Sagittarius A*, or Sgr A*, is a massive black hole at the center of the Milky Way (Credit: EHT Collaboration)

On May 12, 2022, an international team of astronomers led by scientists at the Harvard—Smithsonian Center for Astrophysics revealed the first image of a supermassive black hole at the center of the Milky Way. Researchers have known about the black hole, called Sagittarius A*, or Sgr A* (pronounced sadge-ay-star), since the 1970s. However, until now, its existence had only been deduced from its effect on surrounding stars and dust. The stunning picture is the first direct visual evidence of our galaxy's black hole.

The image does not show the black hole itself. As the name suggests, they are black and 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. Known as the "event horizon," it is where the light bends due to the black hole's powerful gravity.

Sgr A* lies about 27,000 light-years away. It is believed to have a mass of about 4 million times that of our Sun. The black hole was imaged 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.

“It is notoriously difficult to reconstruct images from a widely dispersed array like the EHT, and both rigor and ingenuity have been required to properly understand and quantify uncertainties,” says Colin Lonsdale, director of MIT’s Haystack Observatory. “The result is a milestone in our understanding of black holes in general and the one at the center of our galaxy in particular.

A comparison of the images of the M87 black hole (L) and the Sgr A* black hole (R) (Credit: EHT Collaboration)

This is the second time the team has obtained direct evidence of the existence of a black hole. In 2019, the scientists revealed M87, an even larger black hole at the center of the Virgo Galaxy cluster. Located about 55 million light-years away, it is believed to have a mass of 6.5 billion times that of our sun.

The scientists now plan to compare the two images and gain insights into the black holes and how they interact with their surroundings. They believe understanding the process may help them determine how galaxies form and evolve.

Black holes are regions in space where 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 vary in size and mass. The smallest, called "primordial" black holes, are as tiny as a single atom, but they have 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, like M87 and Sgr A*, are comparable in size to their galaxy. They have a mass greater than millions, or even billions, of suns combined and diameters as large as our Solar System. Researchers believe that the enormous black holes, which exist in the center of every galaxy in the universe, were formed at the same time as their galaxy.