IN 1987, LIGHT FROM AN EXPLODING STAR in a neighbouring galaxy, the Large Magellanic Cloud, reached Earth. Named Supernova 1987A, it was the closest supernova explosion witnessed in almost 400 years, and its proximity has enabled astronomers to study it in unprecedented detail as it evolves.
A team of astronomers has now announced that the supernova debris, which had been fading over the years, is now brightening. This shows that a different “power source” has begun to light up the debris, and marks its transition from a supernova to a supernova remnant.
“Supernova 1987A has become the youngest supernova remnant visible to us,” said Robert Kirshner of the Harvard-Smithsonian Centre for Astrophysics (CfA).
Kirshner leads a long-term study of SN 1987A with NASA’s Hubble Space Telescope. Since its launch in 1990, Hubble has provided a continuous record of the changes in SN 1987A.
A new power source
SN 1987A is surrounded by a ring of gas that blew off the progenitor star thousands of years before it exploded. The ring is about one light-year (10 trillion kilometres) across. Inside that ring, the “guts” of the star are rushing outward in an expanding debris cloud.
Most of a supernova’s light comes from radioactive decay of elements created in the explosion. As a result, it fades over time. However, the debris from SN 1987A has begun to brighten, suggesting that a new power source is lighting it.
“It’s only possible to see this brightening because SN 1987A is so close and Hubble has such sharp vision,” Kirshner said.
A supernova remnant consists of material ejected from an exploding star, as well as the pre-existing material the blast wave sweeps up.
The outflowing debris from SN 1987A is beginning to crash into the surrounding gas ring, creating powerful shock waves that generate X-rays, which have been detected by NASA’s Chandra X-ray Observatory. Those X-rays are illuminating the debris, and shock heating is making it glow.
The same process powers other well-known supernova remnants in our galaxy, such as Cassiopeia A.
Change you can see
Because it’s so young, the remnant of SN 1987A still shows the history of the last few thousand years of the star’s life recorded in the knots and whorls of gas. By studying it further, astronomers may decode that history.
“Young supernova remnants have personality,” Kirshner agreed.
Eventually, that history will be lost when the bulk of the expanding stellar debris hits the surrounding ring and shreds it. Until then, SN 1987A continues to offer an unprecedented opportunity to watch a cosmic object change over the course of a human lifetime. Few other objects in the sky evolve on such short time-scales.
Adapted from information issued by the Harvard-Smithsonian Centre for Astrophysics. Images courtesy NASA / P. Challis (CfA) / David Malin (AAO).
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