Imagine looking up at the sky one night and finding two moons. If it happened in 2017, Twitter would be abuzz with people posting photos. News channels would get astronomers to explain what’s happening, and they’d say it’s not a supernatural phenomenon but likely an exploding star—a supernova. Within hours, telescopes would have nailed down the exact star that suffered the dreadful fate. And then, likely for weeks to come, you’d be able to enjoy the presence of a very, very, very bright star in the sky.
Now imagine seeing the same sight 5,000 years ago. Nobody in your tribe has any clue why there’s suddenly an extra super-bright object in the night sky. There are no records, written or pictorial, to consult. However, curious as you and your tribemates might be, you aren’t going to risk asking someone in the rival tribe nearby. All you could do is wonder about the oddity—and perhaps try to represent it through your favorite artistic medium.
Scientists say this is likely what happened back in 3600 BC. Astrophysicist Mayank Vahia and his colleagues at the Tata Institute of Fundamental Research believe a rock painting found in what is today part of the Kashmir region of south Asia is the oldest record of a supernova and likely the oldest sky chart ever drawn. The artwork shows two bright objects in the sky, with figures of animals and humans underneath. A study detailing the discovery has been published in the Indian Journal of History of Science (pdf). (Vahia also spoke about the discovery for the podcast The Intersection.)
Vahia began the study by taking many steps backwards. Rock art is difficult to date with precision, but Vahia had a solid starting point. The rock was buried in a wall (though hidden from view of residents) of a house that had already been dated to around 2100 BC. The oldest known settlement in the region was founded around 4100 BC. So the rock art is likely to have been made sometime between those two millennia—then inadvertently used to construct a new dwelling.
Next, Vahia needed to understand why someone would draw two bright objects in the sky. It couldn’t be two suns, because we have and have always had only one. It couldn’t be the sun and the moon, because although it’s possible to see both solar objects in the sky at the same time, a full moon can never appear so close to the sun. (From Earth, we see the moon as “full” when it’s on the direct opposite side the planet as the sun.) The only remaining explanation, Vahia figured, was a supernova: if one exploded relatively nearby our solar system (hundreds or few thousands of light years away), it could shine as bright as the sun or the moon.
Of course, this explanation only makes sense if there actually was a supernova bright enough to have been visible on Earth between 4100 BC and 2100 BC. The good news was that Vahia had a way to accurately identify many of supernovas of the past thousands of years.
When a supernova explodes, it releases a lot of energy. The energy we can see with the naked eye—that is, visible light—is only a small fraction of what the explosion produces. The supernova continues to emit high-intensity X-rays for hundreds and thousands of years. Astronomers have been able to track down these supernova remnants and calculate when and how big the stellar explosion would have likely been.
With all constraints set, the database gave Vahia just one option: supernova HB9. It seemed to have all the right characteristics. Its explosion would have been visible on Earth around 3600 BC. At the time of its explosion, it would have appeared to Earthlings as a glowing ball—though not perfectly round—and just a little less bright than a full moon (because it was only about 2,600 light years away).
There’s even better proof to be found when you look more closely at the artwork. The figures underneath the supernova and the moon on the rock painting aren’t part of a hunting scene, as it might seem at first glance. Instead, Vahia’s analysis shows they neatly fit the constellations that surrounded the supernova: The man with the bow and arrow on the left is Orion; the stag is Taurus; the man on the right holding a spear is part of Pisces; and the dog is the Andromeda galaxy. In other words, the rock art is likely a sky chart and, if it is, it would be the oldest sky chart on record.
There is just one problem. Working with the Indira Gandhi National Centre for the Arts, Vahia has studied many more pieces of rock art from the region, but couldn’t find any other sky charts. Though the rock art analyzed here fits quite well with what the sky might have looked like back then, it could also be just a big coincidence. To prove it’s not, Vahia would need a second example. If the people in the region drew a star chart once, they must have drawn it many more times for other kinds of celestial events (such as comets passing or meteor showers).
That is why, on its own, Vahia’s rock painting isn’t enough to definitively prove itself to be the oldest human-made star chart and supernova record. Still, Vahia is confident that as more rock art emerges from the region, he will find the additional evidence needed to solidify the claim.