Astronomy:Sagittarius A*

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Short description: Star cluster orbiting Sagittarius A*
Inferred orbits of 6 stars around the supermassive black hole Sagittarius A* at the Milky Way's center[1]

The Sagittarius A* cluster is the cluster of stars in close orbit around Sagittarius A*, the supermassive black hole at the center of the Milky Way (in the Galactic Center). The individual stars are often listed as "S-stars", but their names and IDs are not formalized, and stars can have different numbers in different catalogues.

One of the most studied stars is S2, a relatively bright star that also passes close by Sgr A*.

As of 2020, S4714 is the current record holder of closest approach to Sagittarius A*, at about 12.6 Astronomy:astronomical unit|astronomical units (1.88×109 km), almost as close as Saturn gets to the Sun, traveling at about 8% of the speed of light. These figures given are approximate, the formal uncertainties being 12.6±9.3 au and 23928±8840 km/s. Its orbital period is 12 years, but an extreme eccentricity of 0.985 gives it the close approach and high velocity.[2]

List of stars

The inferred orbits of stars around the supermassive black hole Sagittarius A* at the Milky Way's center are according to Gillessen et al. 2017,[3] with the exception of S2 which is from GRAVITY 2019,[4] and S4711 up to S4715, which are also from Peißker et al., Aug 2020.[2] Data for S62, reported in Peißker et al. Jan 2020[5], was later found to be further than expected from Sagittarius A* and is now thought to be due to a mistaken observation of S29.[6]

Here id1 is the star's name in the Gillessen catalog and id2 in the catalog of the University of California, Los Angeles. a, e, i, Ω and ω are standard orbital elements, with a measured in arcseconds. Tp is the epoch of pericenter passage, P is the orbital period in years and Kmag is the K-band apparent magnitude of the star. q and v are the pericenter distance in AU and pericenter speed in percent of the speed of light,[7] and Δ indicates the standard deviation of the associated quantities.

Binary Star system D9 was recently found in the S Cluster. [8]

