Given the vastness of the universe and the few short centuries that humans have been gazing starward, it's no surprise that we're still discovering how things work out there. One good example of that process is the "cosmic cow," the unofficial name affectionately used by astronomers to refer to AT2018cow, an unexplained X-ray phenomenon that occurred in 2018.
Today, researchers have a strong hypothesis to explain the cosmic cow and guide future research into black holes, neutron stars and other as yet unexplained events in the farthest reaches of the universe.
How Stars Normally Die
To understand why the cosmic cow caught astronomers' attention, it helps to have an understanding of the life cycle of stars — including star death. Of course, there is a range of star types and sizes, which means there is no normal way for stars to die, and even "dying" isn't an accurate word, as stars simply move from one phase of life to the next.
In any case, it's broadly accurate to say that when most massive stars (much larger than our sun) reach the end of their life cycle and have consumed all of the fuel within their cores, they explode in a supernova and then become either a black hole or neutron star, depending on the original size of the star.
Astronomers have long measured supernovas; the first possible recorded supernova dates back to Indian astronomers in 4500 B.C.E. (plus or minus about 1,000 years). Since then, there have been many notable supernovas, including one noted by Chinese astronomers in 185 C.E., another by Johannes Kepler (and many other astronomers across the globe) in 1604, and dozens more thanks to advancements in telescope science. It's safe to say that astronomers generally understand what they're seeing when a bright emission appears in the sky.
What Makes the Cosmic Cow Different
That's what was so baffling about AT2018cow, the cosmic cow, when astronomers first observed it in June 2018. Astronomers using the ATLAS-HKO telescope at Haleakalā Observatory in Hawaii noted the bright X-ray emission, which persisted for three weeks and glowed ten times more brightly than supernovas that astronomers had previously studied.
Only now, years later, do we have a sense of what may have caused this bright emission: a bouncing bundle of joy for the universe, in the form of either a baby black hole or newborn neutron star.
Astronomers at MIT, led by research scientist Dheeraj "DJ" Pasham at the Kavli Institute for Astrophysics and Space Research in Cambridge, Massachussetts, studied the emissions from the cow for several months, publishing their findings Dec. 13, 2021, in the journal Nature Astronomy. They determined it is likely the result of a massive energy output caused by a black hole or neutron star enjoying its first meal of its origin star. Unlike other supernovas, the energy emitted by the cow occurred a bit differently — hence the bright, long-lasting glow we observed in the sky.
Given the unique data his team was studying, Pasham admitted he was hoping that the explanation would point to a black hole eating an exotic star. “I was a little bit disappointed,” he told ScienceNews. “But I’m more blown away that this could be direct evidence of the birth of a black hole. This is an even cooler result.”
New Ways to Study the Birth of Black Holes and Neutron Stars
The results of this MIT study suggest that astrophysicists might use a similar protocol to examine the data from other unexplained origin phenomena, called fast blue optical transient (FBOT) phenomena. There are about a dozen of these events that have been recorded, and astronomers may now be able to come up with new hypotheses to explain these events in the distant reaches of the universe.
Additionally, the cosmic cow now gives astronomers a guide when it comes to looking for new neutron stars and baby black holes; as studying black holes is a big priority for NASA right now, it's always good to have a better idea of what to look for and to gain a better understanding of the life cycle of black holes too.