This is it! Meet the Supermassive Black Hole at the Heart of the Milky Way
by Matt Williams

On April 10th, 2019, the international consortium known as the Event Horizon Telescope (EHT) announced the first-ever image of a supermassive black hole (SMBH). The image showed the bright disk surrounding the black hole at the center of the M87 galaxy (aka. Virgo A). In 2021, they followed up on this by acquiring an image of the core region of the Centaurus A galaxy and the radio jet emanating from it. Earlier this month, the European Southern Observatory (ESO) announced that the EHT would be sharing the results from its latest campaign – observations of Sagittarius A*!

This supermassive black hole resides at the center of the Milky Way Galaxy, roughly 27,000 light-years from Earth, 44 million km (27.34 million mi) in diameter, and has a mass of 4.31 million Suns. The campaign’s results were shared in an ESO press release and a series of live-streamed press conferences worldwide, including the ESO Headquarters in Munich, Germany. The team’s results (which were shared in six papers) were also published today in a special issue of The Astrophysical Journal Letters.


Since the 1970s, astronomers have speculated about the existence of SMBHs, which were believed to be why massive galaxies had such energetic core regions. Also known as Active Galactic Nuclei (AGNs), or “quasars,” these regions are known to temporarily outshine all of the stars contained in their disks. In some cases, jets of superheated material (relativistic jets) have also been found emanating from them at a fraction of the speed of light.


The study of SMBHs has led to new theories about how galaxies formed and evolved in our Universe and has allowed astronomers to test the laws of physics under the most extreme conditions (i.e., Einstein’s Theory of General Relativity). Until 2019 the study of these massive black holes was confined to observing their effect on the surrounding environment. In particular, astronomers have noted how the gravitational forces of SMBHs cause gas and dust to fall in around their outer edges (the Event Horizon).

This matter is then accelerated to relativistic speed and slowly be accreted onto the faces of the black hole, releasing the tremendous amounts of energy that allow AGNs to outshine their galactic disks. However, visualizing these massive objects in telescopes is extremely difficult because they are nestled within the tightly packed group of stars at the galaxy’s center (aka. the “galactic bulge”), which produces a tremendous amount of light interference.

But thanks to a technique known as Very Long Baseline Interferometry (VLBI), where telescopes worldwide combine light to achieve high-resolution imaging, astronomers at the EHT Collaboration created a virtual telescope with an aperture equivalent in size to the Earth. When concentrated on an object that is difficult to resolve, this telescope can gather light over time and reconstruct an image of what it looks like (similar to a long exposure time with a camera).

This allowed the EHT team to image the bright Event Horizon around the M87 supermassive black hole (M87*) for the first time and has led to new insights on several other SMBHs. For example, the EHT also observed Sgr A* on multiple nights in 2017 and collected data for several hours straight. The latest image shows the Event Horizon of Sagittarius A* (Sgr A* for short) and is the first definitive evidence of this SMBHs existence.


The Atacama Large Millimeter/submillimeter Array (ALMA) looking up at the Milky Way and the location of Sagittarius A*, the supermassive black hole at our galactic center. Credit: ESO

Geoffrey Bower, an EHT Project Scientist from the Academia Sinica’s Institute of Astronomy and Astrophysics, spoke of the results in an ESO press release. “We were stunned by how well the size of the ring agreed with predictions from Einstein’s Theory of General Relativity,” he said. “These unprecedented observations have greatly improved our understanding of what happens at the very center of our galaxy and offer new insights on how these giant black holes interact with their surroundings.”

As noted, the EHT Collaboration is an international effort that includes facilities worldwide, which includes the ESO’s Atacama Large Millimeter/submillimeter Array (ALMA) and Atacama Pathfinder EXperiment (APEX) in northern Chile. ESO Director General Xavier Barcons was also part of the press conference and expressed his support for the Collaboration and its latest results.

“It is very exciting for ESO to have been playing such an important role in unraveling the mysteries of black holes, and of Sgr A* in particular, over so many years,” he said. “ESO not only contributed to the EHT observations through the ALMA and APEX facilities but also enabled, with its other observatories in Chile, some of the previous breakthrough observations of the Galactic center.”

