CTB 1: The Medulla NebulaWhat powers this unusual nebula? CTB 1 is the expanding gas shell that was left when a massive star toward the constellation of Cassiopeia exploded about 10,000 years ago. The star likely detonated when it ran out of elements, near its core, that could create stabilizing pressure with nuclear fusion. Image Credit: Pierre Konzelmann #astrophotography #nebulas #APOD