Why stars turn to red giants before exploding as supernova?
It’s because they have to run out of fuel first. Stars begin their lives burning hydrogen. When stars run out of hydrogen, their cores contract; if they have enough mass, the heat of compression will start fusion of helium to carbon. This releases huge amounts of energy, and the outer layers of the star puff out and get cooler and redder (with the exception of those stars which are so luminous that they lose their outer layers first; see the link). Stars like the Sun stop at carbon and oxygen. Larger stars can keep going, and eventually the cores collapse entirely to neutron stars (and the rebound and neutrino pulse causes a supernova). A red giant phase (and sometimes two) usually comes first. For an example of a star which was blue when it went supernova, see the second link about Sanduleak -69 202. There are also Type 1a supernovae in which a white dwarf core receives matter from a binary companion until it reaches a critical mass, ignites and explodes without leaving a remnant (con
