The Crab Nebula is the remnant of a supernova near enough (six thousand light-years) to have been noticed and recorded by Chinese astrologers in AD 1054. At its center, the energetic pulsar to which the supernova explosion gave birth continues to power the nebula’s extraordinary luminosity across the spectrum from radio to TeV gammas. Seen from Earth at visible wavelengths (figure 1), the still-expanding nebula now subtends an angle of 5 arcminutes, corresponding to a width of about 10 light-years.
The Crab is arguably the most exhaustively studied celestial object beyond the solar system, and the apparent stability of its luminosity at all wavelengths has made it an attractive reference for calibrating observing instruments. The brightness of other objects is often quoted in “millicrabs.” But the startling observation of a powerful gamma-ray flare from the Crab last September belies that vaunted stability, and it challenges the accepted theory of...
