Amateur astronomers have made some pretty amazing observations in the past, including asteroids striking Jupiter, a brand-new system of four Super-Earth exoplanets, and just recently, the rediscovery of a NASA satellite long thought lost. Now, an Argentinian amateur astronomer named Victor Buso is thanking his lucky stars, after capturing on camera the very moment a star went supernova.

Supernovae are pretty common across the sky, but as bright as they are, their unpredictability and transient nature means that the events are usually well underway by the time astronomers spot them. Sometimes they can be picked up early, or even predicted ahead of time thanks to the light-bending phenomenon of gravitational lensing, but these occasions remain very rare. As a result, what's actually happening during the early stages of a supernova are still mysterious.

In the early morning hours of September 20, 2016, Buso was observing the spiral galaxy NGC 613, testing out a new camera for his telescope. After examining a set of short-exposure photos he'd just taken, he noticed something strange on the outskirts of the galaxy – a spot of light that was clearly brightening. It hadn't been visible at all in earlier images of the night, but over the course of an hour or so, it steadily grew brighter.

The images were soon analyzed by astronomers at the Institute of Astrophysics of La Plata, who recognized the significance of the discovery and contacted astronomers around the world to train telescopes on that section of sky.

"Professional astronomers have long been searching for such an event," says Alex Filippenko, co-author of a study describing the rare occurrence. "Observations of stars in the first moments they begin exploding provide information that cannot be directly obtained in any other way. Buso's data are exceptional. This is an outstanding example of a partnership between amateur and professional astronomers."

The event officially came to be known as SN 2016gkg, and its afterglow was visible in the sky for about two months after the explosion. Continued observations and analysis of the original images helped astronomers learn a lot about the star that preceded the explosion, as well as the early stages of supernovae in general.

Filipenko's team used the Lick Observatory in California and the Keck Observatory in Hawaii to examine the spectra of light given off by the explosion, and was able to determine that the event could be classed as a Type IIb supernova. That means the supernova initially emitted hydrogen but that quickly ran out, with helium then becoming the predominant element.

This type of supernova usually means the exploding star has already thrown off much of its hydrogen envelope. Using theoretical models the team estimated that the host star would originally have had a mass about 20 times that of the Sun, but was likely down to only about five solar masses at the time of death. The rest of that mass may have been sucked away by a companion star, the researchers say.

As useful as the chance discovery was, it's incredibly hard to replicate. Buso just happened to be pointing his camera in exactly the right direction at the right time, and the researchers estimate his chances of winning this "cosmic lottery" could have been as low as one in 100 million.

The research was published in the journal Nature.

Source: UC Berkeley

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