On Science:
Hubble Finds
Evidence Of
Cosmic Collision

Debris speeding out from the supernovas is slamming together in a cosmic collision, the likes of which has never before been seen. The images are especially startling because the collision is taking place over a time period of perhaps a few hundred years, a fleeting blink of an eye in the ancient cosmos, said William P. Blair, a Hopkins astrophysicist who led a team of scientists making the discovery.

The astronomers were puzzled when they first spotted the object with a telescope at the Kitt Peak National Observatory in Arizona and with the ROSAT X-ray satellite. It was extremely bright in visible and X-ray light, apparently just like a young supernova, a star much more massive than the sun destroying itself in a titanic explosion.

But further analysis showed that it did not have the proper mixture of elements, and it was expanding too slowly to be a young supernova. Instead, it had all the characteristics of a much older remnant of a supernova, in which the expanding bubble of debris has spread far into space, diffusing into the interstellar gas.

How, then, could it be so bright?

"It was clear that something funny was going on," said Blair, an associate research professor in the Department of Physics and Astronomy.

The scientists solved the puzzle using the Hubble telescope's Wide Field and Planetary Camera 2. The space telescope's superior resolution--its ability to distinguish separate objects that are close to each other--brought the matter into clearer focus.

Whereas the bright point of light had looked like a single supernova from the ground, the Hubble image clearly showed the remnants of two or more objects colliding.

Blair worked with two other astrophysicists: Robert A. Fesen, from Dartmouth College, and Eric M. Schlegel, from the Harvard-Smithsonian Center for Astrophysics. They released details about the discovery last month, during a meeting of the American Astronomical Society.

When a massive star explodes, gas and debris are thrown in all directions at speeds of up to 22 million miles an hour (36 million kilometers per hour), producing a shock wave and compressing the gas into a dense "shell" of material.

The Hubble image captured the shells from two or more supernovas crashing into each other, like "a train wreck," and producing a tremendous light display for an object so far away, Blair said.

Astronomers had predicted the process but because the phenomenon is so short-lived it had never been seen directly.

"It's the first time that we've identified one of these interactions right when the shells are in the process of slamming into each other," he said. "The reason this object is so bright is that we caught it at a very specific time in its evolution. And Hubble's resolving power is what allowed us to see it."

The supernova interaction is taking place in a galaxy known as NGC 6946, located 17 million light-years away in the northern constellation Cepheus. Like the Milky Way, it is a spiral galaxy, but it's about half the Milky Way's mass and size. The exploding stars probably were about 40 light-years apart.

Supernova explosions have been seen in NGC 6946 at an extraordinary rate: astronomers have observed six supernovas in that galaxy since 1917. Only one other galaxy has displayed so many supernovas.

Astronomers might use a new Hubble instrument, the Space Telescope Imaging Spectrograph, to further study the supernova interactions. STIS, installed during the most recent servicing mission, would provide finer detail at the point of interaction, where the individual shells are colliding, Blair said.

Scientists also want to observe stars that are near the supernovas but have not yet exploded, in hopes of learning more about the nature of the stars that have already exploded.

Because the collision also is bright in X-rays, NASA's Advanced X-ray Astrophysics Facility, to be launched next year, will be another tool with which to study the phenomenon, he said.