Hubble images capture colliding supernovas.

Astronomers have for the first time caught supernova remnants in the act of colliding. Although observers had seen evidence of past interactions between these shells of gas and dust blown off by exploded stars, the new images offer a rare glimpse of them "actually pounding into each other," says William P. Blair of Johns Hopkins University in Baltimore.

Such snapshots are not easy to come by. Supernova collisions last only a few hundred years--a mere blink of the eye compared to most processes in astronomy--as shells of material race through each other. Indeed, the fleeting encounter reported last week might have gone unnoticed had astronomers not been intrigued by a point of light in a galaxy 17 million light-years from Earth.

Lying near one of the spiral arms in the galaxy NGC 6946, the point glows brightly in visible light and even more brilliantly in X rays, according to ground-based observations by Blair's team and data from the German-British satellite ROSAT. Blair assumed he had come across a supernova only a few hundred years old--a tantalizing find, as such objects are relatively scarce and provide insight into the massive stars from which they arose.

Visible-light spectra of the object revealed that it could not be young, however. The remnant had neither the enrichment of heavy elements nor the high velocity expected of material recently hurled from an exploded star. "It really took the wind out of my sails," says Blair.

The sharp eye of the Hubble Space Telescope showed that Blair's team had bagged an even rarer quarry. The images depict a moderately young remnant, about 25 light-years in diameter and 2,000 years old, crashing into one or two older, larger remnants that appear as interlocking loops. The collision generates the X-ray and visible-light emissions, Blair says.

Since 1917, astronomers have observed six supernovas in NGC 6946, leading Blair to speculate that the current smashup could be the start of a much larger process in which a slew of colliding supernovas blows a giant network of bubbles within the galaxy (SN: 11/21/92, p. 342).

Blair, along with Robert A. Fesen of Dartmouth College in Hanover, N.H., and Eric M. Schlegel of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., reported the finding at a meeting of the American Astronomical Society in Winston-Salem, N.C.

Such observations, says John C. Raymond of Harvard-Smithsonian, may help determine whether supernova explosions heat a substantial fraction of the interstellar gas in the Milky Way or whether their energy is confined to much smaller regions in the galaxy.