Scenario for Type IIb SN 1993J
This illustration shows the key steps in the evolution of a Type IIb supernova.
Panel 1: Two very hot stars orbit about each other in a binary system.
Panel 2: The slightly more massive member of the pair evolves into a bloated red giant and spills the hydrogen in its outer envelope onto the companion star.
Panel 3: The more massive star explodes as a supernova.
Panel 4: The companion star survives the explosion. Because it has locked up most of the hydrogen in the system, it is a larger and hotter star than when it was born. The fireball of the supernova fades.