Sagittarius A*

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Sagittarius A*
Sagittarius A*.jpg
Sgr A* (center) and two light echoes from a recent explosion (circled)
Observation data
Epoch J2000      Equinox J2000
Constellation Sagittarius
Right ascension  17h 45m 40.0409s
Declination −29° 0′ 28.118″[1]


(4.02±0.16)×106[4] M
8178 ± 13(stat.) ± 22(sys.)[5] pc
Database references

Sagittarius A* (pronounced "Sagittarius A-Star", abbreviated Sgr A*) is a bright and very compact astronomical radio source at the center of the Milky Way, near the border of the constellations Sagittarius and Scorpius. It is likely the location of a supermassive black hole,[6][7][8] similar to those generally accepted to be at the centers of most if not all spiral and elliptical galaxies.

Observations of a number of stars orbiting around Sagittarius A*, most notably the star S2, have been used to provide evidence for the presence of, and produce data about, the Milky Way's hypothesized central supermassive black hole, and have led some scientists to conclude that Sagittarius A* is beyond any reasonable doubt the site of that black hole.[9]

Observation and description[edit]

ALMA observations of molecular-hydrogen-rich gas clouds.[10]

Astronomers have been unable to observe Sgr A* in the optical spectrum because of the effect of 25 magnitudes of extinction by dust and gas between the source and Earth.[11] Several teams of researchers have attempted to image Sgr A* in the radio spectrum using very-long-baseline interferometry (VLBI).[12] The current highest-resolution (approximately 30 μas) measurement, made at a wavelength of 1.3 mm, indicated an overall angular size for the source of 50 μas.[13] At a distance of 26,000 light-years, this yields a diameter of 60 million kilometres. For comparison, Earth is 150 million kilometres from the Sun, and Mercury is 46 million kilometres 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.[14]

In 2017, direct radio images were taken of Sagittarius A* and M87* by the Event Horizon Telescope.[15][16] The Event Horizon Telescope uses interferometry to combine images taken from widely spaced observatories at different places on Earth in order to gain a higher picture resolution. It is hoped the measurements will test Einstein's theory of relativity more rigorously than has previously been done. If discrepancies between the theory of relativity and observations are found, scientists may have identified physical circumstances under which the theory breaks down.[17]

In 2019, measurements made with the High-resolution Airborne Wideband Camera-Plus (HAWC+) revealed that magnetic fields cause the surrounding ring of gas and dust, temperatures of which range from −280 °F (−173.3 °C) to 17,500 °F (9,700 °C),[18] to flow into an orbit around Sagittarius A*, keeping the black hole at bay.[19]


Karl Jansky, considered a father of radio astronomy, discovered in August 1931 that a radio signal was coming from a location at the center of the Milky Way, in the direction of the constellation of Sagittarius;[20] the radio source later became known as Sagittarius A. 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 astronomers Bruce Balick and Robert Brown using the baseline interferometer of the National Radio Astronomy Observatory.[21][22] 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.[23][24]

Detection of an unusually bright X-ray flare from Sgr A*[25]

On October 16, 2002, an international team led by Reinhard Genzel of 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.[26] The observations of S2 used near-infrared (NIR) interferometry (in the K-band, i.e. 2.2 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.

Dusty cloud G2 passes the supermassive black hole at the center of the Milky Way[27]

The VLBI radio observations of Sagittarius A* could also be aligned centrally with the images so S2 could be seen to orbit Sagittarius A*. From examining the Keplerian orbit of S2, they determined the mass of Sagittarius A* to be 2.6±0.2 million solar masses, confined in a volume with a radius no more than 17 light-hours (120 AU).[28] 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 (45 AU) or about 6.7 billion kilometres.[3] S175 passed within a similar distance.[29] For comparison, the Schwarzschild radius is 0.08 AU. 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 (8.0±0.6)×103 parsecs. 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 4.31±0.38 million solar masses. The result was announced in 2008 and published in The Astrophysical Journal in 2009.[2] 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."[30]

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.[25]

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.[31]

Supernova remnant ejecta producing planet-forming material

Central black hole[edit]

NuSTAR has captured these first, focused views of the supermassive black hole at the heart of the Milky Way in high-energy X-rays
A computer simulation of the 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 130 metres 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*.[32][33]

In July 2018, it was reported that S2 orbiting Sgr A* had been recorded at 7,650 km/s, or 2.55% the speed of light, leading up to the pericenter approach, in May 2018, at about 120 AU (approximately 1,400 Schwarzschild radii) from Sgr A*. At that close distance to the black hole, Einstein's theory of general relativity (GR) 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 GR prediction within the 10 percent measurement precision.[34][35]

Assuming that general relativity is still a valid description of gravity near the event horizon, 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.[13] 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. 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.[13]

Recent lower resolution observations revealed that the radio source of Sagittarius A* is symmetrical.[36] Simulations of alternative theories of gravity depict results that may be difficult to distinguish from GR.[37] However, a 2018 paper predicts an image of Sagittarius A* that is in agreement with recent observations; in particular, it explains the small angular size and the symmetrical morphology of the source.[38][39]

The mass of Sagittarius A* has been estimated in two different ways:

  1. 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 4.31±0.38 million solar masses,[2] whereas the American group found 3.7±0.2 million solar masses.[3] Given that this mass is confined inside a 44-million-kilometre-diameter sphere, this yields a density ten times higher than previous estimates.
  2. More recently, measurement of the proper motions of a sample of several thousand stars within approximately one parsec from the black hole, combined with a statistical technique, has yielded both an estimate of the black hole's mass at 3.6+0.2
    M, plus a distributed mass in the central parsec amounting to (1±0.5)×106 M.[40] The latter is thought to be composed of stars and stellar remnants.

