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