Oliver Krause of the University of Arizona and his colleagues analyzed images of Cas A taken over a year's time during routine testing of Spitzer's instruments. In the first shot, the dust surrounding Cas A showed strange, tangled features. Comparisons with ground-based observations made a few months later revealed that the dust appeared to be moving outward at the speed of light. When the space telescope caught its second glance, however, the researchers determined that the dust was stationary, but was being lit up by passing light coming from Cas A. "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," Krause remarks.
The astronomers identified two infrared light echoes from the remnant--one that corresponds to its original supernova explosion and one from a burst of activity that occurred around 1953. The latter is the largest such light echo ever seen and the first observed around a long-dead star. Cas A sent energy spewing out in paths roughly perpendicular to the direction of Earth, which means astronomers observing the object from the ground at the time would not have noticed the outburst. "We had no idea Spitzer would ever see light echoes," co-author George Rieke of the University of Arizona says. "Sometimes you just trip over the biggest discoveries." The scientists report their findings in the current issue of the journal Science.