Scientists Photographed the Milky Way's Four-Million-Solar-Mass Black Hole in 2022

On May 12, 2022, astronomers released a photograph that no human eye had ever seen: a glowing orange ring of superheated gas surrounding a dark central void, 26,673 light-years from Earth. That void is Sagittarius A* (pronounced "Sagittarius A-star"), the supermassive black hole anchoring the center of the Milky Way. The image, produced by the Event Horizon Telescope (EHT) collaboration and published across six simultaneous papers in The Astrophysical Journal Letters, confirmed decades of indirect evidence and gave physicists their first direct visual anchor for the object that governs the rotational fate of our entire galaxy.

The confirmation did not arrive without preamble. Beginning in 1995, two independent research teams, one led by Reinhard Genzel at the Max Planck Institute for Extraterrestrial Physics, the other by Andrea Ghez at UCLA, began painstakingly tracking individual stars near the galactic center using infrared adaptive optics at the European Southern Observatory's Very Large Telescope and the Keck Observatory in Hawaii. Over the following decades they mapped complete orbits. The stars were not drifting; they were racing around an invisible point at speeds that left only one explanation. In 2020, Genzel and Ghez shared the Nobel Prize in Physics for this work.

The Numbers

• Mass: 4.297 million solar masses, derived from stellar orbital dynamics
• Distance from Earth: 26,673 light-years (approximately 8,178 parsecs)
• Schwarzschild radius: roughly 12 million kilometers, smaller than the orbit of Mercury around the Sun
• Angular size from Earth: about 50 microarcseconds, equivalent to seeing a donut on the surface of the Moon
• Star S2 orbital period: approximately 16 years; at closest approach in 2018 it passed just 17 light-hours from the black hole
• S2 peak velocity: 7,650 km/s, roughly 2.55% of the speed of light
• Star S62 orbital period: estimated at just 9.9 years, one of the tightest known stellar orbits around any black hole

How They Imaged It

The EHT is not a single instrument. It is a technique called Very Long Baseline Interferometry (VLBI), in which multiple radio telescopes separated by thousands of kilometers observe the same target simultaneously, recording data on atomic-clock-synchronized hard drives. Those drives are then physically shipped to correlation centers where the signals are combined. The result is a virtual dish with a resolving power equivalent to a telescope spanning the full diameter of Earth.

For the Sagittarius A* image, eight observatories participated: the Atacama Large Millimeter/submillimeter Array (ALMA) and the Atacama Pathfinder Experiment (APEX) in Chile, the IRAM 30-meter telescope in Spain, the James Clerk Maxwell Telescope and Submillimeter Array in Hawaii, the Large Millimeter Telescope in Mexico, the South Pole Telescope in Antarctica, and the Submillimeter Telescope in Arizona. All observed at a wavelength of 1.3 millimeters during coordinated campaigns in April 2017. The EHT had previously imaged M87*, the supermassive black hole in the galaxy Messier 87, releasing that image in 2019.

What the Image Confirms

The ring structure seen in the 2022 image matches predictions from general relativity with remarkable precision. Light bending around the black hole's intense gravity creates a photon ring, the bright annulus surrounding the shadow of the event horizon. The size of that shadow is set entirely by the black hole's mass and spin, and the observed angular diameter of roughly 50 microarcseconds is fully consistent with a 4.297-million-solar-mass object at the measured distance.

Beyond confirming Einstein's equations in the most extreme gravitational environment accessible to us, the Sagittarius A* image provides a calibration baseline for studying supermassive black holes throughout the universe. Galactic centers are now understood to host these objects almost universally, and the feeding and feedback processes around them regulate star formation across cosmic time.