In astronomy terms the difference between supernova and presupernova

is that supernova is a star which explodes, increasing its brightness to typically a billion times that of our sun, though attenuated by the great distance from our sun. Some leave only debris (Type I); others fade to invisibility as neutron stars (Type II) while presupernova is describing the period, and the events, prior to a star becoming a supernova.

As a noun supernova

is a star which explodes, increasing its brightness to typically a billion times that of our sun, though attenuated by the great distance from our sun. Some leave only debris (Type I); others fade to invisibility as neutron stars (Type II).

As an adjective presupernova is

describing the period, and the events, prior to a star becoming a supernova.