Discovery of a star sensitive to the spin of Sagittarius A

Residing in the centre of the Milky Way, Sagittarius A* (Sgr A*) is the closest massive black hole¹ (MBH). Its vicinity has allowed measuring individual stellar orbits around it^2-4. The stars act as test particles and probe the gravitational potential around the 4.3 × 10⁶M_⊙ MBH. These observations have determined the central mass to sub-per-cent precision⁵, and the mildly relativistic motions of stars have given access to the dominant relativistic corrections, the gravitational redshift^6,7, the transverse Doppler effect and the prograde precession imposed by the Schwarzschild metric nature of the potential⁸. These effects are of order β² = (v/c)² (for velocity v and speed of light c). The Kerr metric for a rotating black hole leads to corrections of order β³. Here, we report the discovery of a faint main-sequence star (m_K = 19.3), S301, on an 8.7-year orbit and with small enough a pericentre distance, such that the peak velocity of the star reaches 25,000 km s^-1. Within the measurement abilities of current near-infrared interferometry and future spectroscopy on an extremely large telescope, the motion of S301 is directly sensitive to the spin of Sgr A*. The high eccentricity of S301 suggests that it is the captured component of a binary that was torn apart by the Hills mechanism.