Neutron Star

In A.D. 1054, astronomers in China noted the appearance of a "guest star" in the region of the sky we Figure 1. Illustration of the relationship between a neutron star's rotation axis and its magnetic axis. Illustration by Hans & Cassidy. Courtesy of Gale Group. now call Taurus (the Bull). The new star was bright enough to be visible during the day, but faded within a few months. When we train our telescopes today on the place where this celestial interloper appeared, we see a spectacular cloud of gas, its filaments twisting and ragged. Its shape has led it to be called the Crab Nebula. The Crab is expanding, as if it had been flung violently outward from a central point.

When we observe the Crab with a radio telescope, something even stranger appears: there is an object at its center that flashes on and off like a strobe light, about 30 times per second. So regular are the flashes that they hardly seem to be a natural phenomenon. Indeed, Jocelyn Bell and Antony Hewish, who discovered the first of these flashing objects, toyed with the idea of calling them LGM's—meaning Little Green Men. They eventually settled on the term pulsar, and today about 500 pulsars are known.

Almost a millennium ago, humans stared in wonder at a new star. Today, there is a pulsar, seemingly beaming pulses of radio radiation to us at precise intervals, and a of gas surrounding it. What has happened?