About Stars: Red Supergiants

A red supergiant is the bigger version of a red giant - so far no surprise. But with these stars with more than 8 to 10 solar masses (the exact value is still uncertain) the production of energy doesn't stop at helium or carbon.
A red supergiant is made of several layers. The outer hull of red glowing hydrogen and helium is inactive. Below this is a layer in which hydrogen is fusioned to helium. In the next layer helium is fusioned to carbon. So it goes on until in the core iron is made. The supergiant shines extremely bright, but only for a short time (still several hundred thousand to million years). In the end the phase in which the star fusions sulfur and silicon to iron only lasts a few days to weeks.
From iron no more energy can be made. The core cools down and implodes. The following supernova (of type II) disrupts the star and leaves a tiny neutron star or a black hole behind.

Red supergiants are frequently very unstable, pulsate and often have a strong stellar wind which blows away their hull.

Example: Betelgeuse