Sagittarius

Sagittarius A*, abbreviated as Sgr A* ( 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. Sagittarius A* is a bright and very compact astronomical radio source.

In May 2022, astronomers released the first image of the accretion disk around the event horizon of Sagittarius A*, 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; as light is incapable of escaping the immense gravitational force of a black hole, only nearby objects whose behavior is influenced by the black hole can be observed. The observed radio and infrared energy emanates from gas and dust heated to millions of degrees while falling into the black hole.

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 embedded in a larger, and much brighter, radio source, Sagittarius A (Sgr A).

The observation 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 the mass and the precise radius limits obtained, astronomers concluded that Sagittarius A* was 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 each awarded a quarter share in 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 explanation. Sir Roger Penrose received the other half "for the discovery that black hole formation is a robust prediction of the general theory of relativity".