Twenty-one million years ago in the Pinwheel Galaxy — an elegant, spiral-armed neighbor of our Milky Way — an old, dim star had a very bad day. It exploded and began to blaze like a billion suns.

On Aug. 24, Peter Nugent had a very good day. The Lawrence Berkeley National Laboratory astrophysicist was about to grab lunch when he checked to see if a robotic telescope had spied anything of interest the night before. Bingo. Boom. Supernova.

And not just any supernova. Every night, astronomers spot several exploding stars across the universe. But this one was so close — in cosmic terms — and seen so soon after its light reached Earth that astronomers are calling it the supernova of a generation.

An [url=http://www.astro.caltech.edu/ptf/]automated sky survey]/url] that searches for new nighttime objects with a telescope at California’s storied Palomar Observatory first flagged the supernova.

Soon, telescopes around the world — and beyond, including the Hubble Space Telescope — swiveled to take a peek.