A mystery surrounding the first recorded supernova - seen by Chinese astronomers in AD185 - has been solved.
The supernova RCW 86 lit up the sky for eight months, documented at the time as a "guest star".
In more recent times, astronomers have wondered how it grew so large, so fast.
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 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.
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.
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.
"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.
"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."