Planning for weather on several continents can be a headache.
It can muddle the millimeter-wavelength radio waves to which the EHT’s telescopes are tuned. Researchers’ biggest enemy is water in the atmosphere, such as rain or snow. That’s when the weather at every observatory promises to offer the clearest images of the sky. Guisard/ESO More than the sum of their partsĮHT observing campaigns are best run within about 10 days in late March or early April. It can offer “really solid detections now,” he says, of “anything that you were just barely struggling to detect before.” He’s an astronomer at MIT’s Haystack Observatory in Westford, Mass. “ALMA changed everything,” says Vincent Fish. With a combined dish area larger than an American football field, ALMA collects far more radio waves than other observatories. It is located on a high plateau in northern Chile. Among the newcomers was the Atacama Large Millimeter/submillimeter Array, or ALMA. By 2017, there were eight observing stations in North America, Hawaii, Europe, South America and at the South Pole. Over time, the EHT program recruited new radio observatories. But this small telescope network didn’t yet have the magnifying power to reveal the black hole itself. 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. The EHT was not always the hotshot array that it is today. For the EHT in 2017, that was the distance from the South Pole to Spain. The diameter of that virtual dish is equal to the length of the longest distance, or baseline, between two telescopes in the network. This makes the telescopes effectively work as one giant dish. Instead, astronomers combine radio waves seen by many telescopes at once. “The trick is that you don’t cover the entire Earth with an observatory,” explains Loeb, who wasn’t involved in EHT. So getting a crisp image of even a supermassive black hole needed a planet-sized radio dish. A telescope’s resolution depends on its diameter: The bigger the dish, the clearer the view. Only a telescope with EHT’s resolution could pick out something so tiny. 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. It’s smaller than an orange on the moon as viewed by someone on Earth. But viewed from 55 million light-years away, it appears as the smallest of blips in the sky. It weighs in at around 6.5 billion times the mass of our sun. The supermassive black hole inside M87 is no small fry. By working as one virtual radio dish, their vision would be sharper than that of any single observatory working on its own. The project of imaging M87’s black hole required eight observatories across the globe. And they’re so far away that the halo of light surrounding some of them appears very faint. Black holes take up a minuscule sliver of sky. Though scientists have collected plenty of indirect evidence for black holes over the last half century, he notes that “seeing is believing.”Ĭreating that first-ever portrait of a black hole was tricky, though. He’s an astrophysicist at Harvard University in Cambridge, Mass. “There is nothing better than having an image,” says Avi Loeb. In April 2017, this so-called Event Horizon Telescope, or EHT, collected data that have now yielded the first image of a supermassive black hole. It can be seen against the black hole’s accretion disk. And they have just imaged the silhouette of a black hole’s event horizon - the edge inside which nothing can be seen or escape. Working as one, they effectively make an Earth-sized eye on the sky.
Scientists have now cleverly created a network of eight radio telescopes.
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