Rick Robinson

Apr 18th 2020

Black Hole Images: Shadows From the Event Horizon


What does a black hole look like? Until a few days ago, we could only speculate, but now we’ve seen the first actual black hole images. The black hole at the center of Messier 87 (or M87), a giant galaxy 55 million light-years from Earth, looks like a ring of fire surrounding a dark center.

That dark center is the shadow of the black hole’s event horizon. And that shadow is all we can really ever hope to see of a black hole — an object so massive and dense that it has, in a sense, fallen right out of our universe into a little universe of its own from which nothing can escape, not even light.

Looking Through the Event Horizon

If light can’t escape from a black hole, how can we take a picture of it? How can it be visible at all? The answer is that, while nothing can escape from a black hole, objects of all sorts — including entire stars — can fall into one.

The black hole’s intense gravity field accelerates material falling into it, heating that material up until it glows. This leads to a paradox: The region of space around a black hole can be extremely bright, lit by matter swirling toward the black hole’s event horizon, where it falls out of our universe and out of our view.

What we see in the black hole images is the bright glow of matter falling toward the black hole. At the dark center, as Sky & Telescope explains, we are looking toward the event horizon, where we can see nothing at all — which is why the dark center stands out from the glowing region of space around it. This dark area is aptly called the black hole’s shadow; we see the black hole’s presence even though we cannot actually “see” the black hole itself.

A Mosaic of Images

If obtaining black hole images is challenging in principle, it is even more challenging when practical complications come into play. The image we see is a bit fuzzy, as though it has been highly magnified — and it has been. According to The Atlantic, the silhouette is about 40 microarcseconds across; for comparison, 10 microarcseconds “are equivalent to the size of a coin on the moon, viewed from Earth.”

No single telescope, on Earth or in space, can capture an image with resolution this fine, or even come close. The black hole images are a type of composite, produced by a technique called very long baseline interferometry (VLBI), which allows multiple radio telescopes to combine their results into one image.

As The Atlantic explains, 10 radio telescopes from around the world are harnessed together to form the Event Horizon Telescope. The black hole images we see are combined out of five petabytes — 5,000 terabytes, or 5 million gigabytes — of raw data. Combining the results allows the researchers to mimic a radio telescope nearly the size of the Earth itself, allowing sufficient precision to capture the shadow of a black hole.

In a remarkable final step, adds Sky & Telescope, researchers divided into four teams that each generated an image. Only features that were confirmed by all four images were included in the final composite image that we see.

Seeing Is Believing

Analysis of that fiery ring around darkness is already adding to our understanding of black holes: The imagery indicates that the black hole at the center of M87 has about the mass of 6.5 billion suns.

The Event Horizon Telescope’s next target may be closer to home. According to The Atlantic, our own Milky Way galaxy also has a black hole at its center, called Sagittarius A* (“A-star”). Because it is smaller and spins more rapidly, it is an even more challenging target for VLBI imagery than the black hole in M87, but it will give us yet another look at the edges of the universe within every black hole.

What does a black hole look like? We had theories, but now we have pictures.