S2 (star)

S2, also known as S0–2, is a star in the star cluster close to the supermassive black hole Sagittarius A* (Sgr A*), orbiting it with a period of 16.0518 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. The mass when the star first formed is estimated by the European Southern Observatory (ESO) to have been approximately 14 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 the effort to gather the evidence for the existence of a supermassive black hole in the center of the Milky Way galaxy. By 2008, S2 had been observed for one complete orbit. In 2020, partway through its next orbit, the GRAVITY collaboration released an analysis of the orbit of S2 showing full agreement with Schwarzschild geodesics. 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 it to be 7.94±0.42 kiloparsecs, in close agreement with prior determinations by other methods. S2 was precisely tracked during its May 2018 close approach to Sgr A*, with results in accord with general relativity predictions.