id1 id2 Other name Spectral class Mass (Solar masses) Temperature (K) a Δa e Δe i (°) Δi Ω (°) ΔΩ ω (°) Δω Tp (yr) ΔTp P (yr) ΔP Kmag q (AU) Δq v (%c) Δv Radius (Solar radii)
S1 S0-1 B0-2 V 0.5950 0.0240 0.5560 0.0180 119.14 0.21 342.04 0.32 122.30 1.40 2001.800 0.150 166.0 5.8 14.70 2160.7 6.7 0.55 0.03 4.49
S2 S0-2 B0-2 V 14 0.1251 0.0001 0.8843 0.0001 133.91 0.05 228.07 0.04 66.25 0.04 2018.379 0.001 16.0518 0.0 13.95 118.4 0.2 2.56 0.00 6.36
S4 S0-3 B0-2 V 0.3570 0.0037 0.3905 0.0059 80.33 0.08 258.84 0.07 290.80 1.50 1957.400 1.200 77.0 1.0 14.40 1779.7 25.1 0.57 0.01
S6 S0-7 B0-2 V 0.6574 0.0006 0.8400 0.0003 87.24 0.06 85.07 0.12 116.23 0.07 2108.610 0.030 192.0 0.2 15.40 860.3 4.4 0.94 0.00 4.53
S8 S0-4 B0-2 V 0.4047 0.0014 0.8031 0.0075 74.37 0.30 315.43 0.19 346.70 0.41 1983.640 0.240 92.9 0.4 14.50 651.7 22.5 1.07 0.01 4.47
S9 S0-5 B0-2 V 0.2724 0.0041 0.6440 0.0200 82.41 0.24 156.60 0.10 150.60 1.00 1976.710 0.920 51.3 0.7 15.10 793.2 36.9 0.93 0.02
S10 S0-6 [CRG2004] 12 K5 III 14.26
S12 S0-19 B4-9 V 0.2987 0.0018 0.8883 0.0017 33.56 0.49 230.10 1.80 317.90 1.50 1995.590 0.040 58.9 0.2 15.50 272.9 2.0 1.69 0.01
S13 S0-20 B4-9 V 0.2641 0.0016 0.4250 0.0023 24.70 0.48 74.50 1.70 245.20 2.40 2004.860 0.040 49.0 0.1 15.80 1242.0 2.4 0.69 0.01
S14 S0-16 B4-9 V 0.2863 0.0036 0.9761 0.0037 100.59 0.87 226.38 0.64 334.59 0.87 2000.120 0.060 55.3 0.5 15.70 56.0 3.8 3.83 0.06 3.1789
S17 K0 III 5,000 0.3559 0.0096 0.3970 0.0110 96.83 0.11 191.62 0.21 326.00 1.90 1991.190 0.410 76.6 1.0 15.30 1755.3 16.4 0.57 0.02 9
S18 S0-18 0.2379 0.0015 0.4710 0.0120 110.67 0.18 49.11 0.18 349.46 0.66 1993.860 0.160 41.9 0.2 16.70 1029.3 3.8 0.77 0.01
S19 S0-28 K-M 0.5200 0.0940 0.7500 0.0430 71.96 0.35 344.60 0.62 155.20 2.30 2005.390 0.160 135.0 14.0 16.00 1063.3 4.5 0.83 0.20
S21 0.2190 0.0017 0.7640 0.0140 58.80 1.00 259.64 0.62 166.40 1.10 2027.400 0.170 37.0 0.3 16.90 422.7 3.6 1.32 0.02
S22 1.3100 0.2800 0.4490 0.0880 105.76 0.95 291.70 1.40 95.00 20.00 1996.900 10.200 540.0 63.0 16.60 5903.7 9.7 0.32 0.10
S23 0.2530 0.0120 0.5600 0.1400 48.00 7.10 249.00 13.00 39.00 6.70 2024.700 3.700 45.8 1.6 17.80 910.5 1.6 0.85 0.06
S24 S0-26 0.9440 0.0480 0.8970 0.0049 103.67 0.42 7.93 0.37 290.00 15.00 2024.500 0.030 331.0 16.0 15.60 795.3 30.8 0.99 0.07
S27 S0-27 K3 III 15.74
S29 0.4280 0.0190 0.7280 0.0520 105.80 1.70 161.96 0.80 346.50 5.90 2025.960 0.940 101.0 2.0 16.70 952.2 67.4 0.87 0.05
S31 S0-8 B4-9 V 0.4490 0.0100 0.5497 0.0025 109.03 0.27 137.16 0.30 308.00 3.00 2018.070 0.140 108.0 1.2 15.70 1653.7 14.6 0.63 0.02
S33 S0-33 0.6570 0.0260 0.6080 0.0640 60.50 2.50 100.10 5.50 303.70 1.60 1928.000 12.000 192.0 5.2 16.00 2106.5 179.7 0.56 0.03
S38 S0-38 M III 5.9713 3,330 0.1416 0.0002 0.8201 0.0007 171.10 2.10 101.06 0.24 17.99 0.25 2003.190 0.010 19.2 0.0 17.00 208.4 1.5 1.91 0.01 185.84
S39 0.3700 0.0150 0.9236 0.0021 89.36 0.73 159.03 0.10 23.30 3.80 2000.060 0.060 81.1 1.5 16.80 231.2 3.3 1.86 0.09
S42 0.9500 0.1800 0.5670 0.0830 67.16 0.66 196.14 0.75 35.80 3.20 2008.240 0.750 335.0 58.0 17.50 3364.4 24.8 0.44 0.13
S54 1.2000 0.8700 0.8930 0.0780 62.20 1.40 288.35 0.70 140.80 2.30 2004.460 0.070 477.0 199.0 17.50 1050.2 1.9 0.86 0.78
S55 S0-102 B0-2 V 0.1078 0.0010 0.7209 0.0077 150.10 2.20 325.50 4.00 331.50 3.90 2009.340 0.040 12.8 0.1 17.50 246.1 4.1 1.70 0.02
S60 0.3877 0.0070 0.7179 0.0051 126.87 0.30 170.54 0.85 29.37 0.29 2023.890 0.090 87.1 1.4 16.30 894.5 1.7 0.89 0.02
S66 S1-2 [RGH2007] GEN-0.06-1.01 O-B 1.5020 0.0950 0.1280 0.0430 128.50 1.60 92.30 3.20 134.00 17.00 1771.000 38.000 664.0 37.0 14.80 10712.4 620.5 0.21 0.02
S67 S1-3 O-B 1.1260 0.0260 0.2930 0.0570 136.00 1.10 96.50 6.40 213.50 1.60 1705.000 22.000 431.0 10.0 12.10 6511.2 360.6 0.29 0.01
S71 S0-71 0.9730 0.0400 0.8990 0.0130 74.00 1.30 35.16 0.86 337.80 4.90 1695.000 21.000 346.0 11.0 16.10 803.8 1.4 0.99 0.06
S83 S0-15 [GEO97] W5 O9-9.5 V 33,000 1.4900 0.1900 0.3650 0.0750 127.20 1.40 87.70 1.20 203.60 6.00 2046.800 6.300 656.0 69.0 13.60 7738.6 22.5 0.27 0.05 7.5
S85 M0 III 9.178 3,800 4.6000 3.3000 0.7800 0.1500 84.78 0.29 107.36 0.43 156.30 6.80 1930.200 9.800 3580.0 2550.0 15.60 8277.1 29.6 0.30 0.33 51.008
S87 S1-12 2.7400 0.1600 0.2240 0.0270 119.54 0.87 106.32 0.99 336.10 7.70 611.000 154.000 1640.0 105.0 13.60 17390.5 2572.9 0.17 0.02
S89 1.0810 0.0550 0.6390 0.0380 87.61 0.16 238.99 0.18 126.40 4.00 1783.000 26.000 406.0 27.0 15.30 3191.8 407.2 0.46 0.04
S91 1.9170 0.0890 0.3030 0.0340 114.49 0.32 105.35 0.74 356.40 1.60 1108.000 69.000 958.0 50.0 12.20 10928.4 74.5 0.22 0.02
S96 S0-96 GCIRS 16C Ofpe/WN9 1.4990 0.0570 0.1740 0.0220 126.36 0.96 115.66 0.59 233.60 2.40 1646.000 16.000 662.0 29.0 10.00 10127.0 530.0 0.22 0.02
S97 A S1-16 GCIRS 16SW Ofpe/WN9 50 24,400 2.3200 0.4600 0.3500 0.1100 113.00 1.30 113.20 1.40 28.00 14.00 2132.000 29.000 1270.0 309.0 10.30 12333.9 305.9 0.21 0.08 54.5 × 58.2 × 62.7
S97 B 50 23,500 54.5 × 58.2 × 62.7
S145 M III 5.1216 3,330 1.1200 0.1800 0.5000 0.2500 83.70 1.60 263.92 0.94 185.00 16.00 1808.000 58.000 426.0 71.0 17.50 4580.2 1471.2 0.37 0.10 179.46
S175 0.4140 0.0390 0.9867 0.0018 88.53 0.60 326.83 0.78 68.52 0.40 2009.510 0.010 96.2 5.0 17.50 45.0 0.8 4.27 0.47
S4711 B8-9 V 2.2 11,587.84 0.0751 0.0015 0.768 0.030 114.71 2.92 20.10 3.72 131.59 3.09 2010.85 0.06 7.6 0.3 18.4 143.7 7.4 2.2 0.2 2.59
S4712 B9-A0 V 2.2 10,000 0.451 0.0025 0.365 0.032 117.28 1.31 166.38 3.20 238.08 3.43 2007.12 0.08 112.0 2.9 18.4 2366 120 0.48 0.02
S4713 2.1 0.2 0.095 0.351 0.059 111.07 1.66 95.06 5.15 301.97 8.02 2000.03 0.22 33.2 2.5 18.5 1073 110 0.71 0.05
S4714 2 0.102 0.0003 0.985 0.011 127.7 0.28 129.28 0.63 357.25 0.08 2017.29 0.02 12.0 0.3 17.7 12.6 9.3 8.0 3
S4715 B9 V 2.8 0.1439 0.011 0.247 0.040 129.8 3.72 282.15 2.92 359.99 5.38 2008.05 0.3 20.2 2.4 17.8 894 83 0.75 0.04 2.37
S4716 B7-8 V 12,417.07 1.9300 0.756 161.24 151.54 0.073 4.0 2.45
R34 1.8100 0.1500 0.6410 0.0980 136.00 8.30 330.00 19.00 57.00 8.00 1522.000 52.000 877.0 83.0 14.00 5314.6 856.3 0.36 0.05
R44 3.9000 1.4000 0.2700 0.2700 131.00 5.20 80.50 7.10 217.00 24.00 1963.000 85.000 2730.0 1350.0 14.00 23285.6 901.5 0.15 0.11
id1 id2 a Δa e Δe i (°) Δi Ω (°) ΔΩ ω (°) Δω Tp (yr) ΔTp P (yr) ΔP Kmag q (AU) Δq v (%c) Δv

References

  1. ↑ Eisenhauer, F. (July 20, 2005). "SINFONI in the Galactic Center: Young Stars and Infrared Flares in the Central Light-Month". The Astrophysical Journal 628 (1): 246–259. doi:10.1086/430667. Bibcode: 2005ApJ...628..246E. 
  2. ↑ 2.0 2.1 Peißker, Florian; Eckart, Andreas; Zajaček, Michal; Basel, Ali; Parsa, Marzieh (August 2020). "S62 and S4711: Indications of a Population of Faint Fast-moving Stars inside the S2 Orbit—S4711 on a 7.6 yr Orbit around Sgr A*". The Astrophysical Journal 889 (50): 5. doi:10.3847/1538-4357/ab9c1c. Bibcode: 2020ApJ...899...50P. 
  3. ↑ Gillessen, S. (February 2017). "An Update on Monitoring Stellar Orbits in the Galactic Center". The Astrophysical Journal 837 (1): 30. doi:10.3847/1538-4357/aa5c41. Bibcode: 2017ApJ...837...30G. 
  4. ↑ Abuter, R.; Amorim, A.; Bauböck, M.; Berger, J. P.; Bonnet, H.; Brandner, W.; Clénet, Y.; Coudé Du Foresto, V. et al. (April 2019). "A geometric distance measurement to the Galactic center black hole with 0.3% uncertainty". Astronomy & Astrophysics 625: L10. doi:10.1051/0004-6361/201935656. Bibcode: 2019A&A...625L..10G. 
  5. ↑ Peißker, Florian; Eckart, Andreas; Parsa, Marzieh (January 2020). "S62 on a 9.9 year orbit around SgrA*". The Astrophysical Journal 889 (1): 61. doi:10.3847/1538-4357/ab5afd. Bibcode: 2020ApJ...889...61P. 
  6. ↑ Abuter, R.; Amorim, A.; Bauböck, M.; Berger, J. P.; Bonnet, H.; Brandner, W.; Clénet, Y.; Dallilar, Y. et al. (2021-01-01). "Detection of faint stars near Sagittarius A* with GRAVITY" (in en). Astronomy & Astrophysics 645: A127. doi:10.1051/0004-6361/202039544. ISSN 0004-6361. Bibcode: 2021A&A...645A.127G. 
  7. ↑ Næss, S. (October 4, 2019). "Galactic center S-star orbital parameters". http://folk.uio.no/sigurdkn/s_stars. 
  8. ↑ information@eso.org. "First ever binary star found near our galaxy's supermassive black hole" (in en). https://www.eso.org/public/news/eso2418/. 
Short description: Radio source at center of the Milky Way
This page is about the radio source in the center of the Milky Way. For the black hole, see Sagittarius A*. For the galactic arm, see Sagittarius arm. For the star, see Alpha Sagittarii.

