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Shockwave!

Shockwave Animation

Almost a decade ago, on February 23, 1987, astronomers noticed that a star 167,000 light-years from Earth in the Large Magellanic Cloud had blown apart. This blazing act of self-destruction released an amount of energy equivalent to that of 100 suns over their lifetime. Ever since, scientists have watched the fading aftermath of supernova 1987A--and the second act of the show is now beginning: An immense shockwave from the explosion is smashing into a mysterious ring of gas that surrounds the dead star, causing it to light up once again.

Supernova 1987A was the brightest stellar cataclysm seen since Johannes Kepler recorded his observations of a supernova in the year 1604. At its brightest, Supernova 1987A glowed as intensely as the stars in the Big Dipper constellation. And it is the first supernova observed clearly over a long period with modern instruments. "This is the first time we have been able to observe these events in the span of a human lifetime," says Ann Kinney of the Space Telescope Science Institute, which manages the orbiting Hubble Space Telescope for the National Aeronautics and Space Administration.

Indeed, observing the supernova is like watching a nuclear bomb detonate in extreme slow motion. Because of its vast scale, events that would take a fraction of a second in an explosion on Earth are played out over decades in space. The most recent Hubble observations show that the physical shockwave from the giant blast is just now reaching the innermost of three mysterious gas rings circling the dead star at a distance of two-thirds of a light year.

Jolted by the 40-million mile per hour sledgehammer blow, one knot of gas--100 billion miles in diameter in a piece of the ring--has already begun to glow brightly, as its temperature surges from a few thousand degrees to a million degrees Fahrenheit. Astronomers predict it's only a matter of years before the complete ring becomes ablaze with light as it absorbs the full force of the impact. "Ten years from now, the entire ring will be hundreds of times brighter than today," says Robert Kirshner of the Harvard-Smithsonian Center for Astrophysics, which made the recent images of the supernova ring.

Astronomers do not think that this inner ring is simply debris from the explosion. Rather, they believe that it likely formed about 20,000 years before the star exploded. But it only became visible when it was heated by the burst of x-ray and light energy from the explosion; since then, it has been slowly fading as the gas cools. "This material used to be part of the star--its not shrapnel, its surface stuff that puffed off before the star exploded," says Kirshner.

Supernova 1987A
Image: HUBBLE SPACE TELESCOPE

SUPERNOVA 1987A is encircled by three mysterious rings. The innermost is now revealing the first effects of the spreading shockwave.

The outer two rings are even more puzzling. Hubble images recorded in 1994 revealed that the exploding star had assumed a shape resembling two wine glasses placed base to base. The supernova is located where the stems of the two glasses would intersect. The inner ring of gas is dispersed around the edges of the bases. Then, there are two more, fainter. rings of gas that would be on the edges of the flutes of the glasses.

Because the outer rings appear to be mirror images, astronomers speculate that they were "painted" on the expanding envelope of gas by twin jets of high-energy. A rapidly rotating but unseen companion star--one that collapsed as a neutron star or black hole at the time of the supernova--could have produced these jets. Material falling from the exploded star onto the compact companion would have been heated and blasted back into space in two narrow jets, along with a beam of radiation.

As the compact object spinned, it might have wobbled or precessed about its axis, like a child's top winding down. The twin beam would then trace out great circles like jets of water from a spinning lawn sprinkler. In fact, when Chris Burrows of the European Space Agency did a detailed inspection of the Hubble images, he located a dim object that could be the source of the beams at the predicted location--about one-third light-year from the center of the supernova explosion.

Investigators are waiting for the spreading shockwave to illuminate features that may confirm Burrows' theory and solve other mysteries of dying stars. The shockwave is a physical wallop that will allow scientists to test new theories about the interaction of shockwaves. "We are beginning to see the signature of the collision, the hammer hitting the bell. This event will allow us to validate ideas we have built up over the past ten years of observation," Kirshner adds.

If the theories about the rings are correct, the whole structure surrounding the dead star should be an envelope of low-density gas wherein the matter is too diffuse to be observed. "The ring will form a light bulb emitting visible light and x-rays that will shine through and illuminate the matter outside," says Richard McCray of the University of Colorado in Boulder. "The light from the crash will give us a chance to see this invisible matter for the first time, and then perhaps we can unravel the mystery of the outer rings."

Core Animation
Image: HUBBLE SPACE TELESCOPE

STARDUST. Debris from the core of the shattered star is spreading outward at 2,000 miles a second. Astronomers believe this process seeds galaxies with heavy elements such as iron.

The recent Hubble images are already clarifying how heavy elements that form planets and living organisms spread through the universe. "All the chemical elements in the universe--and our world--are believed to have been created by exploding stars before the solar system was formed," Kirshner says. "We are all from outer space, but we didn't get here on space ships; we arrived an atom at a time from exploding stars."

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