531www.sciencemag.org SCIENCE VOL 304 23 APRIL 2004

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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 fromsight and memory. Chinese records called it a guest star. By the time 18th century observers re-discovered it using telescopes, all they could see was a faint smudge of light. In 1758, the Frenchastronomer Charles Messier mistook it for Halley’s comet; realizing his error, he cataloged it asM1, 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 heardthe throbbing at the heart of the Crab and identified a pulsar—a whirling neutron star sending outsteady pulses of emissions, 30 times per second—at its core. A year earlier, in 1967, Jocelyn Bell Bur-nell had detected similar pulsations coming from four compact sources (see Editorial on p. 489), a dis-

covery that launched the field of pulsar research. Today astronomers have iden-tified more than 1400 pulsars, half of them within the past 6 years.

In this special section, Irion provides a concise history of pulsar research inhis News story (p. 532), and Manchester reviews the demographics and proper-ties of radio pulsars (p. 542). Getting back to basics, Lattimer and Prakash de-scribe the interior of the neutron star (p. 536). A neutron star, with an averagediameter of 12 kilometers and a mass similar to the Sun’s, has an interior domi-nated by neutrons packed as much as 10 times more densely than a typicalatomic nucleus. This form of matter is so exotic that laboratories cannot recreateit; to understand it, neutron star aficionados must depend on theory and astro-nomical observations.

The pulsations that give pulsars their name arethought to be driven by the release of energy alongthe 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). Re-cently, as Irion describes in a second News story (p. 534), theory and obser-vations have revealed the existence of magnetars, extreme pulsars withfields of 1015 gauss. Magnetars may be the source of gamma ray bursts andprovide the missing link between supernovae and gamma ray bursts.

Bizarre and bursty though they are, pulsars have extraordinarily regularhabits. 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 alreadybefuddled theorists, speeding up. It turned out that some of these unsteadypulsars have companions—and very interesting ones, at that. Stairs reviewsbinary systems (p. 547), from the first extrasolar planet ever detected (whichorbits a pulsar) to a two-pulsar system discovered last year. The double-pulsarsystem may provide the best opportunity to study relativity theory.

Pulsars may hold the key to understanding gravity, and they certainly havemuch to tell us about the interstellar medium, odd physics such as super-fluidity and strange matter, and the dynamics of binary systems. New twistsare certain to emerge as researchers continue to probe their surreal reality.

–LINDA ROWAN AND ROBERT COONTZ

The Strange and TwistedWorld of Pulsars

P A G E 5 3 4

T I T L E O F S P E C I A L S E C T I O N

I N T R O D U C T I O N

C O N T E N T S

N E W S

532 The Pulsar MenageriePulsars Surf the Cosmic Waves

534 Crushed by Magnetism

R E V I E W S

536 The Physics of Neutron StarsJ. M. Lattimer and M. Prakash

542 Observational Properties of Pulsars

R. N. Manchester

547 Pulsars in Binary Systems:Probing Binary Stellar Evolution and General Relativity

I. H. Stairs

See also Editorial on p. 489

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