File:Labeled Map of the Milky Way Center.tif

Sagittarius A
Observation data
Equinox J2000.0]] (ICRS)
Constellation Sagittarius
Right ascension  17h 45m 40.0409s[1]
Declination −29° 00′ 28.118″[1]
Astrometry
Radial velocity (Rv)46 km/s
Details
Mass~4.1 million M☉
Radius31.6 R☉
Age+10.000 years
Other designations
AX J1745.6-2900, SAGITTARIUS A, W 24, Cul 1742-28, SGR A, [DGW65] 96, EQ 1742-28, RORF 1742-289, [SKM2002] 28.
Database references
SIMBADdata

Sagittarius A (Sgr A) is a complex radio source at the center of the Milky Way, which contains a supermassive black hole. It is located between Scorpius and Sagittarius, and is hidden from view at optical wavelengths by large clouds of cosmic dust in the spiral arms of the Milky Way. The dust lane that obscures the Galactic Center from a vantage point around the Sun causes the Great Rift through the bright bulge of the galaxy.

The radio source consists of three components: the supernova remnant Sagittarius A East, the spiral structure Sagittarius A West, and a very bright compact radio source at the center of the spiral, Sagittarius A* (read "A-star"). These three overlap: Sagittarius A East is the largest, West appears off-center within East, and A* is at the center of West.

Discovery

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.[2] His observations did not extend quite as far south as we now know to be the Galactic Center.[3] 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,[4] which after further observation with the 80-foot (24-metre) CSIRO radio telescope at Dover Heights was identified in a letter to Nature as the probable Galactic Center.[5] The name Sagittarius A was first used in 1954 by John D. Kraus, Hsien-Ching Ko, and Sean Matt[6] when they included the object in the list of radio sources found with the Ohio State University radio telescope at 250 MHz. As was common practice at the time, sources were named by constellation with capital letters in order of brightness within each constellation, with A denoting the brightest radio source within the constellation.

Sagittarius A East

Sgr A East is a supernova remnant (SNR) located 7 light-years from Sgr A* and spanning approximately 27 light-years in diameter, originating from an explosion between 1,000 and 10,000 years ago.[7] Though its status as a SNR was initially debated,[7] the observed abundance of heavier elements, such as a higher ratio of Mn/Fe and Ni/Fe, closely aligns with theoretical models of SNR.[8] The emission lines of heavier elements in the X-ray band indicate it may have originated from a Type Iax supernova, potentially making it the first of its kind to be observed in the Milky Way.[8]

Sgr A East's morphology varies across spectroscopic observing bands. In radio wavelengths the brightest areas are along the outer edges of the region, whereas in X-ray it is brightest at its center.[7] These morphological variations are most likely from interactions between the SNR and the dense interstellar medium at the Galactic Center.[9]

A separate study showed that the region likely contains overionized plasma, atypical of supernova remnants. This may indicate that the remnant underwent unique evolutionary processes following its initial explosion, such as ionization by charged particles from Sgr A*, rapid cooling, or thermal interactions with nearby material.[10]

It is conjectured that Sgr A East is the remnant of the explosion of a star that was gravitationally compressed as it made a close approach to the central black hole.[9]

Sagittarius A West

Surface brightness and velocity field of the inner part of Sagittarius A West

Sgr A West has the appearance of a three-arm spiral, from the point of view of the Earth. For this reason, it is also known as the "Minispiral". This appearance and nickname are misleading, though: the three-dimensional structure of the Minispiral is not that of a spiral. It is made of several dust and gas clouds, which orbit and fall onto Sagittarius A* at velocities as high as 1,000 kilometers per second. The surface layer of these clouds is ionized. The source of ionisation is the population of massive stars (more than one hundred OB stars have been identified so far) that also occupy the central parsec.

Sgr A West is surrounded by a massive, clumpy torus of cooler molecular gas, the Circumnuclear Disk (CND). The nature and kinematics of the Northern Arm cloud of Sgr A West suggest that it once was a clump in the CND, which fell due to some perturbation, perhaps the supernova explosion responsible for Sgr A East. The Northern Arm appears as a very bright North—South ridge of emission, but it extends far to the East and can be detected as a dim extended source.

The Western Arc (outside the field of view of the image shown in the right) is interpreted as the ionized inner surface of the CND. The Eastern Arm and the Bar seem to be two additional large clouds similar to the Northern Arm, although they do not share the same orbital plane. They have been estimated to amount for about 20 solar masses each.

On top of these large scale structures (of the order of a few light-years in size), many smaller cloudlets and holes inside the large clouds can be seen. The most prominent of these perturbations is the Minicavity, which is interpreted as a bubble blown inside the Northern Arm by the stellar wind of a massive star, which is not clearly identified.

Sagittarius A*

Main article: Astronomy:Sagittarius A*

File:EHT Saggitarius A black hole.tif Astronomers now have evidence that there is a supermassive black hole at the center of the galaxy.[12] Sagittarius A* (abbreviated Sgr A*) is agreed to be the most plausible candidate for the location of this supermassive black hole. The Very Large Telescope at Chile and Keck Telescope at Hawaii have detected stars orbiting Sgr A* at speeds greater than that of any other stars in the galaxy. One star, designated S2, was calculated to orbit Sgr A* at speeds of over 5,000 kilometers per second at its closest approach.[13]

A gas cloud, G2, passed through the Sagittarius A* region in 2014 and managed to do so without disappearing beyond the event horizon, as theorists predicted would happen. Rather, it disintegrated, suggesting that G2 and a previous gas cloud, G1, were star remnants with larger gravitational fields than gas clouds.[14][15]

In September 2019, scientists found that Sagittarius A* had been consuming nearby matter at a much faster rate than usual over the previous year. Researchers speculated that this could mean that the black hole is entering a new phase, or that Sagittarius A* had stripped the outer layer of G2 when it passed through.[16]

