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Strange Ring Found Circling Dead Star

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May 29, 2008: NASA's Spitzer Space Telescope has found a bizarre ring of material around the magnetic remains of a star that blasted itself to smithereens.

The stellar corpse, called SGR 1900+14, belongs to a class of objects known as magnetars. These are the cores of massive stars that blew up in supernova explosions, but unlike most other dead stars, they have tremendously strong magnetic fields.

The ring was found serendipitously. "I was flipping through archived Spitzer data and that's when I noticed SGR 1900+14 was surrounded by a ring we'd never seen before," says Stefanie Wachter of NASA's Spitzer Science Center at the California Institute of Technology. "The universe is a big place and weird things can happen!"

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Above: A ring around SGR 1900+14 observed by the infrared Spitzer Space Telescope. [more]

Wachter and her colleagues think that the ring, which is unlike anything ever seen before, formed in 1998 when the crusty iron surface of the magnetar cracked and erupted in a giant flare. The blast was so powerful, it ionized Earth's upper atmosphere and actually overloaded the instruments of several NASA spacecraft. (For more information about the event, see the 1998 Science@NASA story: Crusty Star Makes its Presence Felt.)

Researchers believe the magnetar was surrounded by a cloud of dust and the explosion excavated that cloud, leaving an outer, dusty ring. The ring is oblong, with dimensions of about seven by three light-years. It appears to be flat, or two dimensional, but the data do not rule out the possibility of a more complex 3-dimensional shell.

"It's as if the magnetar became a huge flaming torch and obliterated the dust around it, creating a massive cavity," says co-investigator Chryssa Kouveliotou of NASA's Marshall Space Flight Center. Nearby stars lit up the ring so that Spitzer could see it--"a ring of fire marking the magnetar for eternity."

Rings and spheres are common in the universe. For instance, young massive stars use their stellar wind to blow bubbles in space, carving clouds of dust into spherical shapes. Later, when those stars die in supernova explosions, their remains are blasted away, forming beautiful orbs called supernova remnants. Rings can also form around exploded stars whose expanding shells of debris ram into pre-existing dust clouds, causing the dust to glow, as is the case with the supernova remnant 1987A.

But the ring around the magnetar SGR 1900+14 fits into none of these categories. For one thing, supernova remnants and the ring around 1987A cry out with X-rays and radio waves. The ring around SGR 1900+14 does not; it only glows at specific infrared wavelengths that Spitzer can see.

Below: A selection of rings and spheres in the Milky Way. From left to right, (1) light echoes from old supernova remnant Cassiopeia A, (2) a blast wave emerging from recent supernova 1987A, and (3) a planetary nebula named The Helix. The ring around SGR 1900+14 is like none of these things.

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At first, the astronomers thought the ring around SGR 1900+14 must be an infrared echo. These occur when an object sends out a blast wave that travels outward, heating up dust and causing it to glow with infrared light. But when they went back to observe SGR 1900+14 later, the ring didn't move outward as an infrared echo would.

A closer analysis revealed that the ring is most likely a carved-out cavity in a dust cloud -- a phenomenon that must be somewhat rare in the universe because it had not been seen before.

The discovery could help scientists figure out if a star's mass influences whether it becomes a magnetar when it dies. Though scientists know that stars above a certain mass will "go supernova," they do not know if mass plays a key role in determining whether the stellar corpse becomes a magnetar or a run-of-the-mill dead star. According to the science team, the glowing ring of dust Spitzer observed connects SGR 1900+14 to a nearby cluster of young stars. By studying the masses of those stars, the scientists might be able to figure out the original mass of SGR 1900+14.

"SGR 1900+14 is interacting with its environment, making a big impact on the region where it was born," concludes astronomer and co-investigator Enrico Ramirez-Ruiz of the University of California, Santa Cruz. "This 'dead star' is still alive in many ways."

Editor: Dr. Tony Phillips | Credit: Science@NASA

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