Hubble eyes new phase of supernova explosion
By Robert Roy Britt
(SPACE.com) -- The most dramatic stellar explosion witnessed in centuries just got more interesting. New images from the Hubble Space Telescope show a dying star's "ring of fire" entering a new phase of brightness.
The remarkable event is the only one of its kind ever recorded by telescopes.
A sequence of Hubble images of supernova 1987A taken from 1994 through late 2003 shows the central star has faded while bright spots in the outer ring are enhanced.
Astronomers first saw the star explode -- an event called a supernova -- in 1987. It shone as bright as 100 million suns for several months.
Robert Kirshner of Harvard University and the Harvard-Smithsonian Center for Astrophysics led the latest observations. He explained what's going on around the star named 1987A. When the star first exploded, ultraviolet light raced outward and lit up a previously unknown ring of gaseous debris that the star had presumably spat out about 20,000 years prior.
"Then there's a blast wave going out from the supernova to the ring," Kirshner said in a telephone interview. "We all knew it was going to hit in a decade or so."
Shock wave hits
In 1996, that shock wave began to plow into the debris ring, which is about a light-year in diameter. It heated the ring material -- gas and dust -- in spots, created an increasing number of bright areas that Hubble has been documenting over the years.
In the latest image, the spots nearly cover the ring like pearls on a necklace. The star, meanwhile, is a million times dimmer than when it first exploded.
The fact that the ring did not light up all at once suggests it is not perfectly round but instead is unstable, with parts of the inner ring closer to the central star than other parts, Kirshner said. He described it as a corrugated structure, or a wall with stalactites sticking inward.
Inside the ring, an amorphous purplish blob surrounds the central, dying star. That blob glows because it's made of radioactive elements forged in the supernova explosion. It is probably radioactive titanium, Kirshner explained, "shredded bits of the star going out at about 3,000 kilometers per second," or 6.7 million mph.
"Looking at the expansion of that [blob] we get a clue to what was happening in [the] first couple of minutes of the explosion of the star," he said.
Scientists have seen no other supernova evolve over time with anything approaching this sort of detail.
The ring around 1987A should continue to brighten for a couple of decades, Kirshner said. The bright spots will merge as the debris is engulfed by the shock wave until it creates a "ring of fire," he said. Watching the evolution should help theorists understand how and why stars explode.
"There's going to be plenty to see," Kirshner said.
But it's uncertain whether 1987A will be monitored continuously.
Like many astronomers, Kirshner said he's disappointed that under NASA's current plans, Hubble won't be around to record 1987A's progress in the latter years of this decade. The activity can be studied by X-ray and radio observatories, but valuable visible-light data would go uncollected.
"The value of the data keeps getting bigger as we get a longer series," Kirshner said. He added that it could be a long time before a similar event is available to astronomers and Hubble "is one of our chief instruments for doing this."
The last supernova to shine so brightly in Earth's skies was spotted by Johannes Kepler 400 years ago.
1987A was generated by a star 20 times more massive than the Sun. It resides in a nearby galaxy called the Large Magellanic Cloud. Because of the time it takes light from the event to reach Hubble, the explosion actually occurred 160,000 years ago, in the time frame of its origin.
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