Supernova 1987A: Fast Forward To The Past

Thesupernova occurred in the Large Magellanic Cloud, a galaxy only 160,000light years from Earth. The outburst was visible to the naked eye, andis the brightest known supernova in almost 400 years. The site of theexplosion was traced to the location of a blue supergiant star calledSanduleak -69º 202 (SK -69 for short) that had a mass estimated atapproximately 20 Suns.

Subsequent optical, ultraviolet and X-rayobservations have enabled astronomers to piece together the followingscenario for SK -69: about ten million years ago the star formed out ofa dark, dense, cloud of dust and gas; roughly a million years ago, thestar lost most of its outer layers in a slowly moving stellar wind thatformed a vast cloud of gas around it; before the star exploded, ahigh-speed wind blowing off its hot surface carved out a cavity in thecool gas cloud.

The intense flash of ultraviolet light from thesupernova illuminated the edge of this cavity to produce the brightring seen by the Hubble Space Telescope. In the meantime the supernovaexplosion sent a shock wave rumbling through the cavity.

In 1999,Chandra imaged this shock wave, and astronomers have waited expectantlyfor the shock wave to hit the edge of the cavity, where it wouldencounter the much denser gas deposited by the red supergiant wind, andproduce a dramatic increase in X-radiation. The latest data fromChandra and the Hubble Space Telescope indicate that thismuch-anticipated event has begun.

Optical hot-spots now encirclethe ring like a necklace of incandescent diamonds (image on right). TheChandra image (left) reveals multimillion-degree gas at the location ofthe optical hot-spots.

X-ray spectra obtained with Chandraprovide evidence that the optical hot-spots and the X-ray producing gasare due to a collision of the outward-moving supernova shock wave withdense fingers of cool gas protruding inward from the circumstellar ring(see illustration). These fingers were produced long ago by theinteraction of the high-speed wind with the dense circumstellar cloud.

Thedense fingers and the visible circumstellar ring represent only theinner edge of a much greater, unknown amount of matter ejected long agoby SK -69. As the shock wave moves into the dense cloud, ultravioletand X-radiation from the shock wave will heat much more of thecircumstellar gas.

Then, as remarked by Richard McCray, one ofthe scientists involved in the Chandra research, "Supernova 1987A willbe illuminating its own past."