References

  1. ↑ 1.0 1.1 Reid, M.J.; Brunthaler, A. (2004). "The Proper Motion of Sagittarius A*". The Astrophysical Journal 616 (2): 874, 883. doi:10.1086/424960. Bibcode: 2004ApJ...616..872R. 
  2. ↑ "Karl Jansky: The Father of Radio Astronomy". 29 August 2012. https://armaghplanet.com/karl-jansky-the-father-of-radio-astronomy.html. 
  3. ↑ Goss, W. M.; McGee, R. X. (1996). "The Discovery of the Radio Source Sagittarius A (Sgr A)". The Galactic Center, Astronomical Society of the Pacific Conference Series 102: 369. Bibcode: 1996ASPC..102..369G. 
  4. ↑ Piddington, J. H.; Minnett, H. C. (1 December 1951). "Observations of Galactic Radiation at Frequencies of 1200 and 3000 Mc/s.". Australian Journal of Scientific Research A 4 (4): 459. doi:10.1071/CH9510459. Bibcode: 1951AuSRA...4..459P. 
  5. ↑ McGee, R. X.; Bolton, J. G. (1 May 1954). "Probable observation of the galactic nucleus at 400 Mc./s.". Nature 173 (4412): 985–987. doi:10.1038/173985b0. ISSN 0028-0836. Bibcode: 1954Natur.173..985M. 
  6. ↑ Kraus, J. D.; Ko, H. C.; Matt, S. (December 1954). "Galactic and localized source observations at 250 megacycles per second.". The Astronomical Journal 59: 439–443. doi:10.1086/107059. ISSN 0004-6256. Bibcode: 1954AJ.....59..439K. 
  7. ↑ 7.0 7.1 7.2 Ehlerová, S.; Palouš, J.; Morris, M. R.; unsch, R. W\"; Barna, B.; Vermot, P. (2022-11-08). "How to create SGR a East". Astronomy & Astrophysics 668: A124. doi:10.1051/0004-6361/202244682. 
  8. ↑ 8.0 8.1 Zhou, Ping; Leung, Shing-Chi; Li, Zhiyuan; Nomoto, Ken’ichi; Vink, Jacco; Chen, Yang (2021-02-01). "Chemical Abundances in Sgr A East: Evidence for a Type Iax Supernova Remnant". The Astrophysical Journal 908 (1): 31. doi:10.3847/1538-4357/abbd45. ISSN 0004-637X. Bibcode: 2021ApJ...908...31Z. 
  9. ↑ 9.0 9.1 Maeda, Y.; Baganoff, F. K.; Feigelson, E. D.; Morris, M.; Bautz, M. W.; Brandt, W. N.; Burrows, D. N.; Doty, J. P. et al. (2002-05-10). "AChandra Study of Sagittarius a East: A Supernova Remnant Regulating the Activity of Our Galactic Center?". The Astrophysical Journal 570 (2): 671–687. doi:10.1086/339773. Bibcode: 2002ApJ...570..671M. 
  10. ↑ XRISM Collaboration; Audard, Marc; Awaki, Hisamitsu; Ballhausen, Ralf; Bamba, Aya; Behar, Ehud; Boissay-Malaquin, Rozenn; Brenneman, Laura et al. (2025-02-01). "Overionized plasma in the supernova remnant Sagittarius A East anchored by XRISM observations". Publications of the Astronomical Society of Japan 77 (1): L1–L8. doi:10.1093/pasj/psae111. ISSN 2053-051X. 
  11. ↑ "Astronomers reveal first image of the black hole at the heart of our galaxy". Event Horizon Telescope. https://eventhorizontelescope.org/blog/astronomers-reveal-first-image-black-hole-heart-our-galaxy. 
  12. ↑ Black hole confirmed in Milky Way , Ghosh, Pallab, BBC News. 2008-12-09. Retrieved 2008-12-10.
  13. ↑ "Surfing a Black Hole - Star Orbiting Massive Milky Way Centre Approaches to within 17 Light-Hours". http://www.eso.org/public/news/eso0226/. 
  14. ↑ Lemonick, Michael (6 November 2014). "Black Hole Fails to Destroy Mystery Cosmic Cloud". National Geographic. https://www.nationalgeographic.com/adventure/article/141106-space-g2-black-hole-science. 
  15. ↑ Cowen, Ron (2017-03-02). "It's Snack Time in the Cosmos". The New York Times. https://www.nytimes.com/2014/02/18/science/its-snack-time-in-the-cosmos.html?_r=0. 
  16. ↑ Devlin, Hannah (2019-09-13). "Black hole at centre of galaxy is getting hungrier, say scientists" (in en-GB). The Guardian. ISSN 0261-3077. https://www.theguardian.com/science/2019/sep/13/black-hole-at-centre-of-our-galaxy-is-getting-hungrier-say-scientists. 

Further reading

Short description: Supermassive black hole at the center of the Milky Way
This page is about the black hole. For the surrounding region, see Sagittarius A.

Sagittarius A*
250px
Sagittarius A* imaged by the Event Horizon Telescope in 2017, released in 2022
Observation data
Equinox J2000.0]] (ICRS)
Constellation Sagittarius
Right ascension  17h 45m 40.0409s
Declination −29° 0′ 28.118″[1]
Details
Mass8.54×1036 kg
4.297×106[2] M☉
Astrometry
Distance26996±33[2] ly
(8277±9[2] pc)
Database references
SIMBADdata

Coordinates: Sky map 17h 45m 40.0409s, −29° 0′ 28.118″ 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. 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*,[8] using the Event Horizon Telescope, a world-wide network of radio observatories.[9] This is the second confirmed image of a black hole, after Messier 87's supermassive black hole in 2019.[10][11] 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.[12]

Sgr A* was discovered in 1974 by Bruce Balick (de) and Robert L. Brown,[13][14] and the asterisk * was assigned in 1982 by Brown,[15] 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.[16] The current best estimate of its mass is 4.297±0.012 million solar masses.[2]

Reinhard Genzel and Andrea Ghez were awarded a half 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".[17]

Observation and description

Sagittarius A* in the constellation of Sagittarius. The black hole is marked with a red circle within the constellation of Sagittarius (The Archer). This map shows most of the stars visible to the unaided eye under good conditions.
Size comparison between Sagittarius A* and M87*. The diameter of Sagittarius A* is smaller than the orbit of Mercury.

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,[10][18] and took five years of calculations to process.[19] 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.[20]

Their result gives an overall angular size for the source of 51.8±2.3 μas.[18] At a distance of 26,000 light-years (8,000 parsecs), this yields a diameter of 51.8 million kilometres (32.2 million miles).[lower-alpha 1] For comparison, Earth is 150 million kilometres (1.0 astronomical unit; 93 million miles) from the Sun, and Mercury is 46 million km (0.31 AU; 29 million mi) 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.[21][22][23] 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.[11]

In 2019, measurements made with the High-resolution Airborne Wideband Camera-Plus (HAWC+) mounted in the SOFIA aircraft[24] 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),[25] to flow into an orbit around Sagittarius A*, keeping black hole emissions low.[26]

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.[27]

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.[28] The radio source later became known as Sagittarius A. His observations did not extend quite as far south as we now know to be the Galactic Center.[29] 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,[30] which after further observation with the 80-foot (24-metre) CSIRO radio telescope at Dover Heights was identified in a letter to Nature as the probable Galactic Center.[31]

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.[13][14] 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.[32][33]

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.[34]

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.[35][36]

Dusty cloud G2 passes the supermassive black hole at the center of the Milky Way.[37] Composition of images taken at different times to show motion; colored blue when approaching the viewer, red when receding; time is left to right. Red cross marks the black hole.

