The Strange and Twisted World of Pulsars

Science  23 Apr 2004:
Vol. 304, Issue 5670, pp. 531
DOI: 10.1126/science.304.5670.531

In 1054 C.E., people around the world witnessed an exploding star. It burst forth on 4 July, brighter than Venus, and stayed visible in daylight for about 23 days before gradually fading from sight and memory. Chinese records called it a guest star. By the time 18th century observers rediscovered it using telescopes, all they could see was a faint smudge of light. In 1758, the French astronomer Charles Messier mistook it for Halley's comet; realizing his error, he cataloged it as M1, the first of his famous sequence of fuzzy objects that shouldn't be confused with comets.

The guest star was a supernova explosion; the fuzzy object, now called the Crab Nebula (see cover), was the remnant of gas and dust it left behind. More than 200 years after Messier, astronomers heard the throbbing at the heart of the Crab and identified a pulsar—a whirling neutron star sending out steady pulses of emissions, 30 times per second—at its core. A year earlier, in 1967, Jocelyn Bell Burnell had detected similar pulsations coming from four compact sources (see Editorial on p. 489), a discovery that launched the field of pulsar research. Today astronomers have identified more than 1400 pulsars, half of them within the past 6 years.

In this special section, Irion provides a concise history of pulsar research in his News story (p. 532), and Manchester reviews the demographics and properties of radio pulsars (p. 542). Getting back to basics, Lattimer and Prakash describe the interior of the neutron star (p. 536). A neutron star, with an average diameter of 12 kilometers and a mass similar to the Sun's, has an interior dominated by neutrons packed as much as 10 times more densely than a typical atomic nucleus. This form of matter is so exotic that laboratories cannot recreate it; to understand it, neutron star aficionados must depend on theory and astronomical observations.

The pulsations that give pulsars their name are thought to be driven by the release of energy along the magnetic poles as the field unwinds and the pulsar spin slows down. All pulsars have relatively strong magnetic fields, around 1012 gauss (compared with Earth's field of about 0.6 gauss). Recently, as Irion describes in a second News story (p. 534), theory and observations have revealed the existence of magnetars, extreme pulsars with fields of 1015 gauss. Magnetars may be the source of gamma ray bursts and provide the missing link between supernovae and gamma ray bursts.

Bizarre and bursty though they are, pulsars have extraordinarily regular habits. Their pulsations can be measured to within about 10 significant digits, making them more precise timers than atomic clocks. In the 1970s, however, astronomers noticed that some were slightly off—or even worse, for already befuddled theorists, speeding up. It turned out that some of these unsteady pulsars have companions—and very interesting ones, at that. Stairs reviews binary systems (p. 547), from the first extrasolar planet ever detected (which orbits a pulsar) to a two-pulsar system discovered last year. The double-pulsar system may provide the best opportunity to study relativity theory.

Pulsars may hold the key to understanding gravity, and they certainly have much to tell us about the interstellar medium, odd physics such as superfluidity and strange matter, and the dynamics of binary systems. New twists are certain to emerge as researchers continue to probe their surreal reality.

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