Sagittarius A* (pronounced: Sagittarius A Star; abbreviated: Sgr A*; a region in the constellation Sagittarius) is a source of radio waves at the center of the Milky Way. According to the current state of radio astronomical research, this is a supermassive black hole of 4.154 ± 0.014 million solar masses, which is 26,673 ± 42 light-years (8,178 ± 13 pc) from Earth. At 24.5 million km, the diameter of its event horizon corresponds to about 21% of the orbital diameter of Mercury in its orbit around the Sun. With the discovery of the Fermi bubbles in 2010 and their possible counterparts in X-ray light, the "eROSITA bubbles" in 2019, there are indications that Sagittarius A* had been active a long time ago and the Milky Way could therefore have been an active galaxy.
In 1932, while systematically investigating a disruption in a transatlantic radio link, Karl Jansky found a strong radio source in the constellation Sagittarius (lat. Sagittarius). On February 13 and 15, 1974, astronomers Bruce Balick and Robert Hanbury Brown discovered Sagittarius A* there using the interferometer at the National Radio Astronomy Observatory. The asterisk "*" was added by Robert Brown to the Latin name of the constellation Sagittarius, analogous to excited-state notation in atomic physics. He suspected that the radio source stimulated its surroundings to emit radiation. The name stuck, even if Brown's guess turned out to be wrong. Independent teams led by Andrea Ghez at the Keck Observatory and Reinhard Genzel at the La Silla Observatory and the Very Large Telescope succeeded in discovering the supermassive black hole in this region over several years of observations from the 1990s. Both received the 2020 Nobel Prize in Physics for this discovery.
The Event Horizon Telescope (EHT) presented images of the supermassive black hole in M87 taken two years earlier in April 2019, showing its shadow. Corresponding images of Sagittarius A* with the EHT were already being processed at that time. Images released in 2019 from a global interconnect of radio telescopes including ALMA in the Global Millimeter VLBI Array (GMVA) observing at 3.5 mm wavelength show a glowing disc and not yet a shadow like M87. The center source had a very symmetrical nearly circular shape and a major axis of 120 microarcseconds. Even better resolution was expected from the EHT observations (at 1.3 mm wavelength), where the effect of interstellar scattering is even smaller and can be corrected. A comparison with computer simulations (Sara Issaoun and Monika Mościbrodzka from the Radboud University Nijmegen) favored accretion disks for the source, but a jet - as observed for example at M 87 - could not be completely ruled out, but it would have to be fairly close to Earth demonstrate.
The actual goal of the EHT was to observe the black hole in the center of the Milky Way, which is why a large number of telescopes were installed and included in the southern hemisphere. On May 12, 2022, the Event Horizon Telescope released an image of Sagittarius A* for the first time, confirming that the object contains a black hole. As with the previously acquired image of the black hole in M87, it shows a central shadow with a bright surrounding ring (diameter around 52 microarcseconds as the angle of view from Earth), which is in good agreement with General Relativity (GR) predictions due to diffraction of light from the surroundings - the diameter is in good agreement with the predictions of the ART, which result from the already known mass. It should be emphasized again that it is only about the observation of the shadow, the event horizon has only e according to the ART according to the mass of the black hole