Last updated March 28, 2018 at 2:01 pm
Happy accidental discovery surprises researchers with what it reveals.
Superimposed series of images obtained at the moment of discovery. The supernova becomes visible as a faint and rapidly brightening object to the South of the host galaxy, NGC 613. Credit: Víctor Buso and Gastón Folatelli
Imagine your surprise when playing with your new camera-phone you see a bright dot in the edge of your test post to instagram. Now imagine the chances of that bright dot being one of the most destructive events in the known Universe. You may well be surprised.
Amateur astronomer Victor Buso made that exact chance photograph as he tested his new camera on a 40-cm Newtonian telescope – discovering the earliest moments of an enormous supernovae explosion.
The odds of catching a supernova in an hour of stargazing are literally less than one in a million.
On 20 September 2016 Victor was observing the galaxy NGC613, over 86 million light years away, chosen simply because it happened to be overhead. Over the course of an hour and a half he took a series of photographs, one of which would propel him into astronomical folklore.
While he was watching, a star exploded, releasing as much energy as the Sun will over its entire ten billion year lifetime (approximately 1051 ergs of energy). His photographs allowed one of the most precise estimates ever made of an exact moment of eruption – a window of time just three hours long.
Through his photographs, astronomers could see a supernovae so soon after exploding that it is literally still brightening before their eyes.
An unexpected explosion
In the space of just 25 minutes the object doubled in brightness – a breakneck change in astronomical terms – which translates into a rate of one hundred million billion times increase in brightness per day (in astronomy terms this is a rate of 43 magnitudes per day).
Related: Alan Duffy’s Astronomy Glossary
The likely cause of such an incredibly rapid rise in brightness is a long-postulated ‘shock-breakout’. This is a rapid phase in the explosion when the star’s outer layers, nearly three and a half Sun’s worth of material, are exploded outward.
The driving force behind that is a titanic roiling mass of energy and exotic particles. Over the course of just minutes these begin to punch through the expanding, and thinning, revealing evermore of the explosive shock’s energy. Subtle differences between the shock-breakout model and the observations suggest that there may well be more to learn about the conditions of space around the supernova eruption too.
The almost immediate detection of the supernova meant that facilities around the world were able to turn and track the progress of the all important cooling peak of the explosion within a day of detection. From X-ray observations with the orbiting Swift spacecraft to ultraviolet and optical telescopes around the world, the supernova SN 2016gkg is now one of the best studied Type IIb supernovae of its kind.
The discovery that keeps surprising
However, the supernova still had some surprises for astronomers. Firstly, the star that caused this supernova isn’t alone, it’s a binary star system!
One possible model for SN 2016gkg is that the larger star was born with a mass 19.5 times that of our Sun, and a nearby companion was 13.5 times larger than the Sun. The larger star evolves more quickly than the smaller companion and over its rock-star lifetime of living fast and dying young, it expands outwards.
The more loosely held outer layers are stolen by the companion and dramatically reduce the main star until it is just 4.6 times the mass of our Sun at the moment that it finally runs out of fuel for fusion.
With no more fuel there is no energy to resist the inward collapse of the star’s own gravity and the layers above come crashing down. The core is compressed into an object of 1.6 times the Sun’s mass, potential as a Neutron Star just 20km across.
The shells above recoil from this super-dense object and fly outwards triggering a violent explosion in the rest of the star’s layers. When the supernovae finally fades away we may yet be able to the companion star, now bloated and overly-luminous through its interstellar theft.
It was those initial moments of that explosion are what, by chance, were photographed.
Astronomers everywhere will be thanking their lucky stars that he happened to choose that galaxy for a test photo.
The analysis of the supernova is published in Nature.
