Probing the mysterious heart of the Crab Nebula, the tattered remains of a stellar cataclysm witnessed more than 900 years ago, astronomers using NASA's Hubble Space Telescope have found that the Crab is even more dynamic than previously understood, based on a cosmic "movie" assembled from a series of Hubble observations.
The results promise to shed new light on a variety of high energy phenomena in the universe, from nearby neutron stars to remote quasars.
Though changes in most astronomical objects are barely perceptible over a human lifetime, Hubble shows that the interior of the nebula "changes its stripes" every few days, according to Jeff Hester of Arizona State University in Tempe, AZ, who leads the team of astronomers that took the Wide Field and Planetary Camera 2 (WFPC2) images.
"We took the images a few weeks apart because we knew that it might be possible to observe slight differences in the Crab over a short time," said Hester. "But I don't think that any of us were prepared for what we saw."
Though ground-based images of the Crab had shown subtle changes in the nebula over months or years, the Hubble movie shows sharp wisp-like features streaming away from the center of the nebula at half the speed of light.
The powerhouse at the center of the nebula responsible for these changes is a rapidly spinning neutron star the compact core of the exploded star. Only about six miles (10 kilometers) across, the neutron star would fit inside a small town, "yet its small size belies its significance and the punch that it packs," said Hester.
As the neutron star spins on its axis 30 times a second, its twin searchlight beams sweep past the Earth, causing the neutron star to blink on and off. Because of this flickering, the neutron star is also called a "pulsar." In addition to the pulses, the neutron star's rapid rotation and intense magnetic field act as an immense slingshot, accelerating subatomic particles to close to the speed of light and flinging them off into space.
In a dramatic series of images assembled over several months of observation, Hubble shows what happens as this magnetic pulsar "wind" runs into the body of the Crab Nebula. The glowing, eerie shifting patterns of light in the center of the Crab are created by electrons and positrons (anti-matter electrons) as they spiral around magnetic field lines and radiate away energy. This lights up the interior volume of the nebula, which is more than 10 light-years across.
The Hubble team finds that material doesn't move away from the pulsar in all directions, but instead is concentrated into two polar "jets" and a wind moving out from the star's equator.
The most dynamical feature in the inner part of the Crab is the point where one of the polar jets runs into the surrounding material forming a shock front. The shape and position of this feature shifts about so rapidly that the astronomers describe it as a "dancing sprite," or "a cat on a hot plate." The equatorial wind appears as a series of wisp-like features that steepen, brighten, then fade as they move away from the pulsar to well out into the main body of the nebula.
"Watching the wisps move outward through the nebula is a lot like watching waves crashing on the beach except that in the Crab the waves are a light-year long and are moving through space at half the speed of light," said Hester. "You don't learn about ocean waves by staring at a snapshot. By their nature waves on the ocean are ever-changing. You learn about ocean waves by sitting on the beach and watching as they roll ashore. This Hubble 'movie' of the Crab is so significant because for the first time we are watching as these 'waves' from the Crab come rolling in."
The Crab Nebula, the result of a supernova explosion witnessed by Chinese astronomers in 1054 AD, also is widely studied because it offers a unique opportunity to study high energy astrophysical phenomena. The physical processes that are at work in the centers of distant active galaxies and quasars are thought to be much like the processes at work in the center of the Crab, only on a vastly larger scale. "The difference is that while astronomers may never truly 'see' into the very heart of an active galaxy, the Crab allows the properties and behavior of high energy winds and jets to be studied up close and personal," Hester said.
"The Hubble results aren't the end of the story," Hester emphasized. "Rather, they are a piece of a larger puzzle. For example, the jets seen streaming away from the pulsar in the Hubble data are of particular interest because they help explain two lobes of X-ray emission seen extending away from the pulsar in images taken with the Einstein and ROSAT X-ray satellites."
In addition to Hester and Paul Scowen of Arizona State University, other members of the team responsible for this work include Ravi Sankrit of Arizona State University, Curt Michel of Rice University, Jay Gallagher of the University of Wisconsin at Madison, James Graham of the University of California at Berkeley, and Alan Watson of New Mexico State University.
NASA Headquarters, Washington, DC
Goddard Space Flight Center, Greenbelt, MD
Space Telescope Science Institute, Baltimore, MD