Editorial NexoScience
How do you photograph an object whose defining property is that light cannot escape it? For most of the twentieth century the honest answer was that you do not. You watch what happens around it, you measure the gravity, and you infer. That is exactly how the black hole at the center of our own galaxy was found, and for decades it remained a deduction rather than a picture.
Sagittarius A* was known long before it was ever seen. Since its discovery as a bright radio source near the Galactic Center, astronomers tracked the three dimensional orbits of individual stars swinging around an invisible point in the innermost arcsecond of the sky. Those orbits pinned down an enormous concentration of mass, roughly 4 million times the mass of the Sun, packed into a region small enough that almost nothing except a black hole could explain it. Compelling evidence, but still indirect. Nobody had an image.
The telescope that finally took the picture is the size of the planet. The Event Horizon Telescope links radio dishes scattered across the globe and synchronizes them with atomic clocks so they behave as a single Earth sized instrument. In April 2017 the array observed Sagittarius A* on five nights using eight facilities spread over six locations, from Chile and Hawai’i to Spain, Arizona, Mexico and Antarctica, all recording at a wavelength of 1.3 millimeters. The results were published in 2022, after five years of calibration and analysis.
What emerged was a bright ring wrapped around a dark center. More than 95 percent of the reconstructed images produced by independent teams using different algorithms show the same structure: a thick emission ring with a diameter of 51.8 plus or minus 2.3 microarcseconds, a fractional width of roughly 30 to 50 percent, mild brightness asymmetry, and a noticeably dim interior. That dark hollow is the black hole shadow, the region where photon paths bend so severely that they fall in rather than reach us.
The ring is exactly the size the stars said it should be. Working from the image alone, the team derived a shadow diameter of 48.7 plus or minus 7.0 microarcseconds and a mass of about 4 million solar masses, at a distance of roughly 8 kiloparsecs, about 27,000 light-years away. Those numbers, obtained from a photograph of the event horizon, agree with numbers obtained from stars orbiting a hundred thousand times farther out. Two completely different measurement techniques converging on the same object is the strongest part of the result.
Consistent with Kerr is not the same as proven beyond doubt. The paper is careful here, and so should we be. The image is consistent with the appearance of a Kerr black hole as general relativity predicts, and it disfavors several scenarios: high inclination viewing angles above 50 degrees, non spinning black holes, retrograde accretion disks, and some specific alternatives such as certain naked singularity and boson star models. But none of the 200 numerical simulations tested passed every single observational constraint, the simulations are consistently more variable than the real source, and other horizonless impostors remain compatible with the data. This is a strong consistency check, not a closed case.
Our black hole is also the twitchy one. Because Sagittarius A* is around 1,500 times less massive than M87*, imaged in 2019, its innermost material completes an orbit in 4 to 30 minutes rather than days or weeks. The source literally changes while you photograph it, which is why the analysis took so long. Comparing both images shows general relativity holding across three orders of magnitude in black hole mass.
Source: Event Horizon Telescope Collaboration, “First Sagittarius A* Event Horizon Telescope Results. I. The Shadow of the Supermassive Black Hole in the Center of the Milky Way”, The Astrophysical Journal Letters (2022). DOI: 10.3847/2041-8213/ac6674
