Surfing the Cosmic Shock Wave

For nearly 18 years, JILA Fellow Dick McCray has been studying the brightest supernova to light up Earth's night skies since the Renaissance. Known as 1987A because it appeared in the southern sky on February 23, 1987, the supernova occurred when a 10-million-year-old blue supergiant star exploded in the Large Magellanic Cloud, a galaxy located 160,000 light years from Earth.

McCray and other astrophysicists have investigated the unfolding story of 1987A in visible, ultraviolet, and X-ray wavelengths, with the goals of better understanding both the star's spectacular demise and its past history. Their observations led them to conclude that about a million years ago, the giant star lost most of its outer layers as a slow-moving stellar wind formed a vast, cool cloud of gas around the star. Then, before the star exploded, a high-speed wind blowing off the star's molten surface carved out a cavity in the gas cloud.

When the star exploded, an intense flash of ultraviolet light illuminated the edge of the cavity, highlighting a bright ring. At the same time, the explosion propelled a shock wave into the cavity toward the gas cloud. In 1991, McCray and his student Ding Luo correctly predicted that the shock wave would crash into the ring of gas around 1997, heating and illuminating denser areas of gas. By 2003, the shock wave crash had created dozens of hot spots encircling the ring's edge like a necklace of dazzling diamonds.

In 1999, the Chandra X-Ray Observatory imaged the shock wave inside the cavity heading for the ring's edge. Soon after, Chandra data indicated the shock wave had penetrated the ring. Recently, Svetozar Zhekov, a JILA research associate, Dick McCray, and colleagues from North Carolina State and Pennsylvania State Universities, analyzed new X-ray spectra to figure out that the shock wave is now interacting with dense fingers of gas that protrude inward from the edge of the circumstellar gas cloud. When the researchers measured the velocity of the X-ray emitting gas, they found it was expanding much more slowly than the shock wave, proving that the X-rays were coming from the fingers. The fingers, shown in the illustration on the previous page, were formed prior to the supernova by the interaction of the dense cloud with the same high-speed wind that sculpted the cavity.

The researchers showed that the shock wave/finger interactions were producing both the X-rays imaged by Chandra and the visible hot spots. McCray said the dense fingers and the visible circumstellar ring are just the inner edge of the cloud of matter ejected long ago from the star. As the shock wave rides further into the circumstellar cloud, it will heat more of the gas, sending information toward Earth as visible, ultraviolet, and X-ray signals.

"When that happens, Supernova 1987A will be illuminating its own past," said McCray, who prepared the composite figure (right) showing the development of the shock wave crash in visible light (Hubble Space Telescope), X-rays (Chandra Observatory), and radio waves (Australia Telescope).

Zhekov, McCray, and their colleagues reported their latest findings on Supernova 1987A in the August 1 (2005) issue of The Astrophysical Journal Letters.   - Julie Phillips