While the image of Sagittarius A* looks remarkably similar to M87*, there are some significant differences between the two. For one, our galaxy’s SMBH is more than a thousand times smaller and less massive than M87* – about 6.5 billion times as massive as our Sun. However, the edges of both black holes look very similar, which the team concludes is the result of General Relativity governing these objects up close. At the same time, differences seen further away are due to differences in the material surrounding them.


Furthermore, imaging Sgr A* was far more difficult than imaging M87*, despite being much closer to Earth. As EHT scientist Chi-kwan Chan, from the Steward Observatory and the University of Arizona Data Science Institute (UoA DSI), explained:

“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*. But where gas takes days to weeks to orbit the larger M87*, in the much smaller Sgr A* it completes an orbit in mere minutes. This means the brightness and pattern of the gas around Sgr A* were changing rapidly as the EHT Collaboration was observing it – a bit like trying to take a clear picture of a puppy quickly chasing its tail.”
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In short, M87* was a steadier target than Sgr A*, where nearly all of the images acquired looked the same because its rapid velocity created the appearance of uniformity. In contrast, the images of Sgr A* the team extracted were markedly different, owing to the slower velocity of matter in the accretion disk. The EHT researchers had to develop new tools to account for this and provide an average of all the different images.

To achieve this, the team worked rigorously for five years, using supercomputers to combine and analyze their data and creating an unprecedented library of simulated black holes to compare with the observations. Because of this,s scientists will now have images of two black holes with very different sizes and offers, which offers opportunities to test the laws of physics under different domains. Scientists have already begun to use the data to test theories and models of how gas behaves around SMBHs.


While this process is not yet well-understood, it is thought to play a key role in the formation and evolution of galaxies. Said EHT scientist Keiichi Asada from the Academia Sinica’s Institute of Astronomy and Astrophysics in Taipei:

“Now we can study the differences between these two supermassive black holes to gain valuable new clues about how this important process works. We have images for two black holes – one at the large end and one at the small end of supermassive black holes in the Universe – so we can go a lot further in testing how gravity behaves in these extreme environments than ever before.”
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The results of the EHT Collaboration’s observation campaign (contained in six studies) were also shared in a special issue of The Astrophysical Journal Letters. Whereas the first paper presents a summary of their overall findings, papers II, III, and IV demonstrate how the lower mass and shorter dynamical scale of Sgr A* led to complexity with the imaging and data analysis. In Paper V, researchers combine the EHT results with extensive multi-wavelength constraints to explore the accretion and outflow physics of Sgr A*.

In Paper VI, researchers present infrared measurements of stellar orbits around Sgr A* to constrain the mass, distance, and ring diameter of Sgr A* to approximately 1% accuracy, enabling precision explorations of gravitational physics. Additional papers not performed by the EHT collaboration are also provided that address the possibility of dynamical VLBI imaging, a new analysis of the data obtained by the 2017 campaign, evaluating simulated images of black holes, and black-hole image reconstruction.


These results were made possible thanks to the over 300 scientists and 80 institutions worldwide that make up the EHT Collaboration. These include the Caltech Submillimeter Observatory (CSO) on the summit of Manua Kea, Hawaii; the Combined Array for Research in Millimeter-wave Astronomy (CARMA) in California; the Kitt Peak National Observatory (KPNO), and the ARO Submillimeter Telescope (SMT) in Arizona; and the Gran Telescopio Milimétrico Alfonso Serrano in Mexico.

Beyond North America, there’s also the NSF’s South Pole Telescope (SPT) at the Amundsen–Scott South Pole Station, Antarctica, and the Institute Radioastronomie Millimetrique‘s (IRAM) 30-meter telescope and NOrthern Extended Millimeter Array (NOEMA) radio telescopes in Spain and France. Additional support is provided by the Max Planck Institute for Radio Astronomy in Germany, which relies on its supercomputer to combine EHT data from multiple observatories.

Further Reading: ESO

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