Impossibly dense, deep, and powerful, black holes reveal the limits of physics. Nothing can escape one, not even light.
But even though black holes excite the imagination like few other concepts in science, the truth is that no astronomer has actually seen one.
"There’s really strong evidence for them, and by now every astronomer believes confidently there are black holes," Peter Edmonds, a NASA astrophysicist, told me. "But there’s no direct image."
Any photo you've seen of a dark mass warping spacetime … well, that's just an illustration:
Why no astronomer has ever seen a black hole directly
The biggest problem with trying to detect a black hole is that even the supermassive ones are relatively tiny.
"The largest one in the sky [is] the black hole in the center of the Milky Way," Dimitrios Psaltis, an astrophysicist at the University of Arizona, writes me in an email. "And taking a picture of it would be equivalent to taking a picture of a DVD on the surface of the moon."
What's more, because of their strong gravity, black holes tend to be surrounded by other bright matter that makes it hard to see the object itself.
That's why, when hunting for black holes, astronomers don't usually try for direct observation. Instead, they look for evidence of the effects of a black hole's gravity and radiation.
"We typically measure the orbits of stars and gas that seem to circle around very dark 'spots' in the sky and measure how much mass is there in that dark spot," Psaltis says. "If we know of no other astrophysical object that can be so massive and so dark as what we just measured, we consider this as very strong evidence that a black hole lies there."
We do have indirect images of black holes, however
Some of the best indirect images of black holes come from the Chandra X-ray Observatory, where Edmonds works. "The friction and the high velocities of material forming out of a black hole naturally produces X-rays," he says. And Chandra is a space telescope specially designed to see those X-rays.
For example, the Chandra observatory documented these X-ray "burps" emanating from the merger of two galaxies around 26 million light-years away. The astrophysicists suspect that these burps came from a massive black hole:
Similarly, the fuchsia blobs on this image are regions of intense X-ray radiation, thought to be black holes that formed when two galaxies (the blue and pink rings) collided:
Similarly, here are X-rays and sound waves emanating from the central region of the Perseus galaxy cluster — more indirect evidence of a black hole:
And in this GIF, the Chandra telescope saw the largest X-ray flare coming from the black hole suspected to lie at the center of the Milky Way galaxy.
And here's a zoomed-out image of that X-ray flare.
We can see black holes spew massive jets of matter into the universe
This composite image (combining data from Hubble and a radio telescope) shows jets of energy and matter being thrown out of the center of the Hercules A galaxy. These jets shoot out at nearly the speed of light, demonstrating the awesome destructive power of the objects.
This next image* shows massive jets that are thought to be propelling away from the black hole at the center of Centaurus A, a galaxy 13 million light-years away. The jets are longer than the galaxy itself.
Astronomers have observed stars orbiting mysterious dark objects, which are most likely black holes
This 16-year time lapse of the stars moving near the center of the Milky Way galaxy is evidence of a black hole there.
Very soon we may see an actual black hole
A true image of a black hole would reveal its event horizon. That's 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.)
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 — and determine what it looks like.
That's because of the Event Horizon Telescope — a global network of sensors that, in effect, forms a telescope as large as the Earth. It's on track to take a snapshot of the black hole by the end of 2017, and is expected to produce the first image of an event horizon. "What they’re hoping to see is the actual shadow, the actual dark region," Edmonds says. "That will be a big deal."
With a direct image of a black hole, scientists will be able to learn more about the effects of extreme gravity, and can further test Einstein's theory of general relativity.
*Correction: This article originally misattributed this image to the Hubble Space Telescope. It's a composite image, combining information from the European Southern Observatory and NASA's radio telescopes.