CTIO / NOIRLab / DOE / NSF
In early December 185 CE, Chinese astronomers recorded a bright “guest star” in the night sky that shone for eight months in the direction of Alpha Centauri before disappearing — likely the first recorded supernova in the historical record. The image above gives us a rare insight into the whole still in tatters of that long-ago explosion, captured by the Dark Energy Camera (DECam), mounted on a four-meter telescope at the Cerro Tololo Pan-American Observatory in the Andes mountains of Chile. . DECam has been in operation since 2012, and although it was originally designed to be part of the ongoing Dark Energy Investigation Program, it is also available for other astronomers to use in their research. This new, broad perspective on the SN 185 remnant should help astronomers learn more about stellar evolution.
As we’ve written before, there are two types of known supernovae, depending on the mass of the star from which they originated. An iron-core supernova explodes with massive stars (more than 10 solar masses), and it collapses so violently that it causes a huge cataclysmic explosion. The temperatures and pressures get so high that the carbon in the star’s core fuses together. This prevents the nucleus from collapsing, at least temporarily, and this process continues, over and over, with heavier and heavier atomic nuclei. When it runs out of fuel completely, the iron core (at that time) collapses into a black hole or neutron star.
Then there is a Type Ia supernova. Smaller stars (up to about eight solar masses) gradually cool into dense ash cores called white dwarfs. If a nuclear-starved white dwarf is part of a binary system, it can siphon matter from its partner, adding mass until its core reaches temperatures high enough for carbon fusion to occur. They are the brightest of all supernovae, and they also shine with remarkably consistent peak brightness, making them invaluable to astronomers for determining “standard candles” for cosmic distances.
There are few valuable details about the SN185 available on file Later Han book, except that it was “about the size of a bamboo rug” and “displayed various colours, pleasing or otherwise”. Astronomers have suspected a possible connection between SN 185 and the remnant structure dubbed RCW 86, but have long assumed that the event that formed RCW 86 was a core implosion supernova, which would take about 10,000 years for the remnant to reach its current shape.
In 2006, new X-ray data collected by the European Space Agency’s XMM-Newton Observatory and NASA’s Chandra X-Ray Observatory showed that RCW 86 is much younger than previously thought: about 2,000 years old. The authors were able to calculate how quickly the shock wave spread in RCW 86. They found that there are denser places where the shock wave travels more slowly, misleading astronomers into thinking the rest is older than it is now. But there are other areas where the shock wave is still inside the bubble – and continues to expand rapidly – that provide a more accurate estimate of RCW 86’s age.
The new age estimate significantly strengthened the case that RCW 86 is the remnant of SN 185. In this case, SN 185 was most likely a Type Ia supernova, a conclusion based in part on the discovery of a significant amount of iron in the remnant. A white dwarf star devouring its partner in a binary system produces high-speed winds that push gas and dust out and create a cavity before the white dwarf explodes. This allowed all of this debris to expand outward at great speed to create the impressive jagged structure it exists today.
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