What is this White blank The blue white hot core of a star that is left behind after its outer layers have expanded and drifted out into space?

White dwarf. Also, for bigger stars would be left with a neutron star for an even more massive star you would be left with a black hole but I'd you really want to impress someone(your teacher) you could mention a pair instability supernova; which is the biggest boom in the universe. It work by being so massive that when it explodes, its gravity squeezes itself together Si tight that it eventually pops, resulting in a massive explosion that leaned nothing behind, not even a brown dwarf.