A long-exposure image of the SOAR telescope with overlaid illustrations of a highly magnetized neutron star (top left) and an accreting black hole (top right).

Image Credit: D. Maturana & NOAO/AURA/NSF; Overlay (top left): NASA/Penn State University/Casey Reed; (top right): NASA Goddard Space Flight Center.

It’s a supernova… It’s a tidal event… It’s a cow?A June 16, 2018 stellar explosion has provided astronomers with a goldmine of new information on stellar deaths. It has also provided them with a wealth of opportunities to insert cow jokes into research talks, given that its official name is AT2018cow.But even as astronomers are united in the jokes, they are divided on what “the cow” actually is—or was, given that it happened in a galaxy 200 million light years away.The cow was initially flagged by the Asteroid Terrestrial-impact Last Alert System (ATLAS), a project carried out by two telescopes in Hawaii that scan the night sky for objects approaching the Earth. On June 16, one of them noticed something unusual—an especially bright spot where there was only darkness the day before.The rapid appearance of this bright object quickly captured the attention of astronomers around the world, and telescope after telescope turned toward the cow. Using ground and space-based instruments, astronomers captured a huge amount of data as the event reached its peak brightness and then faded away over the next few months. But exactly what the data implies depends on who you ask.Astronomers seem to agree on what happened, as indicated by talks at the 233rd American Astronomical Society meeting last week, just now why and how. Here’s what we know:• The cow appeared quickly, reaching its peak brightness in just two days.• Debris from the explosion traveled outward at about 1/10th the speed of light.• The fast-traveling debris interacted with a dense material.• A central power source continued to spew out energy for weeks.“These properties together are essentially unprecedented in anything we’ve ever seen,” astrophysicist Daniel Perley from Liverpool John Moores University said at the meeting. Perley is a researcher and a member of the GROWTH project—the Global Relay of Observatories Watching Transients Happen. Transients are astronomical events like AT2018cow that happen over much shorter time periods than normal astrophysical processes: supernovae, neutron star or black hole merges, and near-Earth asteroids. In fact, the “AT” in AT2018cow stands for astronomical transient (“2018” gives the year of discovery and “cow” comes from the three-letter naming convention for such objects). GROWTH studies transients using a network of small telescopes around the world.On news of ATLAS’s discovery, the GROWTH collaboration used their network to observe the cow almost continuously over its first few weeks, collecting its spectra in ultraviolet, optical, and near infrared light. Their analysis revealed what we know as outlined above, and is supported by observations from lots of other instruments, including x-ray, millimeter, and radio wavelength observatories. From these observations, the GROWTH team concluded that the cow was likely one of two things:• a special kind of supernova—the collapse and violent explosion of a massive star that leaves behind a neutron star or black hole, or• a tidal disruption event—the violent shredding of a star that got too close to a black hole.Astronomers at the meeting presented evidence for both possible explanations, but the trouble is that the data doesn’t fit neatly into one camp or the other. In some ways, that’s even more exciting than having a clear culprit because it means that new information about the universe is on the horizon.Which camp is right, if either, is likely to be determined over time as post-explosion observations explore what’s left in that region of the sky and as astronomers study similar events. Although the cow has an unusual combination of properties, researchers think we’ve actually seen such events before, just not in a context that revealed their differences from more well-known types of stellar explosions. Now that we know what to look for, we can start an intentional search for other objects with the same properties—who knows, there may be a whole herd of them out there.