Cassiopeia A: Dead But Not Quiet
Dual infrared image of Cassiopeia A on Nov. 30, 2003 (top), and Dec. 2, 2004 (bottom).
Credit: NASA/JPL/O. Krause.

An echo has been detected around a star whose death was witnessed 325 years ago. The reverberation--emanating out in light, not sound waves--implies that the stellar remnant let out a burst of energy some 50 years ago.

The dead star in question is Cassiopeia A, whose explosion or supernova was witnessed by Tycho Brahe in 1572. Situated 10,000 light years away, astronomers believe a dense neutron star is all that is left of the original star.

This neutron star remnant was thought to be resting in peace, that is, until this recent discovery of a light echo in infrared images taken by NASA's Spitzer Space Telescope.

"We had thought the stellar remains inside Cassiopeia A were just fading away," said Oliver Krause of the University of Arizona. "Spitzer came along and showed us this exploded star, one of the most intensively studied objects in the sky, is still undergoing death throes before heading to its final grave."

The evidence for this postmortem activity first came in a Spitzer test image that showed glowing dust features around the dead star. Later observations found that the tangled features had moved outward--apparently at the speed of light.

In actuality, the dust hadn't moved, but instead the light waves that were exciting the dust had spread out further. This light echo is the largest one ever observed and the first to be seen around a long-dead star.

By tracing the echo's light waves backwards, the researchers inferred that some sort of bang occurred on the neutron star back in 1953.

This recent activity may mean that Cassiopeia A is an exotic type of neutron star, called a magnetar. These strange objects have surfaces that rupture and quake, letting loose tremendous amounts of high-energy gamma rays.

"Magnetars are very rare and hard to study, especially if they are no longer associated with their place of origin," said George Rieke of the University of Arizona. "If we have indeed uncovered one, then it will be just about the only one for which we know what kind of star it came from and when."

Further observations with Spitzer may reveal more about Cassiopeia A's life after death. Rieke and Krause were two of the authors on a paper describing the observations in this week's issue of the journal Science.

"We had no idea that Spitzer would ever see light echoes," Rieke said. "Sometimes you just trip over the biggest discoveries."

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