It took a ‘virtual’ telescope to actually picture a black hole

Author: Maria Temming / Source: Science News for Students

a photo of the ALMA array, a group of radio telescopes with large dishes — A network of telescopes across the globe — including ALMA in Chile (partially shown here) — teamed up to create the Event Horizon Telescope. It’s a virtual radio dish almost as big as Earth.

Black holes are camera shy. Their extreme gravity prevents light from escaping.

That means that the dark hearts of these cosmic heavy hitters remain entirely invisible. However, supermassive black holes in the centers of galaxies may give themselves away by spewing bright jets of charged particles. Others may be “seen” by the light of the nearby stars that they fling away or rip apart. Up close, these behemoths are surrounded by accretion disks — glowing disks made from the material being sucked into them.

Scientists have now cleverly created a network of eight radio telescopes. Working as one, they effectively make an Earth-sized eye on the sky. And they have just imaged the silhouette of a black hole’s event horizon — the edge inside which nothing can be seen or escape. It can be seen against the black hole’s accretion disk.

In April 2017, this so-called Event Horizon Telescope, or EHT, collected data that has now yielded the first image of a supermassive black hole. It sits inside the galaxy M87.

“There is nothing better than having an image,” says Avi Loeb. He’s an astrophysicist at Harvard University in Cambridge, Mass. Though scientists have collected plenty of indirect evidence for black holes over the last half century, he notes that “seeing is believing.”

Creating that first-ever portrait of a black hole was tricky, though. Black holes take up a minuscule sliver of sky. And they’re so far away that the halo of light surrounding some of them appears very faint. The project of imaging M87’s black hole required eight observatories across the globe. By working as one virtual radio dish, their vision would be sharper than that of any single observatory working on its own.

Putting the ‘solution’ in resolution

The supermassive black hole inside M87 is no small fry. It weighs in at around 6.5 billion times the mass of our sun. But viewed from 55 million light-years away, it appears as the smallest of blips in the sky. It’s smaller than an orange on the moon as viewed by someone on Earth. Still, besides Sagittarius A* — the black hole at the center of our own galaxy — M87’s black hole is the largest black hole silhouetted on the sky.

a map showing the radio telescopes that linked together for the Event Horizon telescope — Getting the first picture of a black hole required connecting pairs of radio telescopes spanning huge distances — almost the entire globe.

Only a telescope with EHT’s resolution could pick out something so tiny. A telescope’s resolution depends on its diameter: The bigger the dish, the clearer the view. So getting a crisp image of even a supermassive black hole needed a planet-sized radio dish.

“The trick is that you don’t cover the entire Earth with an observatory,” explains Loeb, who wasn’t involved in EHT. Instead, astronomers combine radio waves seen by many telescopes at once. This makes the telescopes effectively work as one giant dish. The diameter of that virtual dish is equal to the length of the longest distance, or baseline, between two telescopes in the network. For the EHT in 2017, that was the distance from the South Pole to Spain.

Telescopes, assemble!

The EHT was not always the hotshot array that it is today. In 2009, a smaller network of just four observatories — in Arizona, California and Hawaii — imaged the base of one plasma jet spewing from the center of M87’s black hole. But this small telescope network didn’t yet have the magnifying power to reveal the black hole itself.

Over time, the EHT program recruited new radio observatories. By 2017, there were eight observing stations in North America, Hawaii, Europe, South America and at the South Pole. Among the newcomers was the Atacama Large Millimeter/submillimeter Array, or ALMA. It is located on a high plateau in northern Chile. With a combined dish area larger than an American football field, ALMA collects far more radio waves than other observatories.

“ALMA changed everything,” says Vincent Fish. He’s an astronomer at MIT’s Haystack Observatory in Westford, Mass. It can offer “really solid detections now,” he says, of “anything that you were just barely struggling to detect before.”