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Concept
Sagittarius A*
Summary
Sagittarius A* (ˈeɪ_stɑːr ), abbreviated Sgr A* (ˈsædʒ_ˈeɪ_stɑːr ), is the supermassive black hole at the Galactic Center of the Milky Way. 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* follows from historical reasons. In 1954, John D. Kraus, Hsien-Ching Ko, and Sean Matt listed the radio sources they identified with the Ohio State University radio telescope at 250 MHz. They arranged these sources by constellation and then assigned capital letters in order of brightness within each constellation, with A denoting the brightest radio source within the constellation. The asterisk is a later addition and was added because its discovery was considered "exciting", in parallel with the nomenclature for excited state atoms which are denoted with an asterisk (for example, the excited state of helium would be He*). The asterisk was assigned in 1982 by Robert L. Brown, who understood that the strongest radio emission from the center of the galaxy appeared to be due to a compact nonthermal 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 concluded that Sagittarius A* must be the Milky Way's central supermassive black hole. The current value of its mass is 4.154 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.
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