Behold, the Death of a Star—and the Birth of an Extra-Galactic Cow
New evidence suggests that a mysterious supernova known as “The Cow” may offer a rare glimpse into the creation of a black hole or neutron star.
AT2018cow, an astronomical transient nicknamed "the Cow," is believed to be a supernova that may have given rise to a newborn black hole or neutron star. Here, the Cow is pictured a couple months after its initial discovery in an image taken by the W.M. Keck Observatory in Hawaii. Image Credit: Raffaella Margutti, Northwestern University
Last summer, a dazzling burst of light shook the skies. The explosion, which erupted with the intensity of a newly-ignited camera flashbulb, was neither flare nor firework, but a brilliant, blue-tinged cow.
AT2018cow, to be exact: a mysterious, brain-bendingly powerful supernova, or exploding star, first spotted through a pair of telescopes in Hawaii in June of 2018. The supernova—affectionately nicknamed the Cow—quickly asserted itself as a serious astronomical anomaly. While most stellar eruptions take weeks to reach maximum luminosity, within days of its discovery, the Cow was shining 10 to 100 times brighter than garden-variety supernovae. Then, it rapidly began to disappear. The Cow’s dizzying, ephemeral brightness immediately sparked a fervor among researchers worldwide, several of whom quickly turned their attention to studying the origins of the fast-fading astronomical anomaly.
Raffaella Margutti, an astrophysicist at Northwestern University, was one of many immediately drawn to—and baffled by—the Cow’s enigmatic lure. “We had no idea what we were looking at,” she says.
Today, at the 233rd meeting of the American Astronomical Society in Seattle, an international team of scientists led by Margutti brings us one step closer to pulling back the curtain on the Cow. According to the researchers, the Cow’s spectacular flame-out may offer a rare glimpse into the creation of a black hole or neutron star, born out of the ashes of a massive star’s untimely end.
“They’ve done a really rigorous job here,” says Emily Levesque, an astrophysicist at the University of Washington who was not involved in the study. “I would say it’s our most complete picture yet of this phenomenon.”
When stars die, they tend to go out not with a whimper, but a bang. Stars run on a finite supply of internal fuel, the lifespan of which dictates the longevity of the star. Once it’s burned through its mojo, a star’s core can rapidly collapse inward like a deflating balloon. The end-of-life implosion can then set off a catastrophic chain reaction that literally blows the star’s outer layers into space in a bright, explosive flare, or supernova—a staggering cataclysm that sends shockwaves and stellar matter ricocheting through space, shaking up nearby clouds of gas and birthing the fodder for new suns and galaxies.
But left in the wake of this violent exit is a spent heart—the energetic core that once powered the star’s existence—that’s capable of seeding a very different sort of cosmic life: a petite, tightly-packed neutron star or the ravenous, crushing maw of a black hole.
The machinations behind the creation of these stellar souvenirs, often spotted only by chance, have puzzled stargazers for centuries. Which makes the chance to study stellar death in the act an unmissable opportunity.
AT2018cow is thought to be about 200 million light-years away, in Galaxy CGCG 137-068 in the constellation Hercules. Image Credit: Sloan Digital Sky Survey, Wikimedia Commons
But the Cow’s suite of quirks made it difficult to wrangle from the start. Its eruptive entrance indicated that it was probably some kind of supernova—but one unlike anything that had ever been characterized before.
The Cow’s strange combination of speed and brightness simply didn’t fit neatly into any preconceived notions of stellar death, Margutti explains. Which mean the team had to engage in some spur-of-the-moment sleuthing—and a lot of trial and error. “We went back to the drawing board many times,” she says. “We kept wondering if it was possibility A or B—but it seemed nature had produced possibility C.”
Typical supernovae are powered by the radioactive decay of an isotope of nickel following a star’s inward collapse, explains study author Brian Metzger, an astrophysicist at Columbia University. But the Cow’s brilliance far exceeded anything that nickel could muster on its own. “We immediately knew this supernova couldn’t be powered the way normal supernova were powered,” Metzger explains. In other words, at the heart of the Cow lay an entirely different kind of animal.
The incongruity left many astronomers scratching their heads: There was nothing to explain this strange surge of energy—at least, not at first glance.
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But Margutti’s specialty has always been seeing the unseen. When the Cow first appeared, the easiest data to collect had been wavelengths in the optical range of radiation, or the spectrum visible to the naked eye. So Margutti simply expanded this lens, collecting data with wavelengths much shorter (x-rays) or much longer (radio) than those we can see—the electromagnetic equivalent of upgrading a film from full screen to widescreen.
