The Event Horizon Telescope captured a microwave image of the Milky Way’s central object, a supermassive black hole located 26,000 light-years from Earth in the Sagittarius region, known as Sagittarius A*. The results appear in six articles published in the Astrophysical Journal Letters.
At a private press event held at the ESO headquarters in Munich, astronomers from the European Southern Observatory and the EHT project shared new findings. ESO is an international organization uniting more than ten nations and operating powerful telescopes in the southern hemisphere, with key facilities in Chile. The Event Horizon Telescope is a global array of radio telescopes that achieves unprecedented resolution when observing the regions around supermassive black holes.
In 2019, the project produced an image of the supermassive black hole at the center of the nearby galaxy M87, referred to as M87*. The visualization showed a fuzzy dark circle encircled by a fiery orange ring, a design that drew comparisons to the Eye of Sauron from popular fantasy fiction. The distinctive ring around the shadow of the black hole provided additional support for Einstein’s General Theory of Relativity, which anticipated such a round shadow.
For the initial observations, it was crucial to select a bright, nearby object bright enough to reveal the subtle shadow. One of the giants in this field is Sagittarius A*, a supermassive black hole at the heart of our own galaxy. The EHT collaboration has now released a first composite image of Sgr A*, produced by averaging thousands of frames generated with multiple computational methods. All methods converge on the same essential features, while rarer details are suppressed. Along with the earlier image of M87*, this stands as one of the most carefully calibrated interferometric images ever produced by these researchers.
The smaller mass of the Milky Way’s black hole presents a tougher visualization and analysis task due to its reduced dynamic range. The data analysis employed black hole simulations run on powerful supercomputers around the world. When the observational data are compared with the models, it confirms that Sgr A* is spinning and that observers on Earth view it from the side rather than from above. The estimated diameter of the event horizon is about 25.4 million kilometers. Its mass is approximately 4.3 million solar masses, modest by black hole standards, since many central black holes in other galaxies measure in the billions of solar masses.
Experts emphasize that the result does not claim a discovery of something wholly unexpected; rather, it marks a technically significant milestone. Creating new data processing algorithms was essential to capturing the image, and rapid progress in a few years made high-quality results possible. Such algorithms are expected to find applications across a wide range of radio interferometry observations. A leading researcher described the achievement as a demonstration of all components working in concert and expected advances in the field of radio interferometry [Citation: SAI MGU team].
The broader implication is that general relativity remains consistent with the observations, and the supporting software, telescopes, and observational strategies are functioning well. Even clearer images are anticipated within a year as methods improve and higher-quality data become available. These steps pave the way for more precise tests of gravity and the exploration of new phenomena in black hole physics.
Of particular significance is the ability of EHT to resolve structural features in Sagittarius A*, including the photon ring that appears in the image. This ring, a gravitational lensing effect, matches the angular diameter predicted by general relativity when taking into account the black hole’s mass and distance. Interestingly, the angular size of Sgr A* is only slightly different from that of M87*, despite the latter’s mass being vastly larger and its distance far greater.
Researchers note that the photon ring marks a historic second sighting of such a feature, not a direct photograph of the black hole’s interior, but a representation larger than the disk itself. The mass estimate has been refined, expressed in Schwarzschild radii rather than finer units, underscoring the precision now possible in black hole metrics. The analysis benefited from the Global Telescope Facility’s observations in April 2017 and the collaboration of more than three hundred scientists from around the world. Since then, EHT has expanded its capabilities by adding new stations, increasing bandwidth, and operating at higher frequencies. Ongoing studies of Sgr A* and M87*, together with advances in data analysis and theoretical modeling, are expected to drive further discoveries in black hole physics and the behavior of matter in extreme gravity.