Even in the 21st century, astronomers still turn to ancient texts to help them study supernovae, the explosive deaths of distant stars.
Ancient and medieval stargazers didn’t yet have enough information to fully understand what they saw when a bright new star appeared in the night sky, but many were keen observers. Their descriptions of when and where a “guest star” appeared in the sky can point the way to the cosmic debris left behind by a supernova – a cloud of expanding gas called a nebula. And by describing details like color and brightness, they offer clues about how giant stars collapse and how nebulae evolve over time.
One of the best-documented supernovae in history lit up the sky in 1006. People across the Northern Hemisphere, from Switzerland to China, described a bright new star that flared to life and then slowly faded over several months. Astronomers and historians recognized these stories as descriptions of a supernova – the brightest one ever seen from Earth. And based on those records, astronomers in 1965 pinpointed the cloud of gas left behind by the supernova of 1006; it’s about 7,100 light years away, which means that a star exploded around the time farming was introduced to Europe, and its light reached Earth just 60 years before the Battle of Hastings.
Astronomer Ralph Neuhaeuser and his colleagues recently found another detailed description of the 1006 supernova, buried in a lengthy philosophical tome by medieval scholar Ibn Sina (also known as Avicenna, because European academics of the day didn’t put much effort into spelling). Kitab al-Shifa (Book of Healing), first published in 1027, is a well-known commentary on various natural sciences. But Ibn Sina’s description of the 1006 supernova – buried in a chapter on meteorology – has been overlooked for the last millennium.
“It began to throw out sparks”
Here’s how Ibn Sina described his view – or possibly a view that someone else described to him; he doesn’t specify – of the supernova, from a vantage point in what is now Uzbekistan:
“It remained for close to three months, getting fainter and fainter until it disappeared; at the beginning it was towards a [darkness or faintness] and greenness, then it began to throw out sparks all the time, and then it became more and more whitish and then became fainter and disappeared.” Neuhaeuser and his colleagues say that “throwing out sparks” probably refers to scintillating or twinkling.
He mentions the astronomical event in passing; it’s tucked into the middle of a summary of what the even more ancient Greek philosopher Aristotle had said about transient celestial events – new lights that appear briefly in the night sky and then gradually fade away. These objects are usually either comets or supernovae. But Aristotle and Ibn Sina both claimed they were caused by evaporation in the Earth’s atmosphere; neither philosopher could have guessed “exploding giant balls of plasma many light years away.”
We can’t really blame them. Sometimes reality is wilder than anything we could have imagined.
From Aristotle to standard candles
Ibn Sina described the 1006 supernova’s colors in more detail than any other historical source (that we know about today). According to Neuhaeuser and his colleagues, those seemingly small details could reveal more about the mechanics of the explosion that added a new star to Ibn Sina’s night sky in 1006. And that information, in turn, could eventually help physicists fine-tune their models of the most powerful supernovae.
When astronomers found the nebula SNR 1006 (short for SuperNova Remnant 1006, because astronomical naming prioritizes specificity over creativity), the most interesting part of the discovery may have been what they didn't find: there was no trace of a black hole or a neutron star (the small, incredibly dense remains of a collapsed stellar core, which is all that's left when a very massive star goes supernova). That suggested that the supernova of 1006 had probably been the most powerful type, a type Ia. A type Ia supernova is an explosion powerful enough to vaporize even the collapsed core of the star, leaving behind nothing but a cloud of plasma.
Type Ia supernovae happen when a type of star called a white dwarf — which is, itself, the collapsed remnant of a more average-sized star like our Sun and most of the others in our galaxy — finally pulls in, or accretes, enough mass to collapse even further. Usually the white dwarf accretes this extra mass from its partner in a binary star system.
Because white dwarf stars all explode at the same mass — called the Chandrasekhar limit — type Ia supernovae all burn with the same brightness; some type Ia supernovae only appear dimmer than others to us because they're further away. Because astrophysicists know this, they can use the apparent brightness of a Type Ia supernova to gauge its distance, which is a handy way of measuring how far away another galaxy is. Understanding these powerful stellar explosions better is, therefore, pretty useful.
Of course, that usefulness assumes that Neuhaeuser and his colleagues are correct about what Ibn Sina is describing, and also that his description was accurate. The color he observed may have been the effect of seeing the supernova filtered through layers of atmosphere – similar to the effect that causes the Moon to sometimes appear very orange when it’s low in the sky – as Louisiana State University astronomer Brad Schaefer told National Geographic.
That’s one of the challenges of trying to extract astronomical data from ancient texts, but it’s usually worth modern astronomers’ time to make the effort.
An ancient typo?
Ibn Sina’s eyewitness (or secondhand) supernova account may have escaped notice for nearly a thousand years simply because he buried it in the middle of a discussion of ancient Greek thoughts on natural sciences.
And for centuries, most readers have assumed that the passage referred to a comet, not a supernova.
“The term ‘comet’ was, in former times, used for several kinds of transient objects, including what we today call comets, novae, and supernovae,” wrote Neuhaeuser and his colleagues in their paper, which is currently in press at the journal Astronomical Notes.
There’s also the fact that, according to Neuhaeuser and his colleagues, Ibn Sina got the date wrong.
He wrote Kitab al-Shifa between 1013 and 1023, starting several years after the supernova of 1006 had faded from the night sky. That makes it likely that he was working either from memory or from several-years-old notes. When describing the “star among stars” that appeared in the night sky over what is now Uzbekistan, Ibn Sina writes that it was visible for about three months during spring of the year 397h of the Lunar Hijri calendar, which counts years from the date of the Muslim prophet Mohammed’s migration from Mecca to Medina (the Hijra).
In the more commonly used Gregorian calendar, the spring of 317h would be the spring of 1007 – a year after the supernova, according to every other medieval source. And there’s no record of any astronomical event – comet, supernova, weird glowing atmospheric evaporation, or otherwise – happening in the spring of 1007. Neuhaeuser and his colleagues suggest that 317h may have been a typo, a mistake in the great philosopher’s memory, or a transcription error. But in any case, it may have contributed to the reference being overlooked for so long, even by historians who search for evidence of astronomical events in the ancient world.
We’re all lined up for front-row seats
No one on Earth has seen a supernova with our own eyes since 1987, which is the last time a star exploded close enough to Earth to be visible without the aid of telescopes. (Technically, 1987 was about 168,000 years after the star, a blue giant in the Large Magellanic Cloud, actually exploded; cosmic news only travels at the speed of light.) But we never know when the next one might happen.
And that means we may one day get the same opportunity as Ibn Sina and his contemporaries. Centuries from now, astronomers may be poring over our diaries or social media posts about eclipses – or maybe Betelgeuse finally exploding – to extract some key bit of data to refine a model that doesn’t even exist today.
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