonly pulsating Cepheid variables.
In general, the more massive a star, the shorter its lifespan. For the extremely massive supergiants, then, this means that they are very short-lived in comparison with dwarf stars. A supergiant might exist for less than a million years, and they rarely survive longer than thirty million (a period representing a minuscule fraction of the lifespan of a main sequence star like the Sun).
The core of the star will eventually reach a point where fusion is no longer possible; when the matter in the core is composed primarily of iron, fusion comes to an end, and with it the pressure maintaining the core. At this point the core rapidly collapses, before nuclear forces cause it to suddenly erupt, sending out a shockwave through the surrounding stellar layers. The result is a supernova, in which the outer shells of the star are blasted away from the core at an appreciable fraction of the speed of light. In the wake of a supernova, the core survives as a white dwarf, initially surrounded by the expanding remains of its outer shells in the form of a nebula. The Crab Nebula in Taurus is a particular prominent example of such a supernova remnant.