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 4.1±0.6 million solar masses, confined in a volume with a radius no more than 17 light-hours (120 astronomical unit|AU [18 billion km; 11 billion mi]).[38] 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 [6.7 billion km; 4.2 billion mi]).[39] S175 passed within a similar distance.[40] For comparison, the Schwarzschild radius is 0.08 AU (12 million km; 7.4 million mi). 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 8,000 ± 600 parsecs (30,000 ± 2,000 light-years). 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.[41] 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."[42]

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.[43]

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.[44]

In June 2023, unexplained filaments of radio energy were found associated with Sagittarius A*.[45]

  • ALMA observations of molecular-hydrogen-rich gas clouds, with the area around Sagittarius A* circled[46]
    ALMA observations of molecular-hydrogen-rich gas clouds, with the area around Sagittarius A* circled[46]
  • An unusually bright X-ray flare from Sgr A* was detected in 2013.[43]
    An unusually bright X-ray flare from Sgr A* was detected in 2013.[43]
  • Supernova remnant ejecta producing planet-forming material seen in radio and X-ray
    Supernova remnant ejecta producing planet-forming material seen in radio and X-ray
  • Supernova remnant ejecta producing planet-forming material seen in infrared and X-ray
    Supernova remnant ejecta producing planet-forming material seen in infrared and X-ray

Central black hole

NuSTAR has captured these first, focused views of the supermassive black hole at the heart of the Milky Way in high-energy X-rays.

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 (430 feet) 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*.[12][47]

In July 2018, it was reported that S2 orbiting Sgr A* had been recorded at 7,650 km/s (17.1 million mph), or 2.55% the speed of light, leading up to the pericenter approach, in May 2018, at about 120 astronomical unit|AU (18 billion km; 11 billion mi) (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.[48][49]

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.[50] 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,[50] 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.[51] Simulations of alternative theories of gravity depict results that may be difficult to distinguish from GR.[52] 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.[53]

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,[41] whereas the American group found 4.1±0.6 million solar masses.[39] Given that this mass is confined inside a 44-million-kilometre-diameter sphere, this yields a density ten times higher than previous estimates. #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
    −0.4    ×106     M☉, plus a distributed mass in the central parsec amounting to (1±0.5)×106 M☉.[54] The latter is thought to be composed of stars and stellar remnants.
Magnetar found very close to the supermassive black hole, Sagittarius A*, at the center of the Milky Way galaxy

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.[27]


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.[55][56] In 2011 this conclusion was supported by Japanese astronomers observing the Milky Way's center with the Suzaku satellite.[57]

In July 2019, astronomers reported finding a star, S5-HVS1, traveling 1,755 km/s (3.93 million mph) or 0.006 c. The star is in the Grus (or Crane) constellation in the southern sky, and about 29,000 light-years from Earth, and may have been propelled out of the Milky Way galaxy after interacting with Sagittarius A*.[58][59]

Several values[60][61] have been given for its spin parameter a*=cJGM2; some examples are Fragione & Loeb (2020) a*<0.1[62], Belanger et al. (2006) a*∼0.22,[63] Meyer et al. (2006) a*>0.4,[64] Genzel et al. (2003) a*∼0.52,[65] Daly (2019) a*=0.93±0.15,[66] and Daly et al. (2023) a*=0.90±0.06.[61] Daly et al. (2023) also found that the ratio of the black hole rotational mass component to the irreducible mass component of Sgr A* is Mrot/Mirr=0.62±0.10, which indicates that the black hole is rotating with an angular velocity that is 0.62±0.10 of the maximum possible value, set by the speed of light.[61] [67]

Orbiting stars

Main article: Astronomy:Sagittarius A* cluster
Inferred orbits of six stars around supermassive black hole candidate Sagittarius A* at the Milky Way's center[68]
Stars moving around Sagittarius A*, 20-year timelapse, ending in 2018[69][70]
Stars moving around Sagittarius A* as seen in 2021[71][72][73]

There are a number of stars in close orbit around Sagittarius A*, which are collectively known as "S stars".[74] These stars are observed primarily in K band infrared wavelengths, as interstellar dust drastically limits visibility in visible wavelengths. This is a rapidly changing field—in 2011, the orbits of the most prominent stars then known were plotted in the diagram at left, showing a comparison between their orbits and various orbits in the Solar System.[70] Since then, S62 was thought to approach even more closely than those stars,[75] but later observations of the star have found this not to be the case.[76]

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. An active watch is maintained for the possibility of stars approaching the event horizon close enough to be disrupted, but none of these stars are expected to suffer that fate.

As of 2020, 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. These figures given are approximate, the formal uncertainties being 12.6±9.3 AU and 23,928±8,840 km/s. Its orbital period is 12 years, but an extreme eccentricity of 0.985 gives it the close approach and high velocity.[77]

An excerpt from a table of this cluster (see Sagittarius A* cluster), featuring the most prominent members. In the below table, id1 is the star's name in the Gillessen catalog and id2 in the catalog of the University of California, Los Angeles. a, e, i, Ω and ω are standard orbital elements, with a measured in arcseconds. 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. q and v are the pericenter distance in AU and pericenter speed in percent of the speed of light.[78]

id1 id2 a e i (°) Ω (°) ω (°) Tp (yr) P (yr) Kmag q (AU) v (%c)
S1 S0-1 0.5950 0.5560 119.14 342.04 122.30 2001.800 166.0 14.70 2160.7 0.55
S2 S0-2 0.1251 0.8843 133.91 228.07 66.25 2018.379 16.1 13.95 118.4 2.56
S8 S0-4 0.4047 0.8031 74.37 315.43 346.70 1983.640 92.9 14.50 651.7 1.07
S12 S0-19 0.2987 0.8883 33.56 230.10 317.90 1995.590 58.9 15.50 272.9 1.69
S13 S0-20 0.2641 0.4250 24.70 74.50 245.20 2004.860 49.0 15.80 1242.0 0.69
S14 S0-16 0.2863 0.9761 100.59 226.38 334.59 2000.120 55.3 15.70 56.0 3.83
S4714 0.102 0.985 127.7 129.28 357.25 2017.29 12.0 17.7 12.6 8.0

Discovery of G2 gas cloud on an accretion course

First noticed as something unusual in images of the center of the Milky Way in 2002,[79] 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.[80] 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,[80] 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.[81] In addition to the tidal effects on the cloud itself, it was proposed in May 2013[82] 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.[83]

The average rate of accretion onto Sgr A* is unusually small for a black hole of its mass[84] 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, VLA, INTEGRAL, Swift, Fermi and requested at VLT and Keck.[85]

Simulations of the passage were made before it happened by groups at ESO[86] and Lawrence Livermore National Laboratory (LLNL).[87]

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.[88]

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".[89] 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.[81]

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.[89][90]

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.[81][91]

Artist impression of the accretion of gas cloud G2 onto Sgr A*. Credit: ESO[92]
File:Simulation of gas cloud being ripped apart by the black hole at the centre of the Milky Way.ogv
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.
File:The dusty cloud G2 passes the supermassive black hole at the centre of the Milky Way.webm
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

Notes

  1. ↑ This roughly equates to around 37 times the diameter of the Sun at ~1,400,000 kilometers (~865,000 miles).

