Right now in US2 alpha ,you can have a star made of pure silicates, iron, CO2, everything. Which is impossible in real life, because it wouldn't undergo fusion unless the mass was REALLY big. My best guess is that instead of igniting into a star and growing, it would start to shrink as pressure overcomes electron degeneracy force, essentially becoming a white dwarf. Real white dwarfs are made of unfusible products of stellar fusion, like carbon and oxygen, and are about the same mass as the star they came from. But since they don't have any force coming from fusion, they shrink really small. But it wouldn't get any hotter (brown dwarfs are hot because they fuse deuterium, and white dwarfs don't make any heat of their own)