On this day in 1181, observers in China and Japan looked up at the night sky and saw an unusually bright star in the area we describe as the W-shaped constellation Cassiopeia. They didn’t realize it at the time, but they were witnessing the explosive death of a start 20 or 30 times more massive than our Sun, which had burned up all its inner fuel and collapsed under its own crushing gravity, triggering a cosmic explosion called a supernova. Today, we call that event SN 1181, but to 12th-century Chinese and Japanese observers, it was a “guest star,” a star that appeared for a brief visit in one area of the sky and then went away again.
A Japanese history written in the mid-1200s relates that, “A guest star was seen in the northeast. It was like Saturn [translations differ on whether this refers to size or color] and its color was bluish-red, and it had rays. There had been no other example since that appearing in the 3rd year of Kanko .” The visitor stuck around for about 6 months before fading from view again.
The guest star of 1181 paid its visit during a period when astrology and astronomy were still lumped together in the same field of study — not just in Asia, but in Europe, Africa, and the Middle East as well. The appearance of a new star in the sky meant something, even if astronomers couldn’t agree on exactly what. Fujiwara Kanezane, then a young member of the Japanese imperial court, assumed the worst; in mid-September 1181, he wrote in his diary, “On the 28th day it was heard that since the 25th day a guest star had been present in the inner sky, a sign of abnormality indicating that at any moment we can expect control of the administration to be lost.”
But fellow courtier Yoshia Tsunefusa was more optimistic in his diary: “The 25th day keng-shen of the eighth month [Sept. 26, 1181]. Today was the occasion for making auspicious offerings for a good harvest. A guest star had been seen in the NNE and at the present time it still has not faded away.” Those were just three of a multitude of diaries and official histories from across China (then split into two kingdoms) and Japan which mentioned the astronomical event. Most described the position of the guest star in relation to more familiar stars and constellations, measured the length of its stay, and occasionally speculated on its astrological implications. In fact, it’s a little surprising that no sources from elsewhere in the world mentioned the supernova of 1181, since it marked just one of eight supernovae in recorded history visible to unaided Earthling eyes.
Such accounts offer astronomers a puzzle — something like a cosmic forensics case. A supernova leaves behind an expanding cloud of gas called a nebula, the tattered, blazing remnants of a dead star. Astronomers’ challenge is to match those centuries-old diaries and historical accounts to an actual nebula, then work backward from the nebula’s properties to reconstruct the size and type of the former star. This merger of history and astrophysics is a bit like the cosmic equivalent of searching the seafloor for a shipwreck based on historical accounts of where a ship sank, only the scale involved is many orders of magnitude larger.
Fortunately for modern astronomers, their medieval Chinese and Japanese colleagues didn’t leave behind the kinds of astronomical coordinates we use to describe an object’s position in the sky today, but they did describe where the guest star appeared in relation to five different groups of stars.
“A guest star (k’o hsing) appeared in K’uei hsiu and invading Ch’uan-she until the day keui-yu of the first month of the following year [Feb. 6, 1182], altogether 185 days; only then was it extinguished,” reports the 1345 History of the Sung Dynasty. Other Chinese histories describe the supernova appearing in the night sky “at the North near Wang-liang and guarding Ch’uan-she,” or “”at Hua-kai altogether for 156 days; then it was extinguished.”
And using that information, along with powerful modern telescopes like NASA’s Chandra X-ray observatory, modern astronomers are pretty sure they’ve identified the remains of the star whose death throes lit the night sky in 1181 — although there’s still some debate about whether they are, in fact, looking at the right cosmic corpse. The focus is on a hot, radio-emitting cloud of gas about 6,500 light years away in the Perseus Arm of our Milky Way Galaxy; it lies in the direction of the constellation Cassiopeia, with a pulsar (the burned-out, rapidly rotating core of a dead star) at its center spinning once every 65.68 milliseconds. Astronomer F.R. Stephenson first pointed to the eloquently-named SNR 3C 58 as the possible remnants of the 1181 guest star back in 1971, and astronomers have spent the last 47 years weighing the evidence, gathering new data, and raising more questions.
At first, some astronomers pointed out that SNR 3C 58 was too far away from Earth to be the site of the 1181 supernova. The first distance estimates for the supernova put it at a whopping 27,000 light years ago, and at that distance, the pulsar at the nebula’s center would have had to pump out an impossible amount of energy in order to produce the types and quantities of radiation we observe from Earth. That estimate was based on observations of the velocity of some gas clouds between Earth and the distant nebula; by using the Doppler Effect to measure how fast the clouds move in relation to us, astronomers could also calculate how quickly they orbit the galactic center, which in turn let them calculate distance. But in 2013, a new set of calculations and observations corrected for distortion caused by the shape and position of the Perseus Arm, and astronomer Roland Kothes estimated the actual distance to SNR 3C 58 at around 6,500 light years.
That’s a much more realistic scenario, but it also means the 1181 supernova was a pretty weak blast — relatively speaking, anyway. We’re still talking about an exploding star 20 to 30 times bigger than the Sun, after all. But at its brightest, observers describe it being about as bright as the star Vega, which is the 5th brightest star visible in the night sky. Meanwhile, all of the other four visible-with-the-naked-eye supernovae that have occurred since the year 1000 outshone every star in the sky. If you’re wondering, those supernovae appeared in 1604, 1572, 1054, and 1006; the 1006 event is still the brightest supernova ever recorded.
Astronomers have calculated that the star that exploded in about 5320 BCE, whose final flare of light traveled across the vastness of space to reach Earth in 1181, was a blue star with a surface temperature between 30,000 and 50,000 Kelvin, somewhere between 40,000 and 1 million times brighter than the Sun. Assuming we’re looking at the right nebula, that is. There’s still some uncertainty on that point, because some astronomers say that SNR 3C 58 looks a little too old to be the 1181 guest star. When a star explodes, the blast forces gas outward at incredible velocities, which gradually slow as the nebula ages and loses energy. And radio astronomy observations spanning more than twenty years estimate that the gas cloud in SNR 3C 58 is expanding more slowly than astronomers would expect in such a recent nebula. In fact, it looks like the nebula may be a few thousand years too old to have lit up the skies in 1181.
Other astronomers disagree with that assessment, though, offering alternate explanations for the rapidly-slowing gas cloud. And so far astronomers haven’t found anything else in that region of space that could plausibly be remnants of the 1181 supernova. In a cosmic cold case, it may be the best suspect we have.