The Crab Nebula is one of the brightest sources of high-energy radiation in the sky. It's the expanding remains of an exploded star, a supernova seen in 1054. The supernova left behind a magnetized neutron star -- a pulsar. It's about the size of Washington DC, but it spins 30 times a second. Each rotation sweeps a lighthouse-like beam past us, creating a pulse of electromagnetic energy detectable across the spectrum.