They detected the first evidence of a collision in 1996 [the bright spot at 11 o'clock in the Feb. 6, 1998 image]. Subsequent observations show dozens more "hot spots" as the blast wave slammed into the ring, compressing and heating the gas, and making it glow.

These images, taken by the Wide Field and Planetary Camera 2 and the Advanced Camera for Surveys, are being released to mark the 17th anniversary of the supernova's discovery. The most recent observations, taken Jan. 5, Aug. 12, and Nov. 28, 2003 by the Advanced Camera for Surveys, show many bright spots along a ring of gas, like pearls on a necklace.

The temperature of the hot spots surges from a few thousand degrees to a million degrees Fahrenheit. Curiously, one of the flares on the ring [at 4 o'clock] is just a star that happens to lie along the telescope's line of sight.

Individual hot spots cannot be seen from ground-based telescopes. Only Hubble can resolve them. Since most astronomical events occur over thousands or millions of years, this is a unique opportunity to watch large-scale changes in an object over just a few years. Astronomers also are learning how shock waves really work in the debris of an exploding star.

SN 1987A was the brightest exploding star seen in 400 years when astronomers spotted it on Feb. 23, 1987. The violent death of a star 20 times more massive than the Sun, called a supernova, created this stellar drama.

The star actually exploded about 160,000 years ago, but it has taken that long for its light to reach Earth. The supernova resides in the Large Magellanic Cloud, a nearby small galaxy that is a satellite of our Milky Way galaxy.

The ring, about a light-year across, already existed when the star exploded. Astronomers believe the star shed the ring about 20,000 years before the supernova blast. The ring is glowing in the early Hubble photos because it is being heated by ultraviolet radiation from the exploding star.

As the gas cooled, the ring began to fade, only to be illuminated again by the shock wave, which is delivering a sledgehammer blow to the ring. This is a rare opportunity for astronomers to see exactly how a supernova explosion is transformed into a supernova remnant, the glowing, expanding gaseous remains of a supernova blast.

The elongated and expanding object in the middle of the ring is debris from the supernova blast. The elliptical shape of the supernova debris provides clues to the violent events that took place deep within the exploding star. The glowing debris is being heated by radioactive elements, principally titanium 44, that were created in the supernova explosion. The debris will continue to glow for many decades.

In a few years, the entire ring will be ablaze as it absorbs the full force of the crash. The glowing ring is expected to become bright enough to illuminate the star's surroundings, thus providing astronomers with new information on how the star ejected material before the explosion.

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