Editing Supergiant (section)

==Evolution==
{{Main|Stellar evolution}}
O type main-sequence stars and the most massive of the B type blue-white stars become supergiants. Due to their extreme masses, they have short lifespans, between 30&nbsp;million years and a few hundred thousand years.<ref>{{cite web | last = Richmond | first = Michael | url = http://spiff.rit.edu/classes/phys230/lectures/star_age/star_age.html | title = Stellar evolution on the main sequence | access-date = 2006-08-24 }}</ref> They are observed mainly in young galactic structures such as [[open cluster]]s, in the arms of [[spiral galaxy|spiral galaxies]], and in [[irregular galaxy|irregular galaxies]]. They are less abundant in spiral galaxy bulges, and are rarely observed in [[elliptical galaxy|elliptical galaxies]] or [[globular cluster]]s, which are composed mainly of old stars.

Supergiants develop when massive main-sequence stars run out of hydrogen in their cores, at which point they start to expand, just like lower-mass stars. Unlike lower-mass stars, however, they begin to fuse helium in the core smoothly and not long after exhausting their hydrogen. This means that they do not increase their luminosity as dramatically as lower-mass stars, and they progress nearly horizontally across the HR diagram, becoming red supergiants. Also unlike lower-mass stars, red supergiants are massive enough to fuse elements heavier than helium, so they do not puff off their atmospheres as [[Planetary nebula|planetary nebulae]] after a period of hydrogen and helium shell burning; instead, they continue to burn heavier elements in their cores until they collapse. They cannot lose enough mass to form a white dwarf, so they will leave behind a neutron star or black hole remnant, usually after a core-collapse supernova explosion.

Stars more massive than about {{Solar mass|40}} cannot expand into red supergiants. Because they burn too quickly and lose their outer layers too quickly, they reach the [[blue supergiant]] stage, or perhaps yellow hypergiant, before returning to become hotter stars.  The most massive stars, above about {{Solar mass|100}}, hardly move at all from their position as O main-sequence stars. These convect so efficiently that they mix hydrogen from the surface right down to the core. They continue to fuse hydrogen until it is almost entirely depleted throughout the star, then rapidly evolve through a series of stages of similarly hot and luminous stars: supergiants, slash stars, WNh-, WN-, and possibly WC- or WO-type stars. They are expected to explode as supernovae, but it is not clear how far they evolve before this happens. The existence of these supergiants still burning hydrogen in their cores may necessitate a slightly more complex definition of supergiant: a massive star with increased size and luminosity due to fusion products building up, but still with some hydrogen remaining.<ref name=ekstrom>{{Cite journal|arxiv=1303.1629|author1=Sylvia Ekström|author2=Cyril Georgy|author3=Georges Meynet|author4=Jose Groh|author5=Anahí Granada|title=Red supergiants and stellar evolution|journal=EAS Publications Series|volume=60|pages=31–41|date=2013|doi=10.1051/eas/1360003|bibcode=2013EAS....60...31E|s2cid=118407907}}</ref>

The first stars in the [[universe]] are thought to have been considerably brighter and more massive than the stars in the modern universe. Part of the theorized [[population III]] of stars, their existence is necessary to explain observations of [[chemical element|elements]] other than [[hydrogen]] and [[helium]] in [[quasar]]s. Possibly larger and more luminous than any supergiant known today, they had a quite different structure, with reduced convection and less mass loss. Their very short lives are likely to have ended in violent [[photodisintegration]] or [[pair-instability supernova]]e.