S2 (star)

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**S 2**
Observation data Epoch J2000
S2 and Sgr A
Constellation (pronunciation)	Sagittarius
Right ascension	17h 45m 40.044s
Declination	-29° 00' 28"'
Apparent magnitude (V)	5.28
Characteristics
Spectral type	B1V
U-B color index	?
B-V color index	?
Variable type	None
Astrometry
Details
Mass	? M_☉
Radius	? R_☉
Surface gravity (log g)	?
Luminosity	? L_☉
Temperature	? K
Metallicity	?
Rotation	?
Age	? years
Visual binary orbit
Companion	Sagittarius A*
Period (P)	15.56 +/- 0.35 yr
Semimajor axis (a)	0.05"
Other designations
[CRG2004] 13, [GKM98] S0-2, [PGM2006] E1, [EG97] S2, [GPE2000] 0.15, [SOG2003] 1, S0—2.
Database references
SIMBAD	data

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. Given a recent estimate of 3.7 million M_sun for the mass of Sagittarius A*, the maximum gravitational acceleration experienced by S2 at the pericenter of its orbit would be about 1.5 m/s² or almost one-sixth of Earth's surface gravity.

Its changing apparent position has been monitored since 1995 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 black hole. A large part of one orbit had been observed as of 2004.

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. [1] [2]

The 'S' in 'S2' simply stands for 'star'.