With this strategy, Margutti’s team was able to capture traces of the explosion long after it had faded from view, and home in on an invisible source of power: Spewing from the Cow was a powerful, persistent stream of x-rays. Which indicated that at the core of this fatal explosion was something emitting a dense, highly compacted wellspring of energy—something that might just be a newborn black hole, or a particular type of rapidly spinning neutron star called a magnetar, which boasts a magnetic field over a thousand trillion times stronger than Earth’s.
For Margutti, this was an astounding find. Oftentimes, the births of neutron stars or black holes are almost impossible to see. The ostentatious nature of supernova deaths prompts stars to jettison their sparkly guts into space—and this stellar shrapnel tends to stick around, swallowing most the light energy emitted from the remaining stellar core. With so few signals traversing the millions of lightyears that typically separate moribund stars from Earth, many neutron stars and black holes emerge unobserved. It can take years for the debris to dissipate; by then, it’s too late—and there’s no rewinding the cosmic clock.
AT2018cow was first observed in June of 2018. It was immediately noted as an astronomical anomaly, reaching a peak brightness 10 to 100 times brighter than a typical supernova within just days of its appearance, whereas the average stellar explosion typically takes weeks to rise and fade. Image Credit: Sloan Digital Sky Survey, Wikimedia Commons
The Cow, however, was equipped with an ideal physique. The explosion had resulted in a relatively low amount of debris, Metzger explains, meaning there was less haze blocking Earthbound waves of light; a more substantial cosmic belch would probably have clouded the Cow entirely from view. The eruption also had an aspherical shape, allowing high-energy x-rays to shine through gaps in the star’s gathering shroud like sparse, burning beacons. “It was like we were seeing directly into the heart of the beast,” Metzger says.
The Cow also happened to be in the right place at the right time. Estimates place the Cow’s eruption about 200 million light-years away. While that distance is firmly extra-galactic, by astronomical standards, it’s more or less right around the corner.
Other theories on the Cow’s contents abound, but for now, Margutti thinks neutron stars and black holes appear to be coming out ahead. Distinguishing between the two, however, will likely be challenging. While both emerge from the chaos of stellar explosions, researchers still don’t fully understand the factors that dictate the evolution of these distinct phenomena. “Right now, we can’t say, ‘If you’re born like this, you’re going to die like that, and you’ll form this kind of black hole or this kind of neutron star,’” Margutti says. “This is one of the biggest puzzle pieces missing in our understanding of how stars live and die.”
Part of the issue is that there’s no single recipe for stellar death—not unlike the case with Earth’s organisms. Each end to a star is a uniquely cataclysmic event, brought on by a different set of factors ranging from its chemical makeup to its surrounding environment. But every once in a while, something particularly unusual comes along—a rare disease that fells a star in an unprecedented way. In a way, the circumstances of the Cow’s demise are akin to the foreignness of an exotic illness: poorly understood, but all the more worthy of attention.
“We were never expecting this, but that’s why it’s so fascinating,” says Carla Fröhlich, an astrophysicist at North Carolina State University who was not involved in the study. “It’s like discovering a new species. You generally know there are things out there that you haven’t discovered yet—but it’s hard to come up with a theory for something you don’t know you need a theory for.”
After the Cow's name was announced, a flurry of jokes swept through the astronomical community; in theoretical physics, problems are often simplified to ease calculations—like the description of a spherical cow in a vacuum, allowing researchers to neglect the effects of the cow's actual shape and the effects of air resistance. Image Credit: Abstruse Goose
Still, one particularly prominent question remains: Do cows and dying stars have much in common? “Probably not,” Margutti says with a laugh. The Cow wasn’t purposefully named for any quirky resemblance to our cud-chewing, Earthbound bovine friends; rather, the “cow” suffix was a randomly-generated, three-letter designation from the International Astronomical Union’s Transient Name Server.
“Cows are kind of gentle creatures overall, but this is anything but a gentle event,” Metzger says. “So maybe it’s an enraged cow—more of a bull.” Either way, AT2018cow is no average beefcake.
As the Cow further reveals itself to the world, Levesque adds, more answers on its origins are sure to follow. Until then, what exactly lies at the heart of the intersection of bovine and stellar is probably a moo point.
