Sagittarius A*
Sagittarius A*, abbreviated as Sgr A* (/ ˈ s æ dʒ ˈ eɪ s t ɑːr / SADGE - AY -star), is the supermassive black hole at the Galactic Center of the Milky Way. Viewed from Earth, it is located near the border of the constellations Sagittarius and Scorpius, about 5.6° south of the ecliptic, visually close to the Butterfly Cluster (M6) and Lambda Scorpii.

The object is a bright and very compact astronomical radio source. The name Sagittarius A* distinguishes the compact source from the larger (and much brighter) Sagittarius A (Sgr A) region in which it is embedded. Sgr A* was discovered in 1974 by Bruce Balick and Robert L. Brown, and the asterisk * was assigned in 1982 by Brown, who understood that the strongest radio emission from the center of the galaxy appeared to be due to a compact non-thermal radio object.

The observations of several stars orbiting Sagittarius A*, particularly star S2, have been used to determine the mass and upper limits on the radius of the object. Based on mass and increasingly precise radius limits, astronomers have concluded that Sagittarius A* must be the central supermassive black hole of the Milky Way galaxy. The current best estimate of its mass is 4.297 ± 0.012 million solar masses.

Reinhard Genzel and Andrea Ghez were awarded the 2020 Nobel Prize in Physics for their discovery that Sagittarius A* is a supermassive compact object, for which a black hole was the only plausible explanation at the time.

In May 2022, astronomers released the first image of the accretion disk around the horizon of Sagittarius A*, confirming it to be a black hole, using the Event Horizon Telescope, a world-wide network of radio observatories. This is the second confirmed image of a black hole, after Messier 87's supermassive black hole in 2019. The black hole itself is not seen, only nearby objects whose behavior is influenced by the black hole. The observed radio and infrared energy emanates from gas and dust heated to millions of degrees while falling into the black hole.
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