An exploding star spotted in the Pinwheel galaxy, 21 million light-years from Earth, offers a rare chance to watch a supernova unfold in real time. Telescopes across the world were trained in its direction "within hours of its discovery," said Azalee Bostroem, a researcher in the University of Arizona Steward Observatory, deducing it was probably a type II supernova, in which a supergiant star runs out of fuel and collapses in on itself before exploding. "This is going to be like a Rosetta Stone supernova," Bostroem added.