A Close Look at an Extreme Star
At the heart of our galaxy lies the supermassive black hole Sagittarius A*. A Sun‑like star designated S2 travels on a highly elliptical orbit that brings it as close as the distance of Neptune’s orbit around the Sun. At its closest approach the star exceeds eight percent of the speed of light, making it one of the fastest objects known near a black hole.
These extreme conditions give astronomers a unique laboratory to test predictions of general relativity. According to Einstein, the star’s light should experience a measurable gravitational redshift in the black hole’s strong field, in addition to the ordinary Doppler shift caused by its high velocity. Spectroscopic observations indeed show a combination of both effects that matches the relativistic equations.
Besides the redshift, the advance of S2’s periapsis—its orbital precession—can be measured. The orbit rotates slightly with each revolution, a phenomenon absent in Newtonian gravity but predicted by relativity. The observed precession agrees with theoretical values within the measurement uncertainties, confirming Einstein’s theory even in this intense gravitational environment.
Decades of monitoring S2 have allowed precise determination of the mass and distance of Sagittarius A*. These results not only deepen our understanding of the Milky Way’s centre but also provide stringent tests for alternative theories of gravity that seek to modify or replace general relativity.