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Youngest Supernova in Milky Way Found

Anne Minard
for National Geographic News
May 14, 2008
 
Scientists have peered through a thick shroud of interstellar dust to reveal the youngest supernova ever seen in the Milky Way.

Stephen Reynolds, an astrophysicist at North Carolina State University, and his team suspected that supernova G1.9+0.3 was very young.

So they compared 2007 images of the object from NASA's Chandra X-Ray Observatory with radio observations from the mid-1980s—and their suspicions were confirmed.

Estimated at just 140 years old, G1.9+0.3 is at least 200 years younger than the next oldest known supernova, Cassiopeia A, which was discovered in the 17th century A.D.

"Cas A had been the reigning youngest remnant for so long that it took a while to sink in that we had found something less than half its age," Reynolds said.

If it weren't so obscured by dust, people in the late 1800s would likely have seen G1.9+0.3 appear in the constellation Sagittarius.

As it is, G1.9+0.3—located about 26,000 light-years away—is still expanding at a surprisingly fast rate, and its discovery may pave the way to a greater understanding of exploding stars.

The results will appear in the June 10 edition of Astrophysical Journal Letters.

Galaxy Drivers

Supernovas, or exploding stars, are believed to help drive the life cycles of galaxies. A supernova explosion disperses heavy metals, as well as cosmic rays and high-energy particles that fuel the formation of new stars. (Related: "Brightest Known Supernova Detected" [October 15, 2007].)

The brightness of supernova remnants (SNRs) can easily be obscured from optical telescopes by gas and dust, but are usually visible to x-ray and radio telescopes.

Astronomers have been puzzled, however, by a shortage of young supernova remnants in our galaxy. Only half a dozen have been found, as opposed to the more than 30—roughly two a century—predicted to exist.

More recent supernovas have been discovered in other galaxies, such as the 21-year-old blast called SN 1987A, which is 160,000 light-years away in the Large Magellanic Cloud.

But supernovas in our own Milky Way are easier to study and visible for far longer.

G1.9+0.3 has increased in size by 16 percent in the last 22 years, suggesting that the initial explosion occurred only 140 years ago—or less, if the rate of expansion has been slowing, researchers point out.

"Normally, we deal with older remnants and have to work very hard to see even tiny changes," Reynolds said.

"This supernova is getting brighter, which means it's still on its way up. Studying it will go a long way toward filling in gaps in our knowledge of these events and their effect on galaxies."

Eager for More

Richard Arendt, a scientist with the University of Maryland, Baltimore County, and the Goddard Space Flight Center, said G1.9+0.3 was suspected as being a young supernova but wasn't confirmed until now.

"The results are very interesting and important, but I wouldn't really call them a surprise," he said.

Arendt said most astronomers believe there must be around ten supernova remnants younger than Cassiopeia A.

"So SNR G1.9+0.3 now fills part of that gap," he said. "The real surprises may come in the discovery of and in the nature of the other very young SNRs that are almost certainly out there."

Supernova expert Craig Wheeler is a professor at the University of Texas at Austin and president of the American Astronomical Society. He suggested researchers should now focus on discovering the nature of G1.9+0.3.

"The issues that are top on my mind are how to determine whether this was a Type Ia—an exploding white dwarf—or a core collapse [supernova]," he said, adding that "some means of directly measuring the nearby magnetic field would be great."

If G1.9+0.3 resulted from the core collapse of a star, for example, the explosion could be sending out a powerful energy jet, he pointed out. Otherwise, its boundaries could be limited magnetically.

Reynold said that the best strategy will be to spy on G1.9+0.3 with "every possible astronomical instrument that can observe it."

His team submitted a proposal in March for longer observation time using Chandra.

"We've just never had the opportunity to study a remnant in this phase of its life," Reynolds said.

"Now that we know how fast it's changing, it will be useful to re-observe it again and again and watch its evolution—pretty rare for an astronomical event!"
 

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