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]
Mass (4.31 ± 0.38) × 106[2]M
(4.1 ± 0.6) × 106[3]
(4.02 ± 0.16) × 106[4] M
Distance 7,940 ± 420[5]
7,860±140±0.40[4] pc

Sagittarius A* (pronounced "Sagittarius A-star", standard abbreviation 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 part of a larger astronomical feature known as Sagittarius A. Sagittarius A* is thought to be the location of a supermassive black hole,[6][7][8] like those that are now generally accepted to be at the centers of most spiral and elliptical galaxies. Observations of the star S2 in orbit around Sagittarius A* have been used to show the presence of, and produce data about, the Milky Way's central supermassive black hole, and have led to the conclusion that Sagittarius A* is the site of that black hole.[9]

Observation and description[edit]

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.[10] Several teams of researchers have attempted to image Sagittarius A* in the radio spectrum using very-long-baseline interferometry (VLBI).[11] The current highest-resolution measurement, made at a wavelength of 1.3 mm, indicated an angular diameter for the source of 37 μas.[12] At a distance of 26,000 light-years, this yields a diameter of 44 million kilometers. For comparison, Earth is 150 million kilometers from the Sun, and Mercury is 46 million kilometers 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.[13]


Karl Jansky was the first person to determine that a radio signal was coming from a location at the center of the Milky Way, in the direction of the constellation of Sagittarius.[14] 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.[15] 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.[16][17]

Detection of an unusually bright X-ray flare from Sgr A*.[18]

On October 16, 2002, an international team led by Rainer Schödel of the Max Planck Institute for Extraterrestrial Physics reported the observation of the motion of the star S2 near Sagittarius A* over 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.[19] The observations of S2 used near-infra red (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.[20]

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).[21] 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] 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 three 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. estimate 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 super-massive 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."[22]

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.[18]

Supernova remnant ejecta producing planet-forming material.

Central black hole[edit]

Inferred orbits of 6 stars around supermassive black hole candidate Sagittarius A* at the Milky Way's center.[23]
NuSTAR has captured these first, focused views of the supermassive black hole at the heart of the Milky Way in high-energy X-rays.

If the apparent position of Sagittarius A* were exactly centered on the black hole, it would be possible to see it magnified beyond its actual size, because of gravitational lensing. According to general relativity, this would result in a minimum observed size of at least 5.2 times the black hole's Schwarzschild radius, which, for a black hole of around 4 million solar masses, corresponds to a minimum observed size of approximately 52 μas. This is much larger than the observed size of 37 μas and so suggests that the Sagittarius A* radio emissions are not centered on the 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.[12]

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 km 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.[24] The latter is thought to be composed of stars and stellar remnants.

Astronomers are confident that these observations of Sagittarius A* provide good empirical evidence that the Milky Way has a supermassive black hole at its center, 26,000 light-years from the Solar System[9] because:

  • The star S2 follows an elliptical orbit with a period of 15.2 years and a pericenter (closest distance) of 17 light hours (1.8×1013 m) from the center of the central object.[25]
  • From the motion of star S2, the object's mass can be estimated as 4.1 million solar masses.[3] (The corresponding Schwarzschild radius is 0.08 AU/12 million km/7.4 million miles; 17 times bigger than the radius of the Sun.)
  • The volume of the central object can be further constrained by the orbit of star S0-16 (also known as S14), which came within 45 AU without colliding.
  • Estimates of mass and diameter are now constrained primarily by uncertainties in distance to the objects.

Although strictly speaking there are other mass configurations that would explain the measured mass and size, such an arrangement would collapse into a single supermassive black hole on a timescale much shorter than the age of the Milky Way.[12]

The comparatively small mass of this black hole, along with the low luminosity of the radio and infrared emission lines, imply that the Milky Way is not a Seyfert galaxy.[10]

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. Although other possibilities exist for how these gases emanate energy, such as radiation pressure and interaction with other gas streams, interaction with a massive source of gravity is the simplest explanation.[26] The black hole itself is thought to emit only Hawking radiation at a negligible temperature, on the order of 10−14 kelvin.

Orbital parameters of stars orbiting Sagittarius A*[27]
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 [23]
S2 S0–2 0.1226±0.0025 980±20 0.8760±0.0072 15.24±0.36 2002.315±0.012 [23]
919±23 0.8670±0.0046 14.53±0.65 2002.308±0.013 [28]
S8 S0–4 0.329±0.018 2630±140 0.927±0.019 67.2±5.5 1987.71±0.81 [23]
S12 S0–19 0.286±0.012 2290±100 0.9020±0.0047 54.4±3.5 1995.628±0.016 [23]
1720±110 0.833±0.018 37.3±3.8 1995.758±0.050 [28]
S13 S0–20 0.219±0.058 1750±460 0.395±0.032 36±15 2006.1±1.4 [23]
S14 S0–16 0.225±0.022 1800±180 0.9389±0.0078 38±5.7 2000.156±0.052 [23]
1680±510 0.974±0.016 36±17 2000.201±0.025 [28]
S0–102 S0–102 0.68±0.02 11.5±0.3 2009.5±0.3 [29]