References

  1. ↑ Reid and Brunthaler 2004
  2. ↑ 2.0 2.1 2.2 2.3 The GRAVITY collaboration (September 2023). "Polarimetry and astrometry of NIR flares as event horizon scale, dynamical probes for the mass of Sgr A*". Astronomy & Astrophysics 677: L10. doi:10.1051/0004-6361/202347416. Bibcode: 2023A&A...677L..10G. https://www.aanda.org/articles/aa/full_html/2023/09/aa47416-23/aa47416-23.html. 
  3. ↑ "Astronomers reveal first image of the black hole at the heart of our galaxy" (in en). May 12, 2022. https://eventhorizontelescope.org/blog/astronomers-reveal-first-image-black-hole-heart-our-galaxy. 
  4. ↑ Parsons, Jeff (October 31, 2018). "Scientists find proof a supermassive black hole is lurking at the centre of the Milky Way" (in en-GB). Metro. https://metro.co.uk/2018/10/31/scientists-find-proof-a-supermassive-black-hole-is-lurking-at-the-centre-of-the-milky-way-8092994/. 
  5. ↑ Mosher, Dave (October 31, 2018). "A 'mind-boggling' telescope observation has revealed the point of no return for our galaxy's monster black hole". The Middletown Press (Business Insider). https://www.middletownpress.com/technology/businessinsider/article/Supermassive-black-holes-gorge-themselves-on-a-7971243.php. 
  6. ↑ Plait, Phil (November 7, 2018). "Astronomers See Material Orbiting a Black Hole *Right* at the Edge of Forever" (in en). Syfy Wire. https://www.syfy.com/syfywire/astronomers-see-material-orbiting-a-black-hole-right-at-the-edge-of-forever. 
  7. ↑ Calculated using Equatorial and Ecliptic Coordinates calculator
  8. ↑ Sutter, Paul (2022-05-20). "How did astronomers take a picture of our galaxy's supermassive black hole?" (in en-US). https://www.astronomy.com/science/how-did-astronomers-take-a-picture-of-our-galaxys-supermassive-black-hole/. "The Event Horizon Telescope (EHT) collaboration released the first ever image of the accretion disk around the Milky Way’s supermassive black hole, Sagittarius A* (Sgr A*), on May 12, 2022." 
  9. ↑ Bower, Geoffrey C. (May 2022). "Focus on First Sgr A* Results from the Event Horizon Telescope". The Astrophysical Journal. https://iopscience.iop.org/journal/2041-8205/page/Focus_on_First_Sgr_A_Results. Retrieved May 12, 2022. 
  10. ↑ 10.0 10.1 "Astronomers reveal first image of the black hole at the heart of our galaxy". eso.org. 12 May 2022. https://www.eso.org/public/news/eso2208-eht-mw/. 
  11. ↑ 11.0 11.1 Overbye, Dennis (2022-05-12). "The Milky Way's Black Hole Comes to Light" (in en-US). The New York Times. ISSN 0362-4331. https://www.nytimes.com/2022/05/12/science/black-hole-photo.html. 
  12. ↑ 12.0 12.1 Abuter, R.; Amorim, A.; Bauböck, M.; Berger, J. P.; Bonnet, H.; Brandner, W.; Clénet, Y.; Coudé Du Foresto, V. et al. (2018). "Detection of orbital motions near the last stable circular orbit of the massive black hole SgrA". Astronomy & Astrophysics 618: L10. doi:10.1051/0004-6361/201834294. Bibcode: 2018A&A...618L..10G. 
  13. ↑ 13.0 13.1 Balick, B.; Brown, R. L. (1 December 1974). "Intense sub-arcsecond structure in the galactic center". Astrophysical Journal 194 (1): 265–270. doi:10.1086/153242. Bibcode: 1974ApJ...194..265B. 
  14. ↑ 14.0 14.1 Melia 2007, p. 7
  15. ↑ Brown, Robert L. (November 1, 1982). "Precessing Jets in Sagittarius A: Gas Dynamics in the Central Parsec of the Galaxy". The Astrophysical Journal 262: 110–119. doi:10.1086/160401. Bibcode: 1982ApJ...262..110B. 
  16. ↑ Henderson, Mark (2009-12-09). "Astronomers confirm black hole at the heart of the Milky Way". Times Online. http://www.timesonline.co.uk/tol/news/uk/science/article5316001.ece. 
  17. ↑ "The Nobel Prize in Physics 2020" (in en-us). 6 October 2020. https://www.nobelprize.org/prizes/physics/2020/summary/. 
  18. ↑ 18.0 18.1 The Event Horizon Telescope Collaboration (1 May 2022). "First Sagittarius A* Event Horizon Telescope Results. I. The Shadow of the Supermassive Black Hole in the Center of the Milky Way". The Astrophysical Journal Letters 930 (2): L12. doi:10.3847/2041-8213/ac6674. ISSN 2041-8205. Bibcode: 2022ApJ...930L..12E. 
  19. ↑ Hensley, Kerry (2022-05-12). "First Image of the Milky Way's Supermassive Black Hole" (in en-US). https://aasnova.org/2022/05/12/first-image-of-the-milky-ways-supermassive-black-hole/. 
  20. ↑ The Event Horizon Telescope Collaboration (1 May 2022). "First Sagittarius A* Event Horizon Telescope Results. III. Imaging of the Galactic Center Supermassive Black Hole". The Astrophysical Journal Letters 930 (2): L14. doi:10.3847/2041-8213/ac6429. ISSN 2041-8205. Bibcode: 2022ApJ...930L..14E. 
  21. ↑ Backer and Sramek 1999, § 3
  22. ↑ "Focus on the First Event Horizon Telescope Results – The Astrophysical Journal Letters – IOPscience". https://iopscience.iop.org/journal/2041-8205/page/Focus_on_EHT. 
  23. ↑ Overbye, Dennis (2019-04-10). "Black Hole Picture Revealed for the First Time" (in en-US). The New York Times. ISSN 0362-4331. https://www.nytimes.com/2019/04/10/science/black-hole-picture.html. 
  24. ↑ "HAWC+, the Far-Infrared Camera and Polarimeter for SOFIA". 2018. https://www.sofia.usra.edu/science/instruments/hawc#. 
  25. ↑ "The Milky Way's Monster Black Hole Has a Cool Gas Halo – Literally". Space.com. 5 June 2019. https://www.space.com/milky-way-monster-black-hole-cool-disk.html. 
  26. ↑ "Magnetic Fields May Muzzle Milky Way's Monster Black Hole". Space.com. 14 June 2019. https://www.space.com/milky-way-supermassive-black-hole-magnetic-field-quiet.html. 
  27. ↑ 27.0 27.1 Osterbrock and Ferland 2006, p. 390
  28. ↑ "Karl Jansky: The Father of Radio Astronomy". 29 August 2012. https://armaghplanet.com/karl-jansky-the-father-of-radio-astronomy.html. 
  29. ↑ Goss, W. M.; McGee, R. X. (1996). "The Discovery of the Radio Source Sagittarius A (Sgr A)". The Galactic Center, Astronomical Society of the Pacific Conference Series 102: 369. Bibcode: 1996ASPC..102..369G. https://ui.adsabs.harvard.edu/abs/1996ASPC..102..369G/abstract. Retrieved 25 February 2021. 
