Sagittarius A*, abbreviated as Sgr A* ( ), 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,Calculated using Equatorial and Ecliptic Coordinates calculator 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,Melia 2007, p. 7 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 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".
Observation and description
On May 12, 2022, the first image of Sagittarius A* was released by the Event Horizon Telescope Collaboration. The image, which is based on radio interferometer data taken in 2017, confirms that the object contains a black hole. This is the second image of a black hole, and took five years of calculations to process. The data were collected by eight radio observatories at six geographical sites. Radio images are produced from data by aperture synthesis, usually from night-long observations of stable sources. The radio emission from Sgr A* varies on the order of minutes, complicating the analysis.
Their result gives an overall angular size for the source of . At a distance of , this yields a diameter of . For comparison, Earth is from the Sun, and Mercury is from the Sun at perihelion. The proper motion of Sgr A* is approximately −2.70 mas per year for the right ascension and −5.6 mas per year for the declination.Backer and Sramek 1999, § 3 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.
In 2019, measurements made with the High-resolution Airborne Wideband Camera-Plus (HAWC+) mounted in the SOFIA aircraft revealed that magnetic fields cause the surrounding ring of gas and dust, temperatures of which range from , to flow into an orbit around Sagittarius A*, keeping black hole emissions low.
Astronomers have been unable to observe Sgr A* in the visible spectrum because of the effect of 25 magnitudes of extinction (absorption and scattering) by dust and gas between the source and Earth.Osterbrock and Ferland 2006, p. 390
History In April 1933, Karl Jansky, considered one of the fathers of radio astronomy, discovered that a radio signal was coming from a location in the direction of the constellation of Sagittarius, towards the center of the Milky Way. The radio source later became known as Sagittarius A. His observations did not extend quite as far south as the point we now know to be the Galactic Center. Observations by Jack Piddington and Harry Minnett, using the CSIRO radio telescope at Potts Hill Reservoir in Sydney, discovered a discrete and bright "Sagittarius-Scorpius" radio source, which after further observation with the CSIRO radio telescope at Dover Heights was identified in a letter to Nature as the probable Galactic Center.
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. The name Sgr A* was coined by Brown in a 1982 paper because the radio source was "exciting", and excited states of atoms are denoted with asterisks.
Since the 1980s, it has been evident that the central component of Sgr A* is likely a black hole. In 1994, infrared and sub-millimetre spectroscopy studies by a Berkeley team involving Nobel Laureate Charles H. Townes and future Nobel Prize Winner Reinhard Genzel showed that the mass of Sgr A* was tightly concentrated and on the order of 3 million Suns.
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. The observations of S2 used near-infrared (NIR) interferometry (in the Ks-band, i.e. 2.1 μm) because of reduced interstellar extinction in this band. SiO masers were used to align NIR images with radio observations, as they can be observed in both NIR and radio bands. The rapid motion of S2 (and other nearby stars) easily stood out against slower-moving stars along the line-of-sight so these could be subtracted from the images.Schödel et al. 2002
The VLBI radio observations of Sagittarius A* could also be aligned centrally with the NIR images, so the focus of S2's elliptical orbit was found to coincide with the position of Sagittarius A*. From examining the Keplerian orbit of S2, they determined the mass of Sagittarius A* to be solar masses, confined in a volume with a radius no more than 17 light-hours ().Ghez et al. (2003) "The First Measurement of Spectral Lines in a Short-Period Star Bound to the Galaxy's Central Black Hole: A Paradox of Youth" Astrophysical Journal 586 L127 Later observations of the star S14 showed the mass of the object to be about 4.1 million solar masses within a volume with radius no larger than 6.25 light-hours (). S175 passed within a similar distance. For comparison, the Schwarzschild radius is . They also determined the distance from Earth to the Galactic Center (the rotational center of the Milky Way), which is important in calibrating astronomical distance scales, as . 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*. This black hole of 1,300 solar masses is within a cluster of seven stars. This observation may add support to the idea that supermassive black holes grow by absorbing nearby smaller black holes and stars.
After monitoring stellar orbits around Sagittarius A* for 16 years, Gillessen et al. estimated the object's mass at solar masses. The result was announced in 2008 and published in The Astrophysical Journal in 2009.Gillessen et al. 2009 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 stellar orbits in the Galactic Center show that the central mass concentration of four million solar masses must be a black hole, beyond any reasonable doubt."O'Neill 2008
On January 5, 2015, NASA reported observing an X-ray flare 400 times brighter than usual, a record-breaker, from Sgr A*. 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.
On May 13, 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.
In June 2023, unexplained filaments of radio energy were found associated with Sagittarius A*.
Central black hole In a paper published on October 31, 2018, the discovery of conclusive evidence that Sagittarius A* is a black hole was announced. Using the GRAVITY interferometer and the four telescopes of the Very Large Telescope (VLT) to create a virtual telescope in diameter, astronomers detected clumps of gas moving at about 30% of the speed of light. 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. The flares are thought to originate from magnetic interactions in the very hot gas orbiting very close to Sagittarius A*.
In July 2018, it was reported that S2 orbiting Sgr A* had been recorded at , or 2.55% the speed of light, leading up to the pericenter approach, in May 2018, at about (approximately 1,400 Schwarzschild radii) from Sgr A*. At that close distance to the black hole, Einstein's theory of general relativity predicts that S2 would show a discernible gravitational redshift in addition to the usual velocity redshift. The gravitational redshift was detected, in agreement with the general relativity prediction within the 10 percent measurement precision.
The Sagittarius A* radio emissions are not centered on the black hole, but arise from a bright spot in the region around the black hole, close to the event horizon, possibly in the accretion disc, or a relativistic jet of material ejected from the disc. If the apparent position of Sagittarius A* were exactly centered on the black hole, it would be possible to see it magnified beyond its size, because of gravitational lensing of the black hole. 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). For a black hole of around 4 million solar masses, this corresponds to a size of approximately 52 μas, which is consistent with the observed overall size of about 50 μas, the size (apparent diameter) of the black hole Sgr A* itself being 20 μas.
Lower resolution observations revealed that the radio source of Sagittarius A* is symmetrical. Simulations of alternative theories of gravity depict results that may be difficult to distinguish from GR. A 2018 paper predicted an image of Sagittarius A* that is in agreement with observations. In particular, it explains the small angular size and the symmetrical morphology of the source.
The mass of Sagittarius A* has been estimated in two different ways:
Two groups—in Germany and the U.S.—monitored the orbits of individual stars very near to the black hole and used Kepler's laws to infer the enclosed mass. The German group found a mass of solar masses, whereas the American group found solar masses.