Why are supergiants bright

The luminosity differences between stars are most apparent at low temperatures, where giant stars are much brighter than main-sequence stars. Supergiants have the lowest surface gravities and hence are the largest and brightest at a particular temperature.

Explanation:

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Because supergiants are so massive, the core temperature gets much hotter than in giants, so supergiants can fuse elements heavier than hydrogen and helium. But to support their tremendous mass, supergiants burn up their fuel much more quickly. Eventually, all possible fuels are exhausted, the core collapses, and the star goes supernova.