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A combined image from the Chandra and XMM-Newton X-ray observatories of RCW 86, which was determined to have started out as SN 185 (Credits: ESA/XMM, NASA/CXC, University of Utrecht (J. Vink))

Astronomers are getting a bit of a treat this week—they’re watching a supernova exploding 21 million years ago (that is, 21 million light years away) in the Pinwheel Galaxy. That’s pretty close for a supernova (they’re usually around a billion light years away), and you might even be able to see it with a simple pair of binoculars. But what was the first supernova?

OK, that was a trick question. We can’t know what was the first star to explode. But we can look at the first recorded supernova, SN 185.

In 185 A.D., someone in China looked up in the night sky and saw a new star. It sparkled and did not move, so it couldn’t be a comet. This “guest star” stayed in the sky for eight months and then disappeared forever; it was recorded in the Book of the Later Han, which told the history of China from 25 to 220 A.D.

The guest star was a supernova, a star that had run out of fuel and then collapsed in on itself in a thousandth of a second. The core of the star heated to a billion degrees and destructive gamma rays were produced. Neutrinos were generated in huge quantities. Only a tiny fraction were absorbed by the stellar gas, and they had so much energy they ripped apart the outer layers of the star. This violent explosion, which could have been brighter than an entire galaxy, also produced X-rays, gamma rays and ultraviolet light. The resulting shock wave produced radioactive elements such as cobalt and titanium. Any planet too close to such a destructive event would have been torched.

In 2006, scientists using the Chandra X-ray Observatory and the XMM-Newton Observatory determined that the supernova remnant RCW 86 was the leftover bits of SN 185. They calculated how fast the energized shell of the remnant was moving to estimate the original date of the supernova and determined that the star had gone supernova about 2,000 years ago. Scientists had thought RCW 86 might be SN 185 because the remnant’s location matched historical records of the supernova, but previous calculations gave the remnant an age of 10,000 years. It appears those calculations were based on measurements of a part of the shock wave that had encountered a region of dense matter and slowed down.