| Observation data EpochJ2000 EquinoxJ2000 | |
|---|---|
| Constellation | Sagittarius |
| Right ascension | 17h 45m 40.0409s |
| Declination | −29° 0′ 28.118″ |
| Characteristics | |
| Astrometry | |
| Distance | 26,674±42 ly (8,178±13 pc) |
| Orbit | |
| Companion | Sagittarius A* |
| Period (P) | 12.8±0.1 yr |
| Semi-major axis (a) | 0.1078±0.0010″ |
| Eccentricity (e) | 0.721±0.008 |
| Inclination (i) | 150±2° |
| Longitude of the node (Ω) | 325±4° |
| Periastronepoch (T) | 2009.34±0.04 |
| Argument of periastron (ω) (secondary) | 332±4° |
| Other designations | |
| [GKM98] S0–102. | |
| Database references | |
| SIMBAD | data |
S55 (also known asS0–102) is a star that is located very close to the centre of the Milky Way, near the radio sourceSagittarius A*, orbiting it with an orbital period of 12.8 years. This beat the record of 16 years previously set byS2. The star was identified by aUniversity of California, Los Angeles team headed byAndrea M. Ghez.[1] At itsperiapsis, its speed reaches 1.7% of thespeed of light. At that point it is 246astronomical units (34 light hours, 36.7 billion km) from the centre,[2][3] while the black hole radius is only a small fraction of that size (theSchwarzschild radius is about0.082 au).[4] It passed that point in 2022 and will be there again in 2034.
In 2019 the starS62 was believed to have surpassed S55 to become be the new record holder, but further scrutiny found this star was much further from the star than believed, and earlier observations were consistent with starS29 instead.[5] In 2020, further observation discovered several stars in theSagittarius A* cluster with shorter orbital periods than S55.[6]
Its position in the sky has been monitored from 2000 to 2012 using theW. M. Keck telescope and from 2002 to 2016 with theVLT. One complete orbit has been observed. From Earth's current perspective, it travels in a clockwise direction. Having observed two stars orbiting through complete periods around the centre (S55 and S2), thegravitational potential of SgrA* could be established.[7] It is also possible that there is a great deal ofdark matter around the orbits of these stars.[8] Also general relativistic effects due togravitational redshift should become observable.[7]