The comparatively small mass of this supermassive black hole, along with the low luminosity of the radio and infrared emission lines, imply that the Milky Way is not a Seyfert galaxy.[11]

Ultimately, what is seen is not the black hole itself, but observations that are consistent only if there is a black hole present near Sgr A*. In the case of such a black hole, the observed radio and infrared energy emanates from gas and dust heated to millions of degrees while falling into the black hole.[32] The black hole itself is thought to emit only Hawking radiation at a negligible temperature, on the order of 10−14 kelvin.

MagnetarSGR J1745-2900
Magnetar found very close to the supermassive black hole, Sagittarius A*, at the center of the Milky Way galaxy

The European Space Agency's gamma-ray observatory INTEGRAL observed gamma rays interacting with the nearby giant molecular cloud Sagittarius B2, causing X-ray emission from the cloud. 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.[41][42] In 2011 this conclusion was supported by Japanese astronomers observing the Milky Way's center with the Suzaku satellite.[43]

Orbiting stars[edit]

Inferred orbits of 6 stars around supermassive black hole candidate Sagittarius A* at the Milky Way's center[44]
Orbital parameters of stars orbiting Sagittarius A*[45]
Star Alias a (") a (AU) e P (years) T0 (date) Ref
S1 S0–1 0.412±0.024 3300±190 0.358±0.036 94.1±9.0 2002.6±0.6 [44]
S2 S0–2 0.1226±0.0025 980±20 0.8760±0.0072 15.24±0.36 2002.315±0.012 [44]
919±23 0.8670±0.0046 14.53±0.65 2002.308±0.013 [46]
S8 S0–4 0.329±0.018 2630±140 0.927±0.019 67.2±5.5 1987.71±0.81 [44]
S12 S0–19 0.286±0.012 2290±100 0.9020±0.0047 54.4±3.5 1995.628±0.016 [44]
1720±110 0.833±0.018 37.3±3.8 1995.758±0.050 [46]
S13 S0–20 0.219±0.058 1750±460 0.395±0.032 36±15 2006.1±1.4 [44]
S14 S0–16 0.225±0.022 1800±180 0.9389±0.0078 38±5.7 2000.156±0.052 [44]
1680±510 0.974±0.016 36±17 2000.201±0.025 [46]
S0–102 S0–102 0.68±0.02 11.5±0.3 2009.5±0.3 [47]

Discovery of G2 gas cloud on an accretion course[edit]

First noticed as something unusual in images of the center of the Milky Way in 2002,[48] 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.[49] Predictions of its orbit suggested it would make its closest approach to the black hole (a perinigricon) in early 2014, when the cloud was at a distance of just over 3,000 times the radius of the event horizon (or ≈260 AU, 36 light-hours) from the black hole. G2 has been observed to be disrupting since 2009,[49] 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. Other astronomers suggested the gas cloud could be hiding a dim star, or a binary star merger product, which would hold it together against the tidal forces of Sgr A*, allowing the ensemble to pass by without any effect.[50] In addition to the tidal effects on the cloud itself, it was proposed in May 2013[51] 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.[52]

The average rate of accretion onto Sgr A* is unusually small for a black hole of its mass[53] 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. Several astronomical facilities observed this closest approach, with observations confirmed with Chandra, XMM, EVLA, INTEGRAL, Swift, Fermi and requested at VLT and Keck.[54]

Simulations of the passage were made before it happened by groups at ESO[55] and Lawrence Livermore National Laboratory (LLNL).[56]

As the cloud approached the black hole, Dr. 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.[57]

Nothing was observed during and after the closest approach of the cloud to the black hole, which was described as a lack of "fireworks" and a "flop".[58] Astronomers from the UCLA Galactic Center Group published observations obtained on March 19 and 20, 2014, concluding that G2 was still intact (in contrast to predictions for a simple gas cloud hypothesis) and that the cloud was likely to have a central star.[59]

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. Supporting this hypothesis, G1, a cloud that passed near the black hole 13 years ago, had an orbit almost identical to G2, consistent with both clouds, and a gas tail thought to be trailing G2, all being denser clumps within a large single gas stream.[58][60]

Professor Andrea Ghez et al. suggested in 2014 that G2 is not a gas cloud but rather a pair of binary stars that had been orbiting the black hole in tandem and merged into an extremely large star.[50][61]

Sgr A* is monitored on a daily basis by the X-ray telescope of the Swift satellite.

Artist impression of the accretion of gas cloud G2 onto Sgr A*. Credit: ESO.[62]
This simulation shows a gas cloud, discovered in 2011, as it passes close to the supermassive black hole at the center of the Milky Way
This video sequence shows the motion of the dusty cloud G2 as it closes in on, and then passes, the supermassive black hole at the center of the Milky Way

See also[edit]


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