Whoa: A Black Hole Took a Bite Out of a Star

Black holes have been observed swallowing stars, or getting into tumultuous orbital relationships with them. But today, astronomers based out of the Ohio State University announced their discovery of a star that was clipped by the jaws of a black hole, and escaped to shine on another day. Their observations of the so-called "tidal disruption event" will be published in The Monthly Notices of the Royal Astronomical Society.

"These are very rare," co-author Krzysztof Stanek told me. "Ours is the nearest and the best observed ever because it was relatively close."

"Close" in this case means about 650 million light years away, at the center of a galaxy located in the Big Dipper. The team estimates that the black hole tore a Jupiter-sized chunk off the passing star, representing about one percent of the star's total mass. The chunk lit up the black hole's accretion disc, which is a ring of spiraling gas, dust, and other trapped material surrounding the hole. The sudden flash was what tipped the astronomers off to the event.

"We don't see the mass," explained Stanek. "What we see is the light. The stuff doesn't fall directly into the black hole, but spirals in slowly, and becomes hotter and hotter. That is the light we see. We look at how much energy was produced, and how the mass translates to energy." From there, an estimate of the star's total mass can be calculated.

A star's first encounter with a black hole is usually its last, but in this case, the star may have been massive enough to carry on. "Take a star which is strongly stratified, like a red giant," co-author Christopher Kochanek told me. "The core is very tightly bound—essentially a white dwarf—and the hydrogen envelope is very loosely bound. There will be a range of approach distances where the envelope can be strongly affected by the tidal gravity of the black hole while the core barely even notices the tides because it is so much smaller and denser."

"This can then let you rip the envelope off, while leaving the core alone," he added. "Basically, the core just keeps going along the original orbit of the star."

If the star is lucky, its orbit won't take it near the black hole again. But it might just be gravitationally doomed to circle the hole over and over, losing piece after piece with each successive orbit like some kind of stellar Prometheus. While that sounds like a torturous fate even for an inanimate object like a star, it would be great news for astronomers.

"That's actually a super exciting possibility," said Stanek, "because every time it passes close to the black hole there will be another event."

Indeed, tidal disruption events are not just rare, they are also very useful for understanding how the supermassive black holes perched in the centers of galaxies evolve. "They are probably the only case where you can see a black hole 'turning on' (and off) on a human time scale," said Kochanek.

"One of the longstanding problems in astrophysics right now is actually how [supermassive] black holes grow," added Stanek. "One of the things we want to do in the future is find more of these, to understand the physics better but also to understand the rate—how often do these things happen?"

Stanek, Kochanek, and his team will be delving into that question using the same telescopic survey that caught this particular tidal disruption event: the All-Sky Automated Survey for Supernovae (ASAS-SN, pronounced "assassin"). The survey is just over a year old, and has telescopes based in Hawaii and Chile. The long-term plan is to set up 16 telescopes over four sites that would monitor the entire sky every night.

"We want to take a movie of the sky every night so when things like this happen we will find them no matter where they are on the sky, as long as they are bright enough," said Stanek. "We are the only project right now using 14-cm telescopes that are surveying the whole sky for things like this."

Currently, the tidal disruption event has moved behind the Sun, but the team is eager to get another look at it when it becomes visible again in a few months. In the meantime, the astronomers remain optimistic that ASAS-SN will continue to catch brilliant celestial face-offs and explosions.

"I think the most exciting part is that this is not serendipitous," Stanek said of the discovery. "Either we got very lucky, which is always possible, or we are doing something right, which is surveying the whole sky looking for very rare events."

"Maybe it will turn out that these events are not nearly as rare as people thought," he added. "I was very surprised to see one so quickly."

There you have it: black holes have to settle for crummy Jupiter-sized morsels sometimes, perhaps even often. Even supermassive galactic predators have off days.