Meanwhile, many more observations are on the way. The rate of new TDEs, now about one or two per year, could jump up by an order of magnitude even by the end of this year because of the Zwicky Transient Facility, which started scanning the sky over California’s Palomar Observatory in March. And with the addition of planned observatories, it may increase perhaps another order of magnitude in the years to come.

“The field has really blossomed,” said Suvi Gezari at the University of Maryland, one of the few stubborn pioneers who staked their careers on TDEs during leaner years. She now leads the Zwicky Transient Facility’s TDE-hunting team, which has already snagged unpublished candidates in its opening months, she said. “Now people are really digging in.”

Searching for Star-Taffy

In 1975, the British physicist Jack Hills first dreamed up a black-hole-eats-star scenario as a way to explain what powers quasars—superbright points of light from the distant universe. (Quasars are now known to be supermassive black holes feeding on surrounding gas, not stars.) But in 1988, the British cosmologist Martin Rees realized that black holes snacking on a star would exhibit a sharp flare, not a steady glow. Looking for such flares could let astronomers find and study the black holes themselves, he argued.

Nothing that fit the bill turned up until the late 1990s. That’s when Stefanie Komossa, at the time a graduate student at the Max Planck Institute for Extraterrestrial Physics in Garching, Germany, found massive X-ray flares from the centers of distant galaxies that brightened and dimmed according to the Rees predictions.

The astronomical community responded to these discoveries—based on just a few data points—with caution. Then in the mid-2000s, Gezari, then beginning a postdoc at the California Institute of Technology, searched for and discovered her own handful of TDE candidates. She looked for flashes of ultraviolet light, not X-rays as Komossa had. “In the old days,” Gezari said, “I was just trying to convince people that any of our discoveries were actually due to a tidal disruption.”

Soon, though, she had something to sway even the doubters. In 2010, Gezari discovered an especially clear flare, rising and falling as modelers predicted. She published it in Nature in 2012, catching other astronomers’ attention. In the years since, large surveys in optical light, sifting through the sky for changes in brightness, have taken over the hunt. And like Komossa’s and Gezari’s TDEs, which had both been fished out of missions designed to look for other things, the newest batch showed up as bycatch. “It was, oh, why didn’t we think about looking for these?” said Christopher Kochanek, an astrophysicist at Ohio State University who works on a project designed to search for supernovas.

Suvi Gezari’s work helped convince astronomers that tidal disruption events are real and widespread. John T. Consoli/University of Maryland

Now, with a growing number of TDEs in hand, astrophysicists are within arm’s reach of Rees’s original goal: pinpointing and studying gargantuan black holes. But they still need to learn to interpret these events, divining their basic physics. Unexpectedly, the known TDEs fall into separate classes. Some seem to emit mostly ultraviolet and optical light, as if from gas heated to tens of thousands of degrees. Others glow fiercely with X-rays, suggesting temperatures an order of magnitude higher. Yet presumably they all have the same basic physical root.

To be disrupted, an unlucky star must venture close enough to a black hole that gravitational tides exceed the internal gravity that binds the star together. In other words, the difference in the black hole’s gravitational pull on the near and far sides of the star, along with the inertial pull as the star swings around the black hole, stretches the star out into a stream. “Basically it spaghettifies,” said James Guillochon, an astrophysicist at the Harvard-Smithsonian Center for Astrophysics.