The European Space Agency's gamma-ray observatory INTEGRAL has observed gamma rays interacting with the nearby giant molecular cloud Sagittarius B2, causing X-ray emission from the cloud. This energy was emitted about 350 years earlier by Sgr A*, possibly detectable from Earth around the year 1650.[citation needed] 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.[30][31] In 2011 this conclusion was supported by Japanese astronomers observing the Milky Way's center with the Suzaku satellite.[32]

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

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,[33] 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.[34] 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 3000 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,[34] 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.[35] In addition to the tidal effects on the cloud itself, it was proposed in May 2013[36] 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 into the region surrounding the supermassive black hole at the center of the Milky Way.[37]

The average rate of accretion onto Sgr A* is unusually small for a black hole of its mass[38] 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.[39]

Simulations of the passage were made before it happened by groups at ESO[40] and Lawrence Livermore National Laboratory (LLNL).[41]

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.[42]

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".[43] 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.[44]

An analysis published on 21 July 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 fireworks 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.[43]

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.[35][45]

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.[46]
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]


  1. ^ Reid and Brunthaler 2004
  2. ^ a b c Gillessen et al. 2009
  3. ^ a b c d Ghez, A. M.; et al. (December 2008). "Measuring Distance and Properties of the Milky Way's Central Supermassive Black Hole with Stellar Orbits". Astrophysical Journal. 689 (2): 1044–1062. arXiv:0808.2870Freely accessible. Bibcode:2008ApJ...689.1044G. doi:10.1086/592738. 
  4. ^ a b "An Improved Distance and Mass Estimate for Sgr A* from a Multistar Orbit Analysis". 2016-07-19. Retrieved 2016-07-21. 
  5. ^ Eisenhauer et al. 2003, § 3.1
  6. ^ Reynolds 2008
  7. ^ Overbye, Dennis (8 June 2015). "Black Hole Hunters". NASA. Retrieved 8 June 2015. 
  8. ^ Overbye, Dennis; Corum, Jonathan; Drakeford, Jason (8 June 2015). "Video: Peering Into a Black Hole". New York Times. ISSN 0362-4331. Retrieved 9 June 2015. 
  9. ^ a b Henderson, Mark (December 9, 2008). "Astronomers confirm black hole at the heart of the Milky Way". Times Online. Retrieved 2009-05-17. 
  10. ^ a b Osterbrock and Ferland 2006, p. 390
  11. ^ Falcke H, Melia F, Agol E (2000). "Viewing the Shadow of the Black Hole at the Galactic Center". ApJ Letters. 528 (1): L13–L16. doi:10.1086/312423. PMID 10587484. 
  12. ^ a b c Doeleman et al. 2008
  13. ^ Backer and Sramek 1999, § 3
  14. ^ "Karl Jansky: The Father of Radio Astronomy". Retrieved 2015-10-21. 
  15. ^ Melia 2007, p. 2
  16. ^ "[astro-ph/0305074] The Discovery of Sgr A*". 2003-05-06. Retrieved 2015-02-27. 
  17. ^ "Precessing jets in Sagittarius A - Gas dynamics in the central parsec of the galaxy", R. L. Brown, Astrophysical Journal, Part 1, 262, Nov. 1, 1982, pp. 110-119, Bibcode1982ApJ...262..110B.
  18. ^ a b Chou, Felicia; Anderson, Janet; Watzke, Megan (January 5, 2015). "RELEASE 15-001 - NASA's Chandra Detects Record-Breaking Outburst from Milky Way's Black Hole". NASA. Retrieved January 6, 2015. 
  19. ^ Schödel et al. 2002
  20. ^ "Best View Yet of Dusty Cloud Passing Galactic Centre Black Hole". Retrieved 16 June 2015. 
  21. ^ Ghez et al. 2003
  22. ^ O'Neill 2008
  23. ^ a b c d e f g Eisenhauer, F.; et al. (July 20, 2005). "SINFONI in the Galactic Center: Young Stars and Infrared Flares in the Central Light-Month". The Astrophysical Journal. 628: 246–259. Bibcode:2005ApJ...628..246E. doi:10.1086/430667. 
