In June, a luminous celestial event set the astronomy community buzzing with excitement. The brilliant anomaly, bursting with light 200 million light years away in a distant dwarf galaxy, made it quite unlike a typical exploding star, and scientists have been questioning what the phenomenon could be ever since. Now, astronomers have realized that they captured the exact moment a star collapsed — and perhaps the birth of a black hole or neutron star — for the very first time.
The collapsing star, named AT2018cow — or “the Cow” because of the randomized three-letter naming system used by Astronomer’s Telegram, the online service that first posted about the discovery — was completing the quintessential step before evolving into a compact object, like a neutron star or a black hole. Kate Alexander, Ph.D., a NASA Einstein Fellow at Northwestern University, tells Inverse that the Cow is “highly unusual in many ways.”
“It was incredibly bright, and it got brighter incredibly fast — both of which are hard to explain with typical models of stellar death,” Alexander says. “When most stars die, the resulting supernova takes weeks to reach its peak brightness. AT2018cow reached peak brightness in just a few days and is one of the brightest supernovae ever discovered.”
Alexander is part of the team that’s proposing that “the Cow” was a star’s collapse. They presented their theory Thursday at the 23rd meeting of the American Astronomical Society in Seattle, Washington, but the study detailing their research is not yet published.
When AT2018cow was first detected by the Hawaii-based ATLAS survey, Alexander and co-author Deanne Coppejans, Ph.D., a postdoctoral associate at Northwestern University, set up and processed the radio observations made at the Very Large Array telescope in New Mexico.
A look at "The Cow" 200 million light years from Earth.
These radio observations, Coppejans tells Inverse, were an “important piece of the puzzle” that AT2018cow represented. They revealed that the velocity of the blast wave the Cow launched into space was about 10 percent the speed of light and that the density of the environment around it was much higher than seen around typical massive stars. This high density refuted the idea that it was a disruption of a star around an intermediate-mass black hole, a theory other astronomers had posited.
"Nearly every telescope in the world that could reach it was observing it.
“It was incredibly exciting working on this project,” Coppejans tells Inverse. “Nearly every telescope in the world that could reach it was observing it. But while most groups only had observations at specific wavelengths, our group had observations from radio to gamma-ray wavelengths.”
Each new observation revealed to the team a different aspect of the Cow. Observations made by X-ray, radio, and light revealed that it’s powered by a strong central “engine” — either a new neutron star or a black hole. This so-called engine pumped new energy into the explosion for months. It’s no wonder its brightness was so unusual compared to that of other stellar deaths.
Coppejans says that while scientists know that explosions of massive stars form neutron stars and black holes, AT2018cow represents the first time scientists actually captured the process happening.
“Usually there is so much material blown out into space by the explosion that it blocks out view of the compact object left behind,” she explains. “However, AT2018cow was nearby, bright, and ejected very little material, so we were able to detect the remnant compact object for the first time.”
And in the aftermath of this star death, advances in data processing and worldwide telescope surveys allowed astronomers across the planet to examine AT2018cow’s unusual properties at a quick and detailed pace. Typically, stellar explosions are discovered when they are several days old — but the attention surrounding AT2018cow when it was first spotted was almost immediate. Their rich dataset, says Alexander, is ultimately what allowed the team to “uncover the true uniqueness of AT2018cow.”
