<P> Hotter supergiants show differing levels of nitrogen enrichment . This may be due to different levels of mixing on the main sequence due to rotation or because some blue supergiants are newly evolved from the main sequence while others have previously been through a red supergiant phase . Post-red supergiant stars have a generally higher level of nitrogen relative to carbon due to convection of CNO - processed material to the surface and the complete loss of the outer layers . Surface enhancement of helium is also stronger in post-red supergiants, representing more than a third of the atmosphere . </P> <P> 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 million years and a few hundred thousand years . They are mainly observed in young galactic structures such as open clusters, the arms of spiral galaxies, and in irregular galaxies . They are less abundant in spiral galaxy bulges and are rarely observed in elliptical galaxies, or globular clusters, which are composed mainly of old stars . </P> <P> 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 to become 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 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 . </P> <P> Stars more massive than about 40 M cannot expand into a red supergiant . 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 100 M, 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 . </P>

A star of high surface temperature and low luminosity