S2 (star)

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Orbit of S2.jpg
The clock-wise orbit of 'S2' around Sagittarius A*, which is near the bottom of the ellipse. S2 is shown near the top. An arc of the orbit of another star is shown to the right.
Observation data
Epoch J2000.0      Equinox J2000.0 (ICRS)
Constellation Sagittarius
Right ascension 17h 45m 40.044s[1]
Declination −29° 00′ 28″[1]
Spectral type "B1V[1]
Variable type None
Distance 25900 ± 1400 ly
(7940 ± 420[2] pc)
Companion Sagittarius A*
Period (P) 15.24 ± 0.36 yr
Semi-major axis (a) 0.1226 ± 0.0025″
Eccentricity (e) 0.8760 ± 0.0072
Inclination (i) 131.9 ± 1.3°
Longitude of the node (Ω) 221.9 ± 1.3°
Periastron epoch (T) 2002.315 ± 0.012
Argument of periastron (ω)
62.6 ± 1.4°
Mass 13.6 (+ 2.2, -1.6) M
Other designations
[CRG2004] 13, [GKM98] S0-2, [PGM2006] E1, [EG97] S2, [GPE2000] 0.15, [SOG2003] 1, S0—2.
Database references

Source 2 (abbreviated S2), also known as S0–2, is a star that is located close to the radio source Sagittarius A*, orbiting it with an orbital period of 15.56 ± 0.35 years, a semi-major axis of about 970 AU, and a pericenter distance of 17 light hours (18 Tm or 120 AU)—an orbit with a period only about 30% longer than that of Jupiter around the Sun, but coming no closer than about four times the distance of Neptune from the Sun. As of 2002, its mass was initially estimated by the European Southern Observatory (ESO) to be approximately 14 M.[3]

Its changing apparent position has been monitored since 1995 by two groups (at UCLA and at the Max Planck Institute for Extraterrestrial Physics) as part of an effort to gather evidence for the existence of a supermassive black hole in the center of the Milky Way galaxy. The accumulating evidence points to Sagittarius A* as being the site of such a black hole. By 2008, S2 had been observed for one complete orbit.[4]

A team of astronomers mainly from the Max Planck Institute for Extraterrestrial Physics used observations of S2's orbital dynamics around Sgr A* to measure the distance from the Earth to the galactic center. They determined the distance to be 7.94 ± 0.42 kiloparsecs, in close agreement with prior determinations of the distance by other methods.[2][5]


The orbit of S2 will give astronomers an opportunity to test for various effects predicted by general relativity and even extra-dimensional effects.[6] These effects reach their maximum at closest approach, which will next occur in mid-2018.[7][8] Given a recent estimate of 4.3 million solar masses for the mass of Sagittarius A* and S2's close approach, this makes S2 the fastest known ballistic orbit, reaching speeds exceeding 5000 km/s (11,000,000 mph, or 1/60 the speed of light) and acceleration of about 1.5 m/s2 (almost one-sixth of Earth's surface gravity).[9]

The motion of S2 is also useful for detecting the presence of other objects near to Sagittarius A*. It is believed that there are thousands of stars, as well as dark stellar remnants (stellar black holes, neutron stars, white dwarfs) distributed in the volume through which S2 moves. These objects will perturb S2's orbit, causing it to deviate gradually from the Keplerian ellipse that characterizes motion around a single point mass.[10] So far, the strongest constraint that can be placed on these remnants is that their total mass comprises less than one percent of the mass of the supermassive black hole.[11]


In 2012, a star called S0–102 was found to be orbiting even closer to the Milky Way's central supermassive black hole than does S0-2. At one-sixteenth the brightness of S0-2, S0-102 was not initially recognized because it required many more years of observations to distinguish it from its local infrared background. S0-102 has an orbital period of 11.5 years, even shorter than that of S0-2. Of all the stars orbiting the black hole, only these two have their orbital parameters and trajectories fully known in all three dimensions of space. The discovery of two stars orbiting the central black hole so closely with their orbits fully described is of extreme interest to astronomers, as the pair together will allow much more precise measurements on the nature of gravity and general relativity around the black hole than would be possible from using S0-2 alone.



NAME Right ascension Declination Apparent magnitude (V) Spectral type Database references
Sgr A* 17h 45m 12s −28° 48′ 18″ radio source Simbad


  1. ^ a b c "[EG97] S2". SIMBAD. Centre de données astronomiques de Strasbourg. Retrieved July 15, 2008. 
  2. ^ a b c Eisenhauer, F.; et al. (2003). "A Geometric Determination of the Distance to the Galactic Center". The Astrophysical Journal. 597 (2): L121–L124. arXiv:astro-ph/0306220Freely accessible. Bibcode:2003ApJ...597L.121E. doi:10.1086/380188. 
  3. ^ Habibi, M.; et al. (2009). "Twelve Years of Spectroscopic Monitoring in the Galactic Center: The Closest Look at S-stars near the Black Hole". The Astrophysical Journal. IOP Publishing. 847 (2): 120. arXiv:1708.06353Freely accessible. Bibcode:2017ApJ...847..120H. doi:10.3847/1538-4357/aa876f. 
  4. ^ A short documentary on Sagittarius A* on YouTube
  5. ^ Max-Planck-Institut für extraterrestrische Physik—Infrared/Submillimeter Astronomy—Galactic Center Research
  6. ^ Black Hole as Peephole
  7. ^ Ghez, Andrea M. (speaker) (19 April 2016). Black Holes @ 100 Workshop: Galactic Center (Lecture). Harvard University: Black Hole Initiative. Remarks beginning at 31:55. ...we are '2018 or bust' these days, because at that moment your orbital determination becomes so much better. 
  8. ^ http://www.syfy.com/syfywire/a-star-is-about-to-plunge-head-first-toward-a-monster-black-hole-astronomers-are-ready-to
  9. ^ Surfing a Black Hole
  10. ^ Sabha, Nadeen; Eckart, Andreas; Merritt, David; Mohammad Zamaninasab; Gunther Witzel; Macarena García-Marín; Behrang Jalali; Monica Valencia-S.; et al. (September 2012). "The S-Star Cluster at the Center of the Milky Way: On the nature of diffuse NIR emission in the inner tenth of a parsec". Astronomy and Astrophysics. 545: A70. arXiv:1203.2625Freely accessible. Bibcode:2012A&A...545A..70S. doi:10.1051/0004-6361/201219203. 
  11. ^ Gillessen, S.; et al. (2009). "Monitoring Stellar Orbits Around the Massive Black Hole in the Galactic Center". The Astrophysical Journal. IOP Publishing. 692 (2): 1075–1109. arXiv:0810.4674Freely accessible. Bibcode:2009ApJ...692.1075G. doi:10.1088/0004-637X/692/2/1075. 
  12. ^ "Successful First Observations of Galactic Centre with GRAVITY". Retrieved 28 June 2016. 
  13. ^ Eisenhauer, F.; Genzel, R.; Alexander, T.; Abuter, R.; Paumard, T.; Ott, T.; Gilbert, A.; Gillessen, S.; Horrobin, M.; Trippe, S.; Bonnet, H.; Dumas, C.; Hubin, N.; Kaufer, A.; Kissler‐Patig, M.; Monnet, G.; Strobele, S.; Szeifert, T.; Eckart, A.; Schodel, R.; Zucker, S. (2005). "SINFONI in the Galactic Center: Young Stars and Infrared Flares in the Central Light‐Month". The Astrophysical Journal. 628: 246. arXiv:astro-ph/0502129Freely accessible. Bibcode:2005ApJ...628..246E. doi:10.1086/430667. 

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