S2 is a star that is located close to the radio source Sagittarius A*, orbiting it with an orbital period of 15.2 years and a pericenter distance of 17 light hours.
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.
