A guest star observed by Chinese astronomers for eight months in AD 185 was a supernova whose remnant is now a shell of gas radiating at infrared and X-ray wavelengths. RCW86 is roughly 85 light-years across and at a distance of 8000 light-years it is about as wide as the full Moon in our sky.
This image is a composite of X-ray data from ESA’s XMM-Newton Observatory and NASA’s Chandra X-Ray Observatory, coloured blue and green, and infrared data from NASA’s Spitzer Space Telescope and Wide-Field Infrared Survey Explorer (WISE) is shown in yellow and red. The X-ray data track interstellar gas at millions of degrees, heated by the shock wave from the supernova explosion, and the infrared data maps dust at around 70 K – cool, but a lot hotter than dust usually is in our galaxy.
The remnant is surprisingly big for its age. It also seems to have been a low-density environment for much of its life, suggesting a cavity typically associated with a core-collapse supernova. But this explosion involved a star with a lot of iron, characteristic of a Type 1a supernova, the explosion of a white dwarf star. Researchers suspect that it was indeed a Type 1a event, but one that took place in a low-density region around the white dwarf, a cavity essentially “blown” by stellar winds before the supernova. The cavity allowed the remnant to expand faster than normal, unimpeded by gas or dust, allowing it to get so big, so quickly.