Sagittarius A*
Sagittarius A* (/ˈeɪ stɑːr/ AY star), abbreviated Sgr A* (/ˈsædʒ ˈeɪ stɑːr/ SAJ AY star[3]) is the supermassive black hole[4][5][6] 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,[7] visually close to the Butterfly Cluster (M6) and Lambda Scorpii.
| Observation data Epoch J2000 Equinox J2000 | |
|---|---|
| Constellation | Sagittarius |
| Right ascension | 17h 45m 40.0409s |
| Declination | −29° 0′ 28.118″[1] |
| Details | |
| Mass | 8.26×1036 kg (4.154±0.014)×106[2] M☉ |
| Astrometry | |
| Distance | 26,673±42[2] ly (8,178±13[2] pc) |
| Database references | |
| SIMBAD | data |
The object is a bright and very compact astronomical radio source. The name Sagittarius A* follows from historical reasons. In 1954,[8] 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. The sources were arranged by constellation and the letter assigned to them was arbitrary, with A denoting the brightest radio source within the constellation. The asterisk * is because its discovery was considered "exciting",[9] in parallel with the nomenclature for excited state atoms which are denoted with an asterisk (e.g. the excited state of Helium would be He*). The asterisk was assigned in 1982 by Robert L. Brown,[10] 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 have concluded that Sagittarius A* must be the Milky Way's central supermassive black hole.[11] The current value of its mass is 4.154±0.014 million solar masses.[2]
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.[12]
On May 12, 2022, astronomers, using the Event Horizon Telescope, released the first image of the accretion disk around the horizon of Sagittarius A* produced using a world-wide network of radio observatories made in April 2017,[13] confirming the object to be a black hole. This is the second confirmed image of a black hole, after Messier 87's supermassive black hole in 2019.[14][15]