The first image of the supermassive black hole at the center of our own Milky Way galaxy has been released by astronomers. This result provides overwhelming evidence that the object is a black hole and provides valuable insights into the workings of such massive objects, which are thought to reside at the center of most galaxies.
Today, astronomers revealed the first image of the supermassive black hole at the center of our own Milky Way galaxy at simultaneous press conferences around the world, including at the European Southern Observatory (ESO) headquarters in Germany.
This result provides overwhelming evidence that the object is a black hole and provides valuable insights into the workings of such massive objects, which are thought to reside at the center of most galaxies. The image was created by the Event Horizon Telescope (EHT) Collaboration, a global research team that used observations from a global network of radio telescopes.
The image is a long-awaited look at the massive object at the heart of our galaxy. Previously, scientists observed stars orbiting something invisible, compact, and massive at the center of the Milky Way. This strongly suggested that Sagittarius A* (Sgr A*, pronounced "sage-ay-star") is a black hole, and today's image provides the first direct visual evidence of it.
Although we cannot see the black hole because it is completely dark, the glowing gas surrounding it reveals a distinct signature: a dark central region (called a shadow) surrounded by a bright ring-like structure. The new view captures light being bent by the black hole's powerful gravitational pull, which is four million times the mass of our sun.
"We were surprised how well the ring's size fit the predictions of Einstein's theory of general relativity," said EHT project scientist Jeffrey Bauer of the Academia Sinica Institute of Astronomy and Astrophysics in Taipei. "These unprecedented observations have greatly improved our understanding of what is happening at the heart of our galaxy and provided new insights into how these massive black holes interact with their surroundings." The EHT team's findings were published today in a special issue of The Astrophysical Journal Letters.
Because the black hole is about 27 000 light-years away from Earth, it appears to us to be about the size of a doughnut on the Moon in the sky.
To visualize it, the team developed the powerful EHT, which linked eight existing radio observatories from around the world to form a single "Earth-sized" virtual telescope . The EHT observed Sgr A* on multiple nights in 2017, collecting data for many hours in a row, much like a camera with a long exposure time.
The EHT network of radio observatories includes, among other things, the Atacama Large Millimeter/submillimeter Array (ALMA) and the Atacama Pathfinder EXperiment (APEX) in Chile's the Atacama Desert, which is co-owned and operated by ESO on behalf of its European member states.
Other radio observatories in Europe contribute to EHT observations, including the IRAM 30-meter telescope in Spain and, since 2018, the Northern Extended Millimeter Array (NOEMA) in France, as well as a supercomputer hosted by the Max Planck Institute for Radio Astronomy in Germany. Furthermore, the European Research Council and the Max Planck Society in Germany contributed funding to the EHT consortium project.
"It is very exciting that ESO has played such an important role in unraveling mysteries of black holes, in particular, Sgr A*, for so many years," said ESO General Manager Xavier Barcons. "ESO has not only contributed to EHT observations through the ALMA and APEX facilities but has also enabled some previous paranormal observations of the Galactic Center with its other observatories in Chile."" 
The EHT achievement comes on the heels of the collaboration's 2019 release of the first image of M87*, a black hole at the center of the more distant Messier 87 galaxy.
The two black holes appear strikingly similar, despite the fact that our galaxy's black hole is thousands of times smaller and less massive than M87* .
"We have two completely different types of galaxies and two very different black hole masses, but they look amazingly similar close to the edge of these black holes," says Sera Markoff, Co-Chair of the EHT Science Council and a professor of theoretical astrophysics at the University of Amsterdam in the Netherlands.
"This tells us that general relativity governs these things closely, and any differences we see from a distance must be due to differences in the material surrounding black holes."
Even though Sgr A* is much closer to us, this achievement was significantly more difficult than M87*. "The gas in the vicinity of the black holes moves at the same speed — nearly as fast as light — around both Sgr A* and M87*," says EHT scientist Chi-Kwan ('CK') Chan of the University of Arizona's Steward Observatory and Department of Astronomy and the Data Science Institute.
Whereas gas takes days to weeks to orbit the larger M87*, it completes an orbit in minutes in the much smaller Sgr A*. This means that the brightness and pattern of the gas around Sgr A* were changing rapidly as the EHT Collaboration observed it, similar to trying to capture a clear image of a puppy chasing its tail."
The researchers had to create sophisticated new tools to account for the movement of gas around Sgr A*. While M87* was a simpler, steadier target, with nearly all images looking the same, Sgr A* was not. The image of the Sgr A* black hole is an average of the different images extracted by the team, revealing for the first time the giant lurking at the center of our galaxy.
The EHT Collaboration, which consists of more than 300 researchers from 80 institutes around the world, made the effort possible. In addition to developing complex tools to overcome the challenges of imaging Sgr A*, the team worked tirelessly for five years, combining and analyzing their data on supercomputers while compiling an unprecedented library of simulated black holes to compare with the observations.
Scientists are especially excited because they now have images of two black holes of very different sizes, which allows them to understand how they compare and contrast. They've also started using the new data to put theories and models about how gas behaves around supermassive black holes to the test. This process is not fully understood, but it is thought to play an important role in galaxies' formation and evolution.
"We can now study the differences between these two supermassive black holes to gain valuable new insights into how this important process works," said EHT scientist Keiichi Asada of Academia Sinica's Institute of Astronomy and Astrophysics in Taipei.
"We have images for two black holes — one at the large end and one at the small end of the Universe's supermassive black holes — so we can go much further than ever before in testing how gravity behaves in these extreme environments."
The EHT is still progressing: a major observation campaign in March 2022 will include more telescopes than ever before. In the near future, scientists will be able to share even more impressive images and movies of black holes thanks to the ongoing expansion of the EHT network and significant technological upgrades.