  30. ↑ Piddington, J. H.; Minnett, H. C. (1 December 1951). "Observations of Galactic Radiation at Frequencies of 1200 and 3000 Mc/s.". Australian Journal of Scientific Research A 4 (4): 459. doi:10.1071/CH9510459. Bibcode: 1951AuSRA...4..459P. https://ui.adsabs.harvard.edu/abs/1951AuSRA...4..459P/abstract. Retrieved 25 February 2021. 
  31. ↑ McGee, R. X.; Bolton, J. G. (1 May 1954). "Probable observation of the galactic nucleus at 400 Mc./s.". Nature 173 (4412): 985–987. doi:10.1038/173985b0. ISSN 0028-0836. Bibcode: 1954Natur.173..985M. https://ui.adsabs.harvard.edu/abs/1954Natur.173..985M/abstract. Retrieved 25 February 2021. 
  32. ↑ Goss, W. M.; Brown, Robert L.; Lo, K. Y. (2003-05-06). "The Discovery of Sgr A*". Astronomische Nachrichten 324 (1): 497. doi:10.1002/asna.200385047. Bibcode: 2003ANS...324..497G. 
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  44. ↑ "Our Galaxy's Supermassive Black Hole Has Emitted a Mysteriously Bright Flare". Science Alert. 12 August 2019. https://www.sciencealert.com/our-galaxy-s-supermassive-black-hole-just-mysteriously-got-really-really-bright. 
  45. ↑ Overbye, Dennis (8 June 2023). "Our Local Black Hole Serves Up an 'Awe Moment' – Filaments of radio energy from Sagittarius A*, the black hole at the center of the Milky Way galaxy, are turning astronomers' heads.". The New York Times. Archived from the original on 8 June 2023. https://archive.today/20230608155337/https://www.nytimes.com/2023/06/08/science/astronomy-cosmos-black-hole.html. Retrieved 9 June 2023. 
  46. ↑ "Cloudlets swarm around our local supermassive black hole". https://www.eso.org/public/images/potw1843a/. 
  47. ↑ "Most Detailed Observations of Material Orbiting close to a Black Hole". https://www.eso.org/public/news/eso1835/. 
  48. ↑ Genzel (2018-07-26). "Detection of the gravitational redshift in the orbit of the star S2 near the Galactic centre massive black hole". Astronomy & Astrophysics 615: L15. doi:10.1051/0004-6361/201833718. Bibcode: 2018A&A...615L..15G. https://www.aanda.org/articles/aa/full_html/2018/07/aa33718-18/aa33718-18.html. Retrieved 2018-07-27. 
  49. ↑ "Star spotted speeding near black hole at centre of Milky Way – Chile's Very Large Telescope tracks S2 star as it reaches mind-boggling speeds by supermassive black hole". The Guardian. 2017-07-26. https://www.theguardian.com/science/2018/jul/26/star-spotted-speeding-near-milky-way-black-hole-for-first-time. 
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  51. ↑ Issaoun, S. (January 18, 2019). "The Size, Shape, and Scattering of Sagittarius A* at 86 GHz: First VLBI with ALMA". The Astrophysical Journal 871 (1): 30. doi:10.3847/1538-4357/aaf732. Bibcode: 2019ApJ...871...30I. 
  52. ↑ Rezzolla, Luciano (17 April 2018). "Astrophysicists Test Theories of Gravity with Black Hole Shadows". SciTech Daily. https://scitechdaily.com/astrophysicists-test-theories-of-gravity-with-black-hole-shadows/. 
  53. ↑ "Revealing the black hole at the heart of the galaxy". Netherlands Research School for Astronomy. 22 January 2019. https://www.phys.org/news/2019-01-revealing-black-hole-heart-galaxy.html. 
  54. ↑ Schödel et al. 2009
  55. ↑ "Integral rolls back history of Milky Way's super-massive black hole". Hubble News Desk. January 28, 2005. http://www.esa.int/SPECIALS/Integral/SEMSKPO3E4E_0.html. 
  56. ↑ Revnivtsev, M. G. (October 2004). "Hard X-ray view of the past activity of Sgr A$^{\star}$ in a natural Compton mirror". Astronomy & Astrophysics 425 (3): L49–L52. doi:10.1051/0004-6361:200400064. ISSN 0004-6361. Bibcode: 2004A&A...425L..49R. 
  57. ↑ Nobukawa, M. (2011-09-08). "New Evidence for High Activity of the Supermassive Black Hole in our Galaxy". The Astrophysical Journal 739 (2): L52. doi:10.1088/2041-8205/739/2/L52. ISSN 2041-8205. Bibcode: 2011ApJ...739L..52N. 
  58. ↑ Overbye, Dennis (14 November 2019). "A Black Hole Threw a Star Out of the Milky Way Galaxy – So long, S5-HVS1, we hardly knew you.". The New York Times. https://www.nytimes.com/2019/11/14/science/stars-black-hole-milky-way.html. 
  59. ↑ Koposov, Sergey E (2020-01-11). "Discovery of a nearby 1700 km s−1 star ejected from the Milky Way by Sgr A*" (in en). Monthly Notices of the Royal Astronomical Society 491 (2): 2465–2480. doi:10.1093/mnras/stz3081. ISSN 0035-8711. 
  60. ↑ Eckart, Andreas; Tursunov, A.A.; Zajacek, M.; Parsa, M.; Hosseini, E.; Subroweit, M.; Peissker, F.; Straubmeier, C. et al. (2019-02-01). "Mass, Distance, Spin, Charge, and Orientation of the super massive black hole SgrA*" (in en). Proceedings of Science (Sissa Medialab) 342: 048. doi:10.22323/1.342.0048. https://pos.sissa.it/342/048. 
  61. ↑ 61.0 61.1 61.2 Daly, Ruth A; Donahue, Megan; O'Dea, Christopher P; Sebastian, Biny; Haggard, Daryl; Lu, Anan (2023-10-28). "New black hole spin values for Sagittarius A* obtained with the outflow method" (in en). Monthly Notices of the Royal Astronomical Society 527 (1): 428–436. doi:10.1093/mnras/stad3228. ISSN 0035-8711. https://academic.oup.com/mnras/article/527/1/428/7326786. 
  62. ↑ Fragione, Giacomo; Loeb, Abraham (2020-10-01). "An Upper Limit on the Spin of SgrA* Based on Stellar Orbits in Its Vicinity". The Astrophysical Journal 901 (2): L32. doi:10.3847/2041-8213/abb9b4. ISSN 2041-8213. Bibcode: 2020ApJ...901L..32F. 
  63. ↑ Bélanger, G; Terrier, R; Jager, O C de; Goldwurm, A; Melia, F (2006-12-01). "Periodic Modulations in an X-ray Flare from Sagittarius A*". Journal of Physics: Conference Series 54: 420–426. doi:10.1088/1742-6596/54/1/066. ISSN 1742-6588. Bibcode: 2006JPhCS..54..420B. https://iopscience.iop.org/article/10.1088/1742-6596/54/1/066. 
