Supergiant Stars - Evolution

Evolution

O type main sequence stars and the most massive of the B type blue-white stars become supergiants. Because of their extreme masses they have short lifespans of 30 million years down to 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.

Supergiants develop when massive main sequence stars run out of hydrogen in their cores. They then start to expand, just like lower mass stars, but unlike lower mass stars, they begin to fuse helium in the core almost immediately. 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 when their helium becomes depleted. Furthermore, they cannot lose enough mass to form a white dwarf, so will leave behind a neutron stars or black hole remnant, usually after a core collapse supernova explosion.

Stars more massive than about 40M_☉ cannot expand into a red supergiant. They burn too quickly and lose their outer layers too quickly, so they reach the blue supergiant stage, or perhaps yellow hypergiant, and then return to become hotter stars. The most massive stars, above about 100M_☉, hardly move at all from their position as O main sequence stars. These stars 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 very rapidly evolve through a series of stages of very similar hot and luminous stars, If supergiants, slash stars, WNh stars, WN stars, and possibly WC or WO 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.

The first stars in the universe are thought to have been considerably brighter and more massive than the stars in the modern universe. These stars were part of the theorized population III of stars. Their existence is necessary to explain observations of elements other than hydrogen and helium in quasars. Although they may have been larger and more luminous than any supergiant known today, their structure was quite different, with reduce convection and less mass loss. Their very short lives are likely to have ended in violent photodisintegration or pair instability supernovae.