Sagittarius A*, abbreviated as Sgr A* (/ˈsædʒ ˈeɪ stɑːr/ SADGE-AY-star[3]), is the supermassive black hole[4][5][6] 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,[7] visually close to the Butterfly Cluster (M6) and Lambda Scorpii.

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.

[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.

The observed radio and infrared energy emanates from gas and dust heated to millions of degrees while falling into the black hole.

[20][21][22] The telescope's measurement of these black holes tested Einstein's theory of relativity more rigorously than has previously been done, and the results match perfectly.

[15] In 2019, measurements made with the High-resolution Airborne Wideband Camera-Plus (HAWC+) mounted in the SOFIA aircraft[23] revealed that magnetic fields cause the surrounding ring of gas and dust, temperatures of which range from −280 to 17,500 °F (99.8 to 9,977.6 K; −173.3 to 9,704.4 °C),[24] to flow into an orbit around Sagittarius A*, keeping black hole emissions low.

[25] Astronomers have been unable to observe Sgr A* in the optical spectrum because of the effect of 25 magnitudes of extinction (absorption and scattering) by dust and gas between the source and Earth.

[30] Later observations showed that Sagittarius A actually consists of several overlapping sub-components; a bright and very compact component, Sgr A*, was discovered on February 13 and 15, 1974, by Balick and Robert L. Brown using the baseline interferometer of the National Radio Astronomy Observatory.

[33] On October 16, 2002, an international team led by Reinhard Genzel at the Max Planck Institute for Extraterrestrial Physics reported the observation of the motion of the star S2 near Sagittarius A* throughout a period of ten years.

According to the team's analysis, the data ruled out the possibility that Sgr A* contains a cluster of dark stellar objects or a mass of degenerate fermions, strengthening the evidence for a massive black hole.

In November 2004, a team of astronomers reported the discovery of a potential intermediate-mass black hole, referred to as GCIRS 13E, orbiting 3 light-years from Sagittarius A*.

[40] Reinhard Genzel, team leader of the research, said the study has delivered "what is now considered to be the best empirical evidence that supermassive black holes do really exist.

The unusual event may have been caused by the breaking apart of an asteroid falling into the black hole or by the entanglement of magnetic field lines within gas flowing into Sgr A*, according to astronomers.

[42] On 13 May 2019, astronomers using the Keck Observatory witnessed a sudden brightening of Sgr A*, which became 75 times brighter than usual, suggesting that the supermassive black hole may have encountered another object.

Emission from highly energetic electrons very close to the black hole was visible as three prominent bright flares.

These exactly match theoretical predictions for hot spots orbiting close to a black hole of four million solar masses.

According to general relativity, this would result in a ring-like structure, which has a diameter about 5.2 times the black hole's Schwarzschild radius (10 μas).

The total luminosity from this outburst (L≈1,5×1039 erg/s) is estimated to be a million times stronger than the current output from Sgr A* and is comparable with a typical active galactic nucleus.

[74] The high velocities and close approaches to the supermassive black hole makes these stars useful to establish limits on the physical dimensions of Sagittarius A*, as well as to observe general-relativity associated effects like periapse shift of their orbits.

As of 2020[update], S4714 is the current record holder of closest approach to Sagittarius A*, at about 12.6 AU (1.88 billion km), almost as close as Saturn gets to the Sun, traveling at about 8% of the speed of light.

Tp is the epoch of pericenter passage, P is the orbital period in years and Kmag is the infrared K-band apparent magnitude of the star.

[76] First noticed as something unusual in images of the center of the Milky Way in 2002,[77] the gas cloud G2, which has a mass about three times that of Earth, was confirmed to be likely on a course taking it into the accretion zone of Sgr A* in a paper published in Nature in 2012.

G2 has been observed to be disrupting since 2009,[78] and was predicted by some to be completely destroyed by the encounter, which could have led to a significant brightening of X-ray and other emission from the black hole.

[79] In addition to the tidal effects on the cloud itself, it was proposed in May 2013[80] that, prior to its perinigricon, G2 might experience multiple close encounters with members of the black-hole and neutron-star populations thought to orbit near the Galactic Center, offering some insight to the region surrounding the supermassive black hole at the center of the Milky Way.

[81] The average rate of accretion onto Sgr A* is unusually small for a black hole of its mass[82] and is only detectable because it is so close to Earth.

It was thought that the passage of G2 in 2013 might offer astronomers the chance to learn much more about how material accretes onto supermassive black holes.

[83] Simulations of the passage were made before it happened by groups at ESO[84] and Lawrence Livermore National Laboratory (LLNL).

[85] As the cloud approached the black hole, Daryl Haggard said, "It's exciting to have something that feels more like an experiment", and hoped that the interaction would produce effects that would provide new information and insights.

[79] An analysis published on July 21, 2014, based on observations by the ESO's Very Large Telescope in Chile, concluded alternatively that the cloud, rather than being isolated, might be a dense clump within a continuous but thinner stream of matter, and would act as a constant breeze on the disk of matter orbiting the black hole, rather than sudden gusts that would have caused high brightness as they hit, as originally expected.