  24. ^ Schödel et al. 2009
  25. ^ Schödel, R.; et al. (17 October 2002). "A star in a 15.2-year orbit around the supermassive black hole at the centre of the Milky Way". Nature. 419 (6908): 694–696. arXiv:astro-ph/0210426Freely accessible. Bibcode:2002Natur.419..694S. doi:10.1038/nature01121. PMID 12384690. 
  26. ^ Wheeler 2007, p. 224
  27. ^ "Orbital Parameters of Stars Orbiting Sgr A*". The Astrophysics Spectator (4.10). July 11, 2007. 
  28. ^ a b c Ghez, A. M.; Salim, S.; Hornstein, S. D.; Tanner, A.; Lu, J. R.; Morris, M.; Becklin, E. E.; Duchêne, G. (May 2005). "Stellar Orbits around the Galactic Center Black Hole". The Astrophysical Journal. 620 (2): 744–757. arXiv:astro-ph/0306130Freely accessible. Bibcode:2005ApJ...620..744G. doi:10.1086/427175. 
  29. ^ Meyer, L.; Ghez, A. M.; Schödel, R.; Yelda, S.; Boehle, A.; Lu, J. R.; Do, T.; Morris, M. R.; Becklin, E. E.; Matthews, K. (4 October 2012). "The Shortest Known Period Star Orbiting our Galaxy's Supermassive Black Hole". arXiv:1210.1294Freely accessible. 
  30. ^ "Integral rolls back history of Milky Way's super-massive black hole". Hubble News Desk. January 28, 2005. Retrieved 2007-10-31. 
  31. ^ M. G. Revnivtsev; et al. (2004). "Hard X-ray view of the past activity of Sgr A* in a natural Compton mirror". Astronomy and Astrophysics. 425: L49–L52. arXiv:astro-ph/0408190Freely accessible. Bibcode:2004A&A...425L..49R. doi:10.1051/0004-6361:200400064. 
  32. ^ M. Nobukawa; et al. (2011). "New Evidence for High Activity of the Supermassive Black Hole in our Galaxy". The Astrophysical Journal Letters. 739: L52. arXiv:1109.1950Freely accessible. Bibcode:2011ApJ...739L..52N. doi:10.1088/2041-8205/739/2/L52. 
  33. ^ Matson, John. "Gas Guzzler: Cloud Could Soon Meet Its Demise in Milky Way's Black Hole". Scientific American. Retrieved 2012-10-30. 
  34. ^ a b Gillessen, S.; Genzel; Fritz; Quataert; Alig; Burkert; Cuadra; Eisenhauer; Pfuhl; Dodds-Eden; Gammie; Ott (5 January 2012). "A gas cloud on its way towards the supermassive black hole at the Galactic Centre". Nature. 481: 51–54. doi:10.1038/nature10652. 
  35. ^ a b
  36. ^ Bartos, Imre; Haiman, Zoltán; Kocsis, Bence; Márka, Szabolcs (May 2013). "Gas Cloud G2 Can Illuminate the Black Hole Population Near the Galactic Center". Physical Review Letters. 110 (22): 221102 (5 pages). arXiv:1302.3220Freely accessible. Bibcode:2013PhRvL.110v1102B. doi:10.1103/PhysRevLett.110.221102. 
  37. ^ de la Fuente Marcos, R.; de la Fuente Marcos, C. (August 2013). "Colliding with G2 near the Galactic Centre: a geometrical approach". Monthly Notices of the Royal Astronomical Society: Letters. 435 (1): L19–L23. arXiv:1306.4921Freely accessible. Bibcode:2013MNRAS.435L..19D. doi:10.1093/mnrasl/slt085. 
  38. ^ Morris, Mark (4 January 2012). "Astrophysics: The Final Plunge". Nature. 481: 32–33. doi:10.1038/nature10767. 
  39. ^ Gillessen. "Wiki Page of Proposed Observations of G2 Passage". Retrieved 30 October 2012. 
  40. ^ "A Black Hole's Dinner is Fast Approaching". ESO. 2011-12-14. Retrieved 2015-02-27. 
  41. ^ Robert H Hirschfeld (2012-10-22). "Milky Way's black hole getting ready for snack". [ Lawrence Livermore National Laboratory]. Retrieved 2015-02-27. 
  42. ^, Doomed Space Cloud Nears Milky Way's Black Hole as Scientists Watch, 28 April 2014 "Cosmic encounter that might reveal new secrets on how such supermassive black holes evolve"; "We get to watch it unfolding in a human lifetime, which is very unusual and very exciting"
  43. ^ a b "Why galactic black hole fireworks were a flop : Nature News & Comment". Retrieved 2015-02-27. 
  44. ^ A. M. Ghez; G . Witzel; B. Sitarski; L. Meyer; S. Yelda; A. Boehle; E. E. Becklin; R. Campbell; G. Canalizo; T. Do; J. R. Lu; K. Matthews; M. R. Morris; A. Stockton (2 May 2014). "Detection of Galactic Center Source G2 at 3.8 micron during Periapse Passage Around the Central Black Hole". The Astronomer's Telegram (6110). Retrieved May 3, 2014. 
  45. ^
  46. ^ "Simulation of gas cloud after close approach to the black hole at the centre of the Milky Way". ESO. Retrieved 2015-02-27. 


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