Shock wave revives fading supernova ring.

First came the explosion. Now for the shocker.

Eleven years ago, astronomers witnessed supernova 1987A, the brightest stellar eruption seen from Earth since 1604. Now, a shock wave driven into the surrounding interstellar material has reached an irregular ring of gas encircling the explosion site. Although Hubble Space Telescope images reveal that the collision has thus far lit up only a small section of the ring--the initial point of contact between the innermost protrusion of the ring and the outward-moving shock wave--the finding heralds a major event.

Researchers predict that by 2007, after it absorbs the full brunt of the collision, the entire ring will be ablaze. That fiery glow may literally shed new light on several mysteries surrounding the massive star, whose spectacular death was recorded on Feb. 23, 1987 (SN: 2/22/97, p. 120).

"By lighting up the ring, the supernova is exposing its own past," says Robert P. Kirshner of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass. He, Richard McCray of the University of Colorado at Boulder, and George Sonneborn of NASA's Goddard Space Flight Center in Greenbelt, Md., presented Hubble images and spectra this week at a briefing in Washington, D.C.

X rays and ultraviolet light emitted by the supernova reached the ring a few months after the explosion, but the glow they produced has declined steadily and never illuminated much of the material near the ring, notes McCray. The shock wave "will give us a chance to see the invisible matter for the first time."

The ring may be part of a much larger structure cast off by the bloated, massive star 20,000 years before it exploded as supernova 1987A. In this model, the swollen star would have engulfed a smaller companion, ejecting a disk of material in the process. By 2007, when the collision lights up this material and reveals its shape, astronomers may be able to deduce whether the exploded star indeed had a partner.

Astronomers are eagerly awaiting the coming fireworks, says McCray. "We've never seen a shock wave moving as fast as one-twentieth the speed of light slam into a dense [ring of] gas," he notes.