In the year 1054, ancient astronomers noted the appearance of a bright new star in the constellation Taurus (Bull). It was visible in broad daylight for more than a month before fading from view. This supernova (nova means new) was the explosion that marked the death of a massive star. What we observe in its place today is a supernova remnant, the glowing gaseous remains expelled into space by the exploding star.

The leftover energy from the explosion makes the remnant very bright at all wavelengths. The visible light and radio images are vaguely reminiscent of a Crab and highlight the turbulent filamentary structure of gas. The X-ray image shows a zoom by a factor four of the very central region.

All stars with a birth mass more than eight times that of the Sun die in a spectacular supernova. But if the star wasn't too much heavier than that, the stellar core survives the supernova as a neutron star (it is not massive enough to become a black hole). This is a dense ball of neutral elementary particles. It is somewhat more massive than the Sun, but is squeezed into a space little more than 10 miles across. The Crab supernova produced a neutron star, seen as the dot in the center of the X-ray image. The neutron star spins rapidly and emits a bright beam of radiation from its magnetic poles. Like a cosmic lighthouse, this beams sweeps past the Earth 30 times per second. We observe this as regular pulses and call such a neutron star a pulsar (short for pulsating star).