If you're a hungry black hole, try snacking on a star

Black holes could be seen as the bouncers of the solar system. They hang out and use their brute strength — their mass and energy — to keep all the stars and planets in their galaxy in line. But every once in a while, a star wanders a bit too close. When that happens, it's lights out for the star.

About 290 million years ago, a supermassive black hole in a distant galaxy shredded a sun-like star and swallowed it. Now scientists from the Massachusetts Institute of Technology are coming to understand why.

How does a black hole just swallow a star?

Black holes are objects in the universe of unimaginable mass and energy, and they have such strong gravitational forces that nothing, not even light, can escape. These black holes, especially supermassive ones, exert such a force on the surrounding objects — planets, stars and everything else — that it keeps galaxies together. But sometimes a star can wander a bit too close. When that happens, it will get pulled towards the black hole, eventually crossing the event horizon, the place where nothing can ever escape.

Now, when a star gets swallowed, it doesn't go down without a fight: it will release an intense amount of energy, called a tidal disruption force, that can be seen across the universe.

How was this tidal disruption force detected?

When a star wanders too close to a black hole the whole event results in a bright spot in the sky. One night in November 2014, there was a bright point observed by space telescope SWIFT. Shortly after, a program called ASAS-SN (All-Sky Automated Survey for Supernovae) run by The Ohio State University's Department of Astronomy realized the significance of this bright point, and it caught the attention of Dheeraj Pasham at MIT.

"They [SWIFT and ASAS-SN] take images of this part of the sky, [they see] there was nothing there yesterday, they come back today and all of a sudden there's like a bright point-source there, so, you know, it's pretty obvious that something happened in the sky at that location," said Pasham. 

And it's from that blob that NASA's SWIFT space telescope, as well as others on Earth, started to really train their eye on that part of the sky and measure all sorts of different types of radiation coming from that spot — everything from radio to infrared to X-rays in the electromagnetic spectrum.

What did the data collected tell them about what happened?

One thing that was remarkable about the tidal disruption flare was different telescopes detected a signal from all across the electromagnetic spectrum. It's rare to have that much data from that many different sources. That data allowed Pasham and colleagues to really dig into the mechanics of what happens when a star gets swallowed by a black hole.

"We looked at about a year-long of the monitoring data, in the X-ray wave band, and also in the optical and UV wave band, and what we found was that there was these brightness fluctuations in the optical and the UV that repeated themselves in the X-ray band about thirty days after they appear in the optical and UV band," said Pasham. "This was telling us that the optical UV imaging region was physically distinct from the X-ray imaging region, and we know that X-rays must be coming from very close to the black hole, so now we had a reference that the UV optical emanating sites are roughly thirty days away from the black hole."

What that means is that the sun-like star was getting pulled apart and literally shredded into a debris stream that spiralled around the centre of this black hole. 

It's like a penny swirling around one of those columnar toys at science centres, where it spirals closer and closer and faster and faster towards the centre where it disappears forever. That's exactly what is happening here, but the forces are so strong, the velocities so fast, that something as big as a star is getting ripped apart while it swirls to its death. And the debris is getting flung out into space while the bulk of the sun swirls closer and closer to the centre of the black hole.

How often do these kinds of events happen in the universe?

Not that often, which is why this particular detection was so lucky. It is estimated that this type of event happens only once every 100,000 years in a galaxy.

In fact, in 2014, the supermassive black hole in the centre of our own Milky Way galaxy was getting a little hungry. There was a star that seemed to get perilously close and astronomers were waiting with bated breath for it to be engulfed. But, for whatever reason it never happened. 

Why these events occur is still unclear. Is it some wobble or disruption in the gravitational field? Is it something that comes from the black hole or other parts of the universe?  We simply don't know. But now, with this data, we understand more about the forces underlying this behaviour and now understand more about black holes which have always been impossible to detect.