The first written record of a supernova comes from Chinese astrologers in the year 185. Those records say a ‘guest star’ lit up the sky for about eight months. We now know that it was a supernova.
All that remains is a ring of debris named RCW 86, and astronomers working with the DECam (Dark Energy Camera) used it to examine the debris ring and the aftermath of the supernova.
Chinese astrologers recorded SN 185 in The Book of the Later Han, or as the Chinese call it, the Hou Han shu. There’s uncertainty around ancient records of astronomical events, and in the Hou Han shu’s case, the uncertainty is amplified by the fact that it was written 200 years after the events that transpired. Ancient Romans may have recorded the supernova explosion too, but that’s less certain.
Ancient records of celestial events can also be uncertain because of confusion between supernovae and comets. In the Hou Han shu, there’s no record of the guest star moving, and the location the Chinese recorded agrees with the position of RCW 86, the debris ring from the SN. Modern astronomers are pretty sure the Hou Han shu recorded SN 185, especially since modern high-tech observations help confirm it.
SN 185 exploded more than 8,000 light-years away in the rough direction of our nearest stellar neighbour Alpha Centauri. It’s a fascinating object because astronomers can observe the aftermath of a supernova explosion, one of nature’s most climactic events. RCW 86 is just a tattered remnant of SN 185 now, an increasingly misshapen ring of gas and dust. SN 185 was a Type 1a supernova, and unlike other types of supernovae, it left nothing behind other than the expanding, dissipating ring of debris.
But astronomers didn’t know all that at first. They had to figure it all out, and RCW 86 was misleading because of its size.
Its large size led astronomers to believe that SN 185 was a core-collapse supernova. That type of supernova would take about 10,000 years to form the remnant we see today. So astronomers weren’t certain that RCW 86 was associated with SN 185. The timing was way off by over 8,000 years.
Then in 2006, a study showed that an extremely high expansion velocity was behind RCW 86, meaning it is temporally associated with SN 185. That study was based on x-ray observations. They showed that along some portions of the expanding shell, there was a peculiar mixture of both thermal x-ray radiation and synchrotron x-ray radiation. Simply put, thermal x-rays are generated by heat, and synchrotron x-rays are generated by movement. The presence of synchrotron x-rays suggests a much higher velocity in the shell since charged particles need to travel at relativistic speeds to produce them.
This study corrected RCW 86’s age to about 2,000 years old, right in line with SN 185. “Finally,” the authors of the 2006 paper wrote, “we show that the derived shock velocity strengthens the case that RCW 86 is the remnant of SN 185.”
But that didn’t explain why RCW 86 is expanding so fast. Once again, x-ray data led to an explanation. X-ray observations showed a higher-than-expected level of iron in the remnant shell. Type 1a supernovae produce an excessive amount of iron due to their physics. In fact, two-thirds of the iron in our blood and in the Earth itself was produced by type 1a supernovae. Since type 1a supernova can account for the increased iron, and since RCW 86 is expanding so rapidly, astronomers determined that it is indeed SN 185’s remnant.
A type 1a supernova consists of a binary pair, including a white dwarf and another star that could be anything from another smaller white dwarf to a giant star. As the two get close, the white dwarf siphons off material from the companion star. The white dwarf’s pressure and temperature both rise and the star ejects material at a high velocity. This material forms part of the expanding shell called RCS 86.
But unlike a main sequence star, which can expand and cool to compensate, the white dwarf keeps getting hotter until it eventually explodes. The previously ejected material created an empty shell around the white dwarf that made room for the material from the supernova explosion to expand. The result, 1800 years later, is the tattered, bedraggled ring of debris that we see today.
Of course, the ancients had no idea about any of this. They just witnessed a blazing light in the sky that shone for 8 months and then disappeared. Who knows what impact it had on regular people?
It’s fascinating when modern astronomy intersects with what the ancients saw. It’s like a one-way conversation between the past and the future. SN 185/RCW 86 is just one example of it.
A 2021 study examined ancient literature for 3,000 years of records of auroras to help understand Earth’s magnetosphere over time. A 2018 paper showed that a meteor explosion over the Dead Sea 3,700 years ago could explain the Biblical story of Sodom. There are lots of other examples.
Thanks to modern observing capabilities, we can untangle the complex physics behind things like supernovae and understand them in detail. The Dark Energy Camera’s wide-angle image makes it easy for us to relate to them. The aftermath is spread across our screens in intriguing detail.
If you want to go even deeper, download the full-size .tif from NOIRLab’s website.