When Victor Buso was photographing the night sky in September 2016, he had a one in 10 million chance that he’d capture a supernova. The universe made sure he took it.
An artist’s impression of the star KSN 2011d going supernova. Credit: NASA Ames, STScI/G. Bacon
When a heavy star burns out with a giant explosion, it’s a supernova. Astronomers have observed supernovae but haven’t caught many of them in the moment of their birth itself. More specifically, we don’t have any records of the first few hours of a type IIb supernova explosion – where the progenitor star first loses a large part of its outer layers before blowing up.
Fortune amended this gap in 2016.
On a calm and clear night in September that year, Victor Buso, an amateur astronomer and astrophotographer, was testing a camera he had mounted on his telescope in Rosario, Argentina. He pointed his setup at NGC 613, a spiral galaxy over 65 million lightyears away, and took a series of pictures. Each short had 20 seconds of exposure over 90 minutes, from 1:30 am to 3 am local time. Buso didn’t realise what he’d managed to get on camera until later: the first ever visible-light images of a supernova shockwave.
The first 20 minutes of Buso’s observations show a quiet sky to the galaxy’s south. After 45 minutes – and some 40 photos into Buso’s sequence – a supernova erupts into life. It can be seen getting bigger and brighter as the sequence continues, with maximum brightness nearly tripling in the last few pictures. This is the first time a supernova has been photographed in such quick succession.
In the middle of his shoot, Buso had noticed that a new spot of light had appeared in his pictures that kept getting brighter. He realised he could be looking at a developing event and attempted to contact the Astronomical Observatory of Córdoba.
An animation showing what Buso was able to capture. The supernova is visible towards the bottom-right. Credit: Víctor Buso and Gastón Folatelli
When that didn’t work – possibly because it was the middle of the night – he called Sebastián Otero of the American Association of Variable Star Observers. The two put out an announcement soon. Otero also reported the bright spot to the Transient Name Server, the official body for reporting temporary events, or transients, with short lifespans. Soon after, the All Sky Automated Survey for SuperNovae, an automated program with 20 telescopes around the world searching for supernovae, and the Asteroid Terrestrial-impact Last Alert System telescope, Hawaii, looking for moving objects in the sky, confirmed the event for what it was: a type IIb supernova.
It was designated SN 2016gkg. The detection was announced immediately and a large-scale monitoring effort began the next day, September 21, 2016. It was observed in the visible, X-ray and ultraviolet parts of the spectrum, and quickly became one of the most-studied supernovae in history.
The wave of death
Buso’s photos contained a particularly unique gem, an astrophysics Arkenstone, if you will: the signs of a shock breakout. In the last few moments of their lives, stars weighing 10x the Sun collapse under their own weight. This happens because the star has run out of lighter elements to fuse and starts trying to fuse iron. However, unlike lighter elements, iron fusion consumes more energy than it produces, forcing the star to eat itself.
The result is a shockwave from the core that ripples out through the star’s body, heating everything up by millions of degrees, eventually reaching the surface in a blinding flash of energy. “The shock breakout phase is like the first flash of light from when you light a matchstick,” Parshati Patel, an education and outreach coordinator at the Centre for Planetary Science and Exploration, Western University, Canada, told The Wire. “It is a wave of death starting from the core and reaching the surface.”
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Astronomers have expected shock breakouts to occur because their theories and computer models predicted it – but they weren’t observed until recently. In 2008, Kevin Schawinksi of the University of Oxford and his team observed the shockwave’s precursor (a UV emission) followed by the shockwave itself shooting through SNLS-04D2dc, a supernova about 2.5 billion lightyears away.
In 2016, the Kepler space telescope caught the first shock breakout of a type II supernova in visible light. Buso’s pictures are the second such set of recordings, and the first of a type IIb supernova. “What is new here is that we have seen the explosion within a few hours of it occurring, meaning we have been able to watch the shock breakout as it happens,” said Alan Duffy, an astrophysicist and associate professor at Swinburne University, Australia.
Buso’s album is all the more remarkable because supernovae are unpredictable. The only reason humans have never been able to regularly see the first few seconds of one is because we don’t know where to look. By the time we do, the star has been dead for hours, even days. Besides, shock breakouts occur for up to ~20 minutes only. The entire explosion itself lasts about three hours before the supernova begins cooling. “The images really are a big deal. It is one of the first times we have seen these moments,” Duffy said.
Everyone can look up at the night sky, fewer still have a camera with them. But the subset of them that go on to capture the birth of a supernova as well as its shock breakout is astonishingly smaller. The authors of a paper published (paywall) in Nature this week say the odds are at most one in 10 million. According to Patel, “A lot of the recent imaging of supernovae have missed the three-hour mark. The key to understanding these processes and improve our theories is always early detection. So these photographs provide a lot of value.”
Stellar gluttony
We’ve seen a few supernovae progenitors just before they exploded, and we’ve currently identified about 30 that could explode soon. After comparing Buso’s images, data acquired by telescopes and archived shots of the same patch of sky, astronomers have concluded that the progenitor is part of a binary star-system.
One guess for how this supernova happened, per the Nature paper’s authors, is that there were originally two stars weighing 19.5x and 13.5x the Sun. As the heavier star became bigger and brighter, its outer layers expanded into the sphere of gravitational influence of the second star. The second star then siphoned in these layers unto itself, reducing the first star to 4.6 solar masses. At this point, the star was light enough for its 1.6-solar-mass core to run out of fuel and trigger a collapse.
To find out whether they’ve guessed right, we wait for the explosion to subside and then observe the companion star, still young but now luminous and engorged from all the material it stole. Astrophysicists have predicted the result of its supernova will give birth to a neutron star about 20 km wide – although when is not clear.
Sandhya Ramesh is a science writer focusing on astronomy and earth science.
