Hubble telescope reveals dancing crab.

A whirling dervish lies at the heart of the Crab nebula, the giant remnant of a blazing stellar explosion that Chinese astronomers witnessed 942 years ago. Though less than a ten thousandth the size of Earth, the rapidly rotating powerhouse-the shrunken core of the exploded star-lights up the entire nebula, which is big enough to hold more than 600 solar systems.

Astronomers discovered nearly 3 decades ago that the compact engine powering the Crab is a pulsar, so named because the body's twin searchlights sweep across Earth 30 times a second. Yet even though the Crab ranks as one of the most thoroughly studied objects in the sky, ground-based observers have never fully documented how the pulsar imparts its energy to the nebula. Now, a movie made from a series of Hubble Space Telescope images reveals that the region around the pulsar changes dramatically in just a few days, much faster than Earth-based observations had indicated. The movie shows that just above the pulsar lies a flickering blob of light, nicknamed a "dancing sprite" because of its capricious variation in brightness and position. The film also shows equatorial wisps, concentric waves of light generated by charged particles flung from the pulsar's equator at nearly the speed of light. "In astronomy, normally if you can see something change over the course of your career, you're excited," says J. Jeffrey Hester of Arizona State University in Tempe. Using Hubble, he and his colleagues "found that the Crab is so dynamic that if you wait more than about a week to take your next look, you're going to get the wrong impression because things have changed so terribly much." Hester and his Arizona State colleague Paul A. Scowen presented the findings last week at a NASA press briefing in Washington, D.C. The images, Hester says, demonstrate that the pulsar doesn't spew its energy in all directions. Rather, much of the energy shows up around the equator, emanating from a wind of electrons and positrons generated by the pulsar's intense magnetic field. The spiraling particles radiate light, illuminating the Crab's interior and traveling far into the nebula. Similarly, high-speed jets of electrons and positrons travel out from the pulsar's polar regions. The sprite, Hester points out, marks the region where material from one of these jets blasts into slower-moving material in the nebula. "We've thrown down the gauntlet," declares Hester. "Any model of a pulsar and its wind is going to have to come to grips with what we see going on in the Crab."

Although Andrew S. Fruchter of the Space Telescope Science Institute in Baltimore cautions that the analysis of the images "is not set in stone," he says the pictures offer "the best close-up view of a pulsar wind that we've ever had."

The processes unveiled in the Crab may be a scaled-down version of those at work in the centers of active galaxies that lie too far away to be studied at high resolution. "The Crab allows the properties and behavior of high-energy winds to be studied up close and personal," says Hester.