An international team of scientists collaborated to find the first-ever visual evidence of a supermassive black hole in a faraway galaxy (Credit: EHT Collaboration et al 2019)

The existence of black holes, first proposed by Albert Einstein in his 1916 general theory of relativity, has been known for decades. However, astrophysicists have thus far relied on indirect evidence, such as the stars orbiting a large and invisible object in the center of the Milky Way galaxy, to prove their presence. That changed on April 10, 2019, with the release of the first-ever direct visual evidence of a black hole in the center of the galaxy M87, located 55 million light-years from Earth.

The culmination of many years of hard work and collaboration by an international team of more than 200 astrophysicists, the image does not show the black hole itself. That's because, as the name suggests, they are black, and hence, invisible against the backdrop of space. Instead, the scientists used radio signals to capture the black hole's "shadow" — the bright ring that forms around its boundary, or "event horizon," where light bends due to the hole's extreme gravitational forces.

"We now have visual evidence for a black hole," Event Horizon Telescope project director Sheperd Doeleman told reporters at a press conference in Washington, DC. "It is also consistent, the shape of this shadow ... with Einstein's predictions."

The EHT comprises eight powerful ground-based radio telescopes spread over six geographic locations (Credit: EHT Collaboration et al 2019)

The M87 black hole, believed to have a mass 6.5 billion times that of our sun, was captured using the Event Horizon Telescope (EHT) — a network of eight powerful ground-based telescopes linked to form a radio array as wide as the Earth. The observatories, located in Hawaii, Arizona, Chile, Mexico, Spain, and Antarctica, individually captured the black hole's radio signals over four nights in April 2017, when the weather was optimal in all six regions. The data, stored on physical hard drives, was transported to a central location where it was "stitched" together by a supercomputer to form the image of the black hole's shadow.

The researchers, who targeted M87's black hole due to its enormous mass and relative proximity to Earth, say while the image produced is slightly blurry, future photos will be clearer as more telescopes are added to the EHT. The team also believes they will be able to fine-tune the photos further. "We think we can make the image perhaps a little sharper through algorithms," Doeleman said.

The radio signals from the M87 black hole were collected over four "optimal weather" days in April 2017 (Credit: EHT Collaboration et al 2019)

The EHT team has also been trying to image a black hole closer to home — the Sagittarius A, which resides 25,000 light-years away from Earth at the center of our Milky Way galaxy. Though much smaller than M87's black hole, it still boasts an impressive mass of about 4 million times that of our sun. The scientists are currently processing the data collected by the telescopes and hope to release a photo in the very near future.

Black holes are regions in space where the pulling strength of gravity is so powerful that even light is unable to escape. The intense gravity is caused by matter that is compressed into a small space. Black holes can vary widely in size and mass. The smallest, called "primordial" black holes, are as tiny as a single atom but with a mass of a giant mountain. "Stellar" black holes, which are the most common, have a diameter of about 10 miles and a mass 20 times that of our sun. The largest, or "supermassive," black holes, similar to that of M87's, are proportionate in size to their galaxy. They can boast a mass greater than millions, or even billions, of suns combined and diameters as large as our solar system. Researchers believe that the massive black holes, which exist in the center of every galaxy in the universe, were formed the same time as their galaxy.