Don Savage
Headquarters, Washington, DC	May 30, 1996
(Phone: 202/358-1547)

Tammy Jones
Goddard Space Flight Center, Greenbelt, MD (Phone: 301/286-5566)

Ray Villard
Space Telescope Science Institute, Baltimore, MD (Phone: 410/338-4514)

RELEASE: 96-104

Hubble Astronomers Unveil "Crab Nebula -- The Movie" 

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, Tempe, AZ, who leads the team of astronomers that took the images, which were separated by 
only a few weeks. 

"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," Hester said.

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 also is 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 ten 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 dynamic 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 at Tempe, Curt Michel of Rice University, 
Houston, TX, 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 at University Park. 

The Space Telescope Science Institute is operated by the Association of Universities for Research in 
Astronomy, Inc. for NASA, under contract with the Goddard Space Flight Center, Greenbelt, MD. The 
Hubble Space Telescope is a project of international cooperation between NASA and the European Space 
Agency.

-end-

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