S2 (star)
 The clock-wise orbit of 'S2' around Sagittarius A* |
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| Observation data Epoch J2000.0 Equinox J2000.0 (ICRS) |
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|---|---|
| Constellation | Sagittarius |
| Right ascension | 17h 45m 40.044s |
| Declination | −29° 00′ 28″ |
| Characteristics | |
| Spectral type | "B1V |
| Variable type | None |
| Astrometry | |
| Distance | 25900 ± 1400 ly (7940 ± 420pc) |
| Orbit | |
| 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 |
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Argument of periastron (ω) (secondary) |
62.6 ± 1.4° |
| Details | |
| Mass | 15 M☉ |
| Other designations | |
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[CRG2004] 13, [GKM98] S0-2, [PGM2006] E1, [EG97] S2, [GPE2000] 0.15, [SOG2003] 1, S0—2.
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| Database references | |
| SIMBAD | data |
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 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 15 M☉.
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.
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.
The orbit of S2 will give astronomers an opportunity to test for various effects predicted by general relativity and even extra-dimensional effects. These effects reach their maximum at closest approach, which will next occur in mid-2018. 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).
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