What Is a ‘Stealth Black Hole,’ and Should We Worry One Will Gobble Us Up?

An explainer on the "stealth black hole" scientists announced this week.

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Jun 30 2016, 5:36pm

Image: X-ray: NASA/CXC/Univ. of Alberta/B.Tetarenko et al; Optical: NASA/STScI; Radio: NSF/NRAO/VLA/Curtin Univ./J. Miller-Jones

This week, scientists announced they'd found what's been called a "stealth black hole" lurking in Milky Way, which seems to be slowly munching away at its companion star. Their research suggests our galaxy could be peppered with black holes we didn't know about before, like a galactic piece of Swiss cheese.

So, what is a "stealth black hole" anyway?

It's a black hole that seems to give off radio waves. This one didn't have the telltale X-ray radiation that black holes usually give off when they suck in material from nearby stars, forming a disc that glows brightly. That's because it eats its companion star's material very slowly, gently grazing on it instead of fully chowing down, for reasons that are poorly understood. Where there's one slow eater, there likely are many others.

How was it discovered?

Scientists found it by noticing that a pattern of radio waves emanating from a group of stars was behaving very weirdly, so they reanalyzed them. The radio waves, which were thought to come from a star cluster known as M15, were known to scientists for some 20 years, but this team discovered their source was moving much more quickly across the sky than they'd expected—indicating the waves couldn't be coming from M15 after all.

Turns out the source is a black hole 7,000 light-years away.And chances are, there's lots more of these sneaky black holes out there. There could be 170 million, just in our galaxy.

That's potentially a lot of black holes we didn't know about, right?

To narrow it down a little, our Milky Way galaxy has anywhere from 26,000 to millions and millions of these stealth black holes. The closest one could be anywhere from 25 to 500 light-years away—a very short distance in space terms.

It's hard to know how exactly many of these invisible black holes are lurking in the galaxy. It took the combined powers of several observatories to find even one. And it was a lucky break: data only existed in most cases because the nearby M15 star cluster is such an interesting object in itself and the black hole happened to be in the same field of view. Scientists want to look for more.

Any chance one will devour us?

We're completely safe. Statistically speaking, the closest one would be 25 light-years away. Even if there was a nearby invisible black hole 10,000 times the mass of the sun (a typical mass), it would have to live at a fraction of a light-year of us to pose a threat. In other words, we'd already know about it or be dead.