On June 16, 2018, astronomers around the world witnessed a cosmic flash unlike anything they’d ever seen before. Now, in a forthcoming paper to be published in the Astrophysical Journal, an international team of 45 scientists argue that what they saw that day was likely the exact moment a star collapsed into a black hole. If they’re right, it would be the first time that this transition was ever witnessed by human scientists.
When a star runs out of nuclear fuel, some of its mass flows into its core, which eventually gets so dense that it collapses from its own gravitational force. This creates an explosion known as a supernova, which astronomers on Earth can detect as a bright flash of light.
The event witnessed last June, now known as “The Cow” in homage to the event’s official name—AT2018cow—originally caught the eye of astronomers around the world because it was ten times brighter than a typical supernova.
The flash of light from the Cow peaked over the course of three days and then gradually faded over the next 10 months. This was also unusual because the Cow’s brightness quickly peaked, whereas typical supernovas slowly get brighter. The fact that the light from the explosion also stuck around for so long was also mysterious.
In November, two research teams used independent observations of the Cow to explain the longevity of the explosion. Although both research teams posited that there was a “central engine” that continued to stoke the original explosion, they differed on the nature of this engine. One research team suggested that the engine fueling this explosion was a newly formed black hole sucking in matter while the other team suggested it was the work of a rapidly rotating neutron star.
If the Cow turns out to be a black hole or a neutron star—both of which are known as “compact bodies,” in astronomer-speak—it would be unlike anything astronomers have ever observed before. Normally the gas and debris produced during a supernova block astronomers from seeing the object at the center of the explosion, but in this case the Cow also provided a burst of strong X-ray emissions that gave astronomers a window into the center of the blast.
“If we’re seeing the birth of a compact object in real time, this could be the start of a new chapter in our understanding of stellar evolution,” NASA scientist Brian Grefenstette said in a statement. “We looked at this object with many different observatories, and of course the more windows you open onto an object, the more you can learn about it. But as we’re seeing with the Cow, that doesn’t necessarily mean the solution will be simple.”
For now, the jury’s still out on whether astronomers saw the birth of a black hole or a neutron star and some scientists think it may be none of the above. According to a team of astronomers whose research will be published in a forthcoming issue of Monthly Notices of the Royal Astronomical Society, there’s also a chance that the Cow event was a massive black hole ripping apart a white dwarf star—an astronomical event known as a tidal disruption.
According to one of the authors of that research, University of Maryland astronomer Amy Lien, the Cow is a big event for astronomy regardless of the result. Indeed, Lien summarized the sentiment of the astronomy community at large in a recent statement when she said “we’ve never seen anything exactly like the Cow, which is very exciting.”