Complex sentence: It is one of the most luminous stars known, a supergiant that has used up the hydrogen fuel in its core and has entered the evolutionary stage where it is fusing helium atoms to form carbon.
Split into simpler sentences: It is one of the most luminous stars known. This is a supergiant star that has used up the Hydrogen fuel in its core and has entered the evolutionary stage where it is fusing Helium atoms to form Carbon.