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We can't directly see black holes. But this time lapse shows the awesome power of their gravity.

Watch: stars orbit the supermassive black hole in the center of our galaxy.

UCLA Galactic Center Group
Brian Resnick is Vox’s science and health editor, and is the co-creator of Unexplainable, Vox's podcast about unanswered questions in science. Previously, Brian was a reporter at Vox and at National Journal.

No astronomer has even seen a black hole through a telescope. They’re dark, often occluded by matter, and actually very small compared with many of the stars in the sky. We’ve never directly seen the ring of material around a black hole, known as the “event horizon,” that represents the boundary beyond which no light can escape. Yet we know black holes are real.

One way we know: Last year, physicists “heard” for the first time the gravitational waves produced from two black holes colliding.

Another way we know: We can observe the effects of a black hole’s extreme gravity on the objects around it. Here’s a very cool illustration of that.

You’re looking at 20 years of data on the stars that live near the supermassive black hole at the center of the Milky Way galaxy, called Sagittarius A. And yes, stars — some many times more massive than our sun -- are orbiting it.

Star S2, which is marked in the video with a yellow line, is around 15 times as massive as our sun. That’s big. But it’s nothing compared with the black hole, which is estimated to be some 4 million times more massive than our sun. The gravity it produces whips S2’s orbit to around 11 million miles per hour, which is about 200 times the speed the Earth orbits around the sun. (S2 completes one orbit in around 16 Earth years.)

We haven’t directly observed this black hole, but we know it’s there, because nothing else can explain the orbits of these stars.

“These orbits, and a simple application of Kepler's Laws, provide the best evidence yet for a supermassive black hole, which has a mass of 4 million times the mass of the Sun,” explains UCLA’s Galactic Center Group, which produced the animation.

Here’s another look at the same phenomenon. This video includes 16 years of observations from the European Southern Observatory. This isn’t an animation. It’s real images of stars sped up by a factor of 32 million. Watch them dance around a mysterious blank center.

Very soon we may be able to see the black hole itself

A true image of a black hole would reveal its event horizon — the perimeter beyond which nothing can escape. Scientists speculate the event horizon would look like what you see in illustrations: a sudden boundary between bright lights in space and a void.

A true image might also show an accretion disk — a bright ring of matter that swirls around it. (The black hole in the movie Interstellar shows an accretion disk.) We don’t have an image yet because the black hole is so relatively small, dark, and surrounded by a lot of material. “Taking a picture of it would be equivalent to taking a picture of a DVD on the surface of the moon," Dimitrios Psaltis, an astrophysicist at the University of Arizona, told me last year.

To see something that small and occluded, we need a huge telescope.

What's exciting is that in the next few years, scientists hope to be able to confirm the existence of the black hole in the center of the Milky Way — with an enormous telescope.

The Event Horizon Telescope is a global network of sensors that, in effect, forms a telescope as large as the Earth. It's on track to come online later this year, and is expected to produce the first image of the event horizon of the black hole in the middle of our galaxy.

This is what astronomers think it might look like:

More on black holes:

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