See the ghosts of dead stars

NASA's Spitzer Space Telescope bears witness this week to two stellar hauntings - bizarre and beautiful phenomena sparked by dead giant stars.

First, Spitzer trained its infrared eye on the pulsing, undead corpse of a strongly magnetic star called SGR 1900+14 - revealing a bizarre ring of dusty material that couldn't be seen in visible wavelengths. Then, just today, Spitzer's scientists reported on the ghostly echoes of light emanating from Cassiopeia A, a supernova that blew up 300 years ago.

The report on the ring appears in today's issue of the journal Nature, but is based on a review of readings from Spitzer going back almost five years.

SGR 1900+14 has been an object of interest since 1998, when the star blew up, sending a powerful blast of gamma rays in Earth's direction. What's left behind is something known as a magnetar - a dead star that slowly pulsates with X-rays and hums with a strong magnetic field.

"The universe is a big place, and weird things can happen," the Nature paper's lead author, Stefanie Wachter of NASA's Spitzer Space Science Center, said in a news release issued Wednesday. "I was flipping through archived Spitzer data of the object, and that's when I noticed it was surrounded by a ring we'd never seen before."

Wachter and her colleagues surmise that the ring was created during the blast: The magnetar's crusty surface might have cracked and sent out a flare so strong that it blasted away at a nearby cloud of dust, leaving an oblong ring behind that measures about seven light-years in its widest dimension.

"It's as if the magnetar became a huge flaming torch and obliterated the dust around it, creating a massive cavity," said Chryssa Kouveliotou, senior astrophysicist at NASA's Marshall Space Flight Center and a co-author of the paper. "Then the stars nearby lit up a ring of fire around the dead star, marking it for eternity."

The scientists believe the ring isn't being lit up by the magnetar itself, but instead by radiation from nearby massive stars.

The effect may be beautiful, but a magnetar's gamma-ray burst is no walk in the park. In fact, some researchers believe the 1998 burst from SGR 1900+14 was one of Earth's closest brushes with doom. In their book "Rare Earth," astronomer Donald Browlee and paleontologist Peter Ward note that the blast occurred more than 20,000 light-years away - and yet, the radiation penetrated to within 30 miles of Earth's surface before dissipating, requiring the pre-emptive shutdown of two orbiting satellites.

"Had the magnetar in question been only 10,000 light-years away, the energy reaching Earth would have been four times stronger - perhaps strong enough to damage the ozone layer," they wrote. "Did this particular event sterilize worlds within a light-year or less? ... Perhaps life can flourish only in neighborhoods far from magnetars."

That goes for supernovae in general, for that matter. A supernova leaves a beautiful corpse, but you wouldn't want to be next to the star when it blows up.

Echoes from a supernova

Spitzer's other study of the week pays tribute to the long-lasting effects of a supernova - specifically, Cassiopeia A, which was in the record books as the Milky Way galaxy's youngest supernova remnant until this month.

Cass A is about 11,000 light-years away, and its supernova blast reached Earth in the year 1680. Scientists didn't think they'd be seeing anything more from that blast - but starting in 2005, they picked up on some curious upticks in the heat emanating from dusty clouds surrounding the dead star.

The scientists figured out that these were "infrared echoes" - caused when the radiation from the initial blast heated up the clouds and caused them to re-radiate in infrared wavelengths. Because the clouds are not in a direct line with the supernova's central star, the path taken by the echoing radiation is longer than the direct route, and as a result we're just now observing the glow.

Spitzer's infrared camera is perfectly suited to catch such echoes. The Spitzer team took the infrared readings and matched them up with faint echoes in visible-light wavelengths, captured by Japan's Subaru Telescope in Hawaii and other ground-based telescopes. These echoes were directly reflected off the surrounding clouds, like yodels reflected off an Alpine mountainside.

In Friday's issue of the journal Science, the researchers report that they were able to get enough observations from those echoes to analyze the spectrum of light from the original explosion, seen from Earth 300 years ago.

"The echoes of light we found around Cassiopeia A provide us with a time machine to go back and see its past," the study's lead author, Oliver Krause of the Max Planck Institute for Astronomy in Germany, said in today's Spitzer news release.

The spectrum showed that Cassiopeia A was a red supergiant star that became a Type IIb supernova. That was something researchers never knew before.

"Cassiopeia A has been studied extensively with many telescopes over a wide range of wavelengths," said Alex Filippenko of the University of California at Berkeley, a supernova expert who was not affiliated with the study. "It is gratifying that we finally know what kind of star exploded so long ago."

NASA said the Type IIb determination could explain a longstanding mystery: A supernova that close should have registered as a noticeably bright star, but very few sightings of Cassiopeia A were recorded back in 1680. Why didn't more people notice the supernova?

"Type IIb supernovas fade quickly," said George Rieke of the University of Arizona in Tucson, a co-author of the Science study. "This, plus a few cloudy nights, might explain the historical enigma around Cassiopeia A."

In addition to Wachter and Kouveliotou, co-authors of the Nature paper about the magnetar include Enrico Ramirez-Ruiz of the University of California at Santa Cruz; V. Dwarkadas of the University of Chicago; J. Granot of the University of Hertfordshire; S.K. Patel of the Optical Sciences Corp.; and D. Figer of the Rochester Institute of Technology.

In addition to Krause and Rieke, co-authors of the Science paper about Cassiopeia A include Stephan Birkmann and Miwa Goto of the Max Planck Institute for Astronomy; Tomonori Usuda and Takashi Hattori of the National Astronomical Observatory of Japan in Hawaii; and Karl Misselt of the University of Arizona.