S2 - the star that measured the Galactic centre

node · earth · Galactic centre, constellation Sagittarius as seen from Earth

A hot, young star on a highly eccentric sixteen-year orbit around Sagittarius A*, and the single most productive test particle in the study of black holes. At closest approach it passes within roughly 120 astronomical units - on the order of a thousand Schwarzschild radii - and reaches about two and a half per cent of the speed of light. Because the orbit is short enough to be watched in full and close enough for relativistic corrections to be measurable, two decades of infrared astrometry and spectroscopy have used it to weigh the central mass, to confirm that the mass is concentrated within the pericentre distance, and then to detect general-relativistic effects directly: the gravitational redshift of its light at the 2018 pericentre passage, and the Schwarzschild precession of the orbit itself, reported in 2020. Its existence is also a puzzle in its own right, since a massive young star should not easily form so close to a supermassive black hole. Not a place on Earth.