How do we know red supergiants like Antares and Betelgeuse aren’t just main-sequence stars like the Sun who are in the end of their life?

Question

I’ve always been told that massive stars undergo supernova events, while smaller ones like the sun just grow into red giants, to then just shrink back to a white dwarf. My point is: how do we know red supergiants aren’t just regular “Suns” that are just on their giant phase?

In: 9

Answer 1

Mainly brightness. If you know how far away the star is you can calculate its brightness and a red supergiant will be thousands of times brighter than a red giant.