One September evening amateur astronomer Victor Buso hauled his 40-centimeter telescope up to his rooftop observatory in Rosario, Argentina, and pointed it at a relatively nearby galaxy called NGC 613. He says he chose it at random, remarking that “among the ones that were in that part of the sky, NGC 613 has a beautiful shape with ringlets of bright and dark clouds.”
Photographing a galaxy requires a lot of light-gathering; Buso spent an hour and a half braving city lights to create an exposure of the spiral galaxy and its twisting arms. A complete astronomical picture is often built from a series of images composited together, and Buso’s first 40 images showed nothing extraordinary.
After a 45-minute break he resumed his observations—and that’s when he captured something big, brightening by the minute. A star that had long been siphoning mass from its binary sibling had collapsed, and as the outer layers of the star’s atmosphere were stripped away it created a giant shock wave. Buso captured the event as its light reached Earth on the night of September 20, 2016.
He sent notices to members of the astronomy community, who immediately jumped into action. Buso had caught the very first stages of a type IIb supernova, in which two stars eight to 10 times the mass of our sun get locked in a deadly gravitational embrace until one of them explodes. Word spread quickly, and getting follow-up observations became a worldwide effort, with Australian observers picking up where others had left off. Melina Bersten and colleague Gastón Folatelli at the Institute of Astrophysics La Plata chatted with Buso by Skype the next day to begin their own observations from their perch an hour’s drive away from Rosario. “The sky is very bright, and he was lucky that this night was clear with a good sky,” Folatelli says.
The initial results of that collaboration were published Wednesday in Nature. It is the first time the initial shock wave from this kind of supernova has been captured in time to allow astronomers to get a thorough peek into the first few hours of these powerful stellar events. (NASA’s Kepler telescope captured the official first flash of a similar supernova in 2016, but that event’s distance and timing made follow-ups difficult.) Buso had earlier identified a new variable star and a previously undiscovered asteroid, “but discovering and registering such an early supernova, the earliest one in the history of mankind until now, is something that moves me very deeply,” he wrote via e-mail.
Bersten, who is lead author of the new paper, says the supernova—called SN 2016gkg—is now calming down enough for astronomers to hunt for the remains of its sibling star. A nebula—a glowing shell of ionized gas and dust produced by the explosion—should soon cool and fade to relative transparency, allowing the battered remaining star to be seen. “We expect it to be there, more or less in the same location that the supernova was,” Berstem says. “The only thing we need to do is wait a bit for the supernova to be [settled] enough.”
Fifty stars are thought to explode every second in the universe but most are too distant to be seen from Earth, according to astrophysicist Brad Tucker of The Australian National University, who studies supernovae. Others, Tucker says, go unspotted because they are so close: Some of those are blotted out by interstellar dust and others are too glaring for professional observatory telescopes to focus on; they only appear as bloblike patches of brightness. Many large observatories are designed for more distant targets or those whose light appears extremely faint from Earth. So a more energetic event might be more like a flashlight shining directly into one’s eye, rather than turning on a lamp in the astronomical living room.
These factors leave amateur astronomers uniquely suited to capture relatively clear supernova. Their telescopes and imaging equipment are more attuned to brighter objects, and they tend to focus on just a few targets over the course of a night rather than a large-observatory survey that monitors the entire night sky. Tucker leads a project called Supernova Sighting on Zooniverse.org, a citizen science Web site, to enlist amateurs for this very reason. “Because amateurs can focus on observing just a few galaxies every night, if they’re going to catch something, they’re going to catch it quickly,” Tucker says.
Sometimes a professional observatory may discover an emerging supernova, and then turn to amateurs to help gather more much-needed data. Some amateur astronomers might use only the optical wavelengths human eyes can see whereas others may try to see it in other frequencies such as infrared. Still some set up their own spectrometers—sensors that measure chemicals traces given off by a star or other object. “They don’t discover as many [supernovae] as the professional surveys,” Tucker says. “But usually when they discover something, it’s way more important than what we discover.”
The Main Event
A type IIb is not the rarest form, as far as supernovae go. A few have been spotted over the past few decades; most stars are born in pairs, and theirs is not always a symbiotic, give-and-take relationship. What is rare, though, is to catch one early enough to watch its very first moments, when the more massive star’s core collapses into a dense neutron star or black hole and strips away the soon-to-be-supernova’s hydrogen envelope in what astronomers call a “shock breakout.”
SN 2016gkg is a standard type IIb supernova with little difference from what is usually seen in the aftermath, “which makes this early detection even more useful,” says stellar astrophysicist Ashley Pagnotta of the College of Charleston, who was not involved in the new paper. By capturing the first few moments, astronomers can fit observations to predicted models of how a supernova evolves. If a supernova is a book, we usually witness just the middle and end. Now astronomers have a beginning. “You have this initial collapse of the core on the inside, so as it hits and bounces back out, you get the shock from that,” Pagnotta says. “We don’t know a whole lot about [the shock], partly because we don’t typically see it. The key is that because it’s this initial shock escaping from the core, it’s extremely fast and you get this very fast rise in brightness.”
The shock breakout takes place over a timescale of hours, and capturing as much of it as possible is critical to understanding these beginning phases. This initial event is explosive, in this case likely leaving behind a neutron star from the exploding star, according to Bersten.
The latest captured event, like most supernovae, did not stay bright for long. “It’s going down and it has become really, really faint now—hundreds or thousands times fainter than when it was discovered,” says Folatelli, who was a co-author of the paper. Upcoming Hubble Space Telescope observations could reveal what’s left of SN 2016gkg, picking apart the remaining nebula for signs of the binary star speeding off into parts unknown.
The next supernova of this type may be spotted by an instrument like the Zwicky Transient Facility at the Palomar Observatory, an automated telescope designed just for the purpose of monitoring the sky for short-lived events. Or maybe a lucky observer (professional or amateur) will serendipitously stumble across the next occurrence. But whoever catches it—and wherever it is caught—amateurs could well prove essential to gathering some of the more nuanced data of the big stellar boom. Especially if happens in our own galaxy.
“If [the Milky Way has] the next visible galactic supernova, none of the professional telescopes will be able to see it; it’s too bright,” Pagnotta says. “We will be, especially in the early days, almost entirely reliant on the amateurs.”