Mystery Closes Over A 2,000 Year Old Supernova

In 185 A.D., Chinese astronomers witnessed what they called a mysterious "guest star" that appeared in the sky and lingered for about eight months. It wasn't until the 1960s that scientists determined that this cosmic object was the first documented observation of a supernova that signaled the violent death of a distant star.Now, infrared views of the supernova from NASA's Spitzer Space Telescope and the Wide-field Infrared Survey Explorer (WISE) reveal that the star explosion detonated inside a region of space that was relatively free of gas and dust. This allowed the star's explosion to travel out much farther and faster than expected, researchers said.

"This supernova remnant got really big, really fast," said Brian Williams, an astronomer at North Carolina State University in Raleigh, in a statement. "It's two to three times bigger than we would expect for a supernova that was witnessed exploding nearly 2,000 years ago. Now, we've been able to finally pinpoint the cause."Space telescope observations now suggest that before exploding, a wind of material from the star blew a cavity around it, into which the supernova could expand much more quickly.

The findings, published in the Astrophysical Journal, combine existing data from the Chandra X-ray telescope and the XMM-Newton Observatory with recent images from the US space agency Nasa's Spitzer and Wide-field Infrared Survey (Wise) telescopes.Both of these telescopes are sensitive to infrared light, giving a picture of the conditions of material that is at fairly low temperatures in the supernova remnant. The supernova, about 8,000 light-years away, is huge - if the infrared light it emits could be seen by our eyes, it would appear to be as large in the sky as the full Moon.

Taken together, the data show that the supernova initially expanded into a comparatively empty "cavity", meaning its material could quickly expand unimpeded.However, the case is not closed for RCW 86; these cavities are associated only with what are called core-collapse supernovas, but the Chandra and XMM-Newton observations show evidence of a great deal of iron in the remnant - associated instead with Type 1A supernovas.

"Now, with multiple observatories extending our senses in space, we can fully appreciate the remarkable physics behind this star's death throes, yet still be as in awe of the cosmos as the ancient astronomers.""Modern astronomers unveiled one secret of a two-millennia-old cosmic mystery only to reveal another," said Bill Danchi, a programme scientist for the Spitzer and Wise missions.