  64. ↑ Meyer, L.; Eckart, A.; Schödel, R.; Duschl, W. J.; Mužić, K.; Dovčiak, M.; Karas, V. (December 2006). "Near-infrared polarimetry setting constraints on the orbiting spot model for Sgr A* flares". Astronomy & Astrophysics 460 (1): 15–21. doi:10.1051/0004-6361:20065925. ISSN 0004-6361. Bibcode: 2006A&A...460...15M. http://www.aanda.org/10.1051/0004-6361:20065925. 
  65. ↑ Genzel, R.; Schödel, R.; Ott, T.; Eckart, A.; Alexander, T.; Lacombe, F.; Rouan, D.; Aschenbach, B. (November 2003). "Near-infrared flares from accreting gas around the supermassive black hole at the Galactic Centre" (in en). Nature 425 (6961): 934–937. doi:10.1038/nature02065. ISSN 0028-0836. PMID 14586462. Bibcode: 2003Natur.425..934G. https://www.nature.com/articles/nature02065. 
  66. ↑ Daly, Ruth A. (2019-11-15). "Black Hole Spin and Accretion Disk Magnetic Field Strength Estimates for More Than 750 Active Galactic Nuclei and Multiple Galactic Black Holes" (in en). The Astrophysical Journal 886 (1): 37. doi:10.3847/1538-4357/ab35e6. ISSN 0004-637X. Bibcode: 2019ApJ...886...37D. 
  67. ↑ Daly, Ruth A (2022-10-20). "Robust supermassive black hole spin mass-energy characteristics: a new method and results" (in en). Monthly Notices of the Royal Astronomical Society 517 (4): 5144–5159. doi:10.1093/mnras/stac2976. ISSN 0035-8711. https://academic.oup.com/mnras/article/517/4/5144/6764739. 
  68. ↑ Eisenhauer, F. (July 20, 2005). "SINFONI in the Galactic Center: Young Stars and Infrared Flares in the Central Light-Month". The Astrophysical Journal 628 (1): 246–259. doi:10.1086/430667. Bibcode: 2005ApJ...628..246E. 
  69. ↑ "First Successful Test of Einstein's General Relativity Near Supermassive Black Hole – Culmination of 26 years of ESO observations of the heart of the Milky Way" (in en). https://www.eso.org/public/news/eso1825/. 
  70. ↑ 70.0 70.1 GRAVITY Collaboration; Abuter, R.; Amorim, A.; Anugu, N.; Bauböck, M.; Benisty, M.; Berger, J. P.; Blind, N. et al. (July 2018). "Detection of the gravitational redshift in the orbit of the star S2 near the Galactic centre massive black hole". Astronomy & Astrophysics 615: L15. doi:10.1051/0004-6361/201833718. ISSN 0004-6361. Bibcode: 2018A&A...615L..15G. https://www.aanda.org/10.1051/0004-6361/201833718. Retrieved 2021-12-15. 
  71. ↑ "Watch stars move around the Milky Way's supermassive black hole in deepest images yet" (in en). https://www.eso.org/public/news/eso2119/. 
  72. ↑ GRAVITY Collaboration; Stadler, J.; Drescher, A. (2021-12-14). "Deep images of the Galactic center with GRAVITY". Astronomy & Astrophysics 657: A82. doi:10.1051/0004-6361/202142459. ISSN 0004-6361. 
  73. ↑ GRAVITY Collaboration; Abuter, R.; Aimar, N.; Amorim, A.; Ball, J.; Bauböck, M.; Gillessen, S.; Widmann, F. et al. (2021-12-14). "Mass distribution in the Galactic Center based on interferometric astrometry of multiple stellar orbits". Astronomy & Astrophysics 657: L12. doi:10.1051/0004-6361/202142465. ISSN 0004-6361. 
  74. ↑ Eckart, A.; Genzel, R.; Ott, T.; Schödel, R. (2002-04-11). "Stellar orbits near Sagittarius A*". Monthly Notices of the Royal Astronomical Society 331 (4): 917–934. doi:10.1046/j.1365-8711.2002.05237.x. ISSN 0035-8711. Bibcode: 2002MNRAS.331..917E. 
  75. ↑ Peissker, Florian; Eckart, Andreas; Parsa, Marzieh (January 2020). "S62 on a 9.9 year orbit around SgrA*". The Astrophysical Journal 889 (1): 61. doi:10.3847/1538-4357/ab5afd. Bibcode: 2020ApJ...889...61P. 
  76. ↑ GRAVITY Collaboration (2021). "Detection of faint stars near Sagittarius A* with GRAVITY". Astronomy & Astrophysics 645: A127. doi:10.1051/0004-6361/202039544. ISSN 0004-6361. Bibcode: 2021A&A...645A.127G. https://www.aanda.org/articles/aa/pdf/2021/01/aa39544-20.pdf. Retrieved 2024-12-29. 
  77. ↑ Peißker, Florian; Eckart, Andreas; Zajaček, Michal; Basel, Ali; Parsa, Marzieh (August 2020). "S62 and S4711: Indications of a Population of Faint Fast-moving Stars inside the S2 Orbit—S4711 on a 7.6 yr Orbit around Sgr A*". The Astrophysical Journal 889 (50): 5. doi:10.3847/1538-4357/ab9c1c. Bibcode: 2020ApJ...899...50P. 
  78. ↑ Næss, S. (October 4, 2019). "Galactic center S-star orbital parameters". http://folk.uio.no/sigurdkn/s_stars. 
  79. ↑ Matson, John (2012-10-22). "Gas Guzzler: Cloud Could Soon Meet Its Demise in Milky Way's Black Hole". Scientific American. https://www.llnl.gov/news/newsreleases/2012/Oct/NR-12-10-07.html. 
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  81. ↑ 81.0 81.1 81.2 Witzel, G.; Ghez, A. M.; Morris, M. R.; Sitarski, B. N.; Boehle, A.; Naoz, S.; Campbell, R.; Becklin, E. E. et al. (1 January 2014). "Detection of Galactic Center Source G2 at 3.8 μm during Periapse Passage". Astrophysical Journal Letters 796 (1): L8. doi:10.1088/2041-8205/796/1/L8. Bibcode: 2014ApJ...796L...8W. 
  82. ↑ 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). doi:10.1103/PhysRevLett.110.221102. PMID 23767710. Bibcode: 2013PhRvL.110v1102B. 
  83. ↑ 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. doi:10.1093/mnrasl/slt085. Bibcode: 2013MNRAS.435L..19D. 
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  85. ↑ Gillessen. "Wiki Page of Proposed Observations of G2 Passage". https://wiki.mpe.mpg.de/gascloud/ProposalList. 
  86. ↑ "A Black Hole's Dinner is Fast Approaching". ESO. 2011-12-14. http://www.eso.org/public/news/eso1151/. 
  87. ↑ H Hirschfeld, Robert (2012-10-22). "Milky Way's black hole getting ready for snack". Lawrence Livermore National Laboratory. https://www.llnl.gov/news/newsreleases/2012/Oct/NR-12-10-07.html. 
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  92. ↑ "Simulation of gas cloud after close approach to the black hole at the centre of the Milky Way". ESO. http://www.eso.org/public/images/eso1151a/. 

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