A sequence of colorful images detailing a stellar explosion in another galaxy show a supernova's massive, morphing shock wave, providing scientists with a case study in the structure and evolution of the events.

The images of SN1993J, discussed in the Jan. 7 issue of the journal Science, represent the first long-term supernova sequence ever obtained. (The first five of the 20 images were published in 1997, providing for great viewing but relatively little detailed information.)

The last known supernovas in our own galaxy exploded in the 1700s. The effect on Earth of a nearby stellar explosion could be traumatic, and the new images may help researchers understand supernovas in general as well as the remnants of ancient supernovas in the Milky Way.

"The next (Milky Way) supernova is clearly overdue," said Norbert Bartel of York University in Canada and lead author of Science article. "A supernova in our Milky Way would probably evolve similar to SN1993J."

Bartel told space.com that a nearby supernova -- one occurring a mere 10 or 20 light-years away in our neck of the Milky Way -- would "bombard us with high-energy radiation that could seriously affect our atmosphere and perhaps even significantly harm life on Earth."

The star generating explosion, known as the progenitor star, was probably about 15 times the size of our sun, Bartel said. Near the end of its life, the star had burnt all its fuel.

"No material was left that could be fused to deliver the energy that makes the star shine and counterbalances the star's enormous gravitational collapsing forces," he explained. "As a result the inner zones of the star imploded, condensing the core to unimaginable densities."

The implosion generated an enormous shock wave that zoomed to the surface of the star and broke through on March 28, 1993. Spanish amateur astronomer Francisco Garcia Diaz spotted it.

Within hours the supernova was billions of times brighter than our sun. The shock wave fled the scene at 12,427 miles each second (20,000 kps).

"After 50 days the supernova has expanded to about 13 times the size of our planetary system," Bartel said. That's when the first image was taken, on April 27, 1993. (The event, being so far away, actually occurred a long time ago -- the light only began reaching us in 1993.)

Even with a resolving power 100 times better than the Hubble telescope (using a global array of radio telescopes called the Very Long Baseline) no details were yet visible on the distant object, which lies in a relatively nearby galaxy called M81, roughly 12 million light-years away. Only the object's size could be measured.

"After a half year first details became visible," Bartel said. "A round shell developed. But the shell was not uniformly bright; it was shining in kind of a horseshoe pattern. Furthermore this brightness pattern rotated with time."

After two years, hot spots appeared around the outer edges. Researchers don't know why they occur, but Bartel thinks the shock wave may be catching up with knots of other material blown off the star thousands of years before it exploded.

By this time, the shock wave's speed had decreased, and the deceleration itself increased with time. And by the time of the final image, on June 3, 1998, the shock wave moved at 5,282 miles-per-second (8,500 kps).

While the explosion of a star might seem a violent and frightening event, Bartel reminds us how necessary the process is:

"Supernovas are the source of the heavier elements, the material we are made of," he said. "The explosion of a star therefore is fundamental to our own life as well as to any other life forms that may exist in the universe."