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Hubble is first to spot colliding supernovas

Johns Hopkins University

Astronomers using the Hubble Space Telescope have taken the first images clearly showing interactions between two or more exploding stars, called supernovas, which are producing a tremendous display in a galaxy 17 million light years from Earth.

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 Johns Hopkins University astrophysicist who led a team of scientists making the discovery.

Blair worked with two other astrophysicists: Robert A. Fesen, from Dartmouth College, and Eric M. Schlegel, from the Harvard-Smithsonian Center for Astrophysics. Their findings will be detailed in a poster paper to be presented on Tuesday, June 10, during a meeting of the American Astronomical Society. The paper will be on display from 10 a.m. to 6:30 p.m. in the South Main Hall of the Benton Convention Center in Winston-Salem, N.C.

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 optical and X-ray light -- 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 was not expanding rapidly enough for it 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?

They found the answer after 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.

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 one 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 resolution 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.

The spiral galaxy NGC 6946 lies at a distance of 17 million light years. Adjacent to its bright northern spiral arm is a bright point-like source with characteristics of an extremely bright remnant of a supernova explosion. But astronomers have learned that the object actually is debris from two or more supernovas slamming together in a cosmic collision.

The Hubble Space Telescope Wide Field and Planetary Camera 2 image of the region, resolves the source into several interlocking loops of emission. Most of the light comes from a small crescent of emission at the lower right in the Hubble image, where the expanding shell from a relatively recent supernova explosion is plowing into an older, larger shell from a previous explosion. Astronomers have never seen this phase of such an interaction directly.

This material was presented to the American Astronomical Society meeting in Winston-Salem, NC, on June 10, 1997.

Supernova explosions have been see 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.

"It indicates that not only is there a lot of star formation going on but a lot of those stars are massive," Blair said. "They are evolving quickly, and they are exploding as supernovas."

Note: Journalists may get a hard copy of this image by contacting Emil Venere, in the Johns Hopkins Office of News and Information, at the above phone number and e-mail address.

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