A single star confirmed Einstein's gravity around our galaxy's monster black hole
For nearly three decades, astronomers meticulously tracked a star named S2 as it looped around Sagittarius A*, the supermassive black hole at the center of the Milky Way. Orbiting once every 16 years, S2 screams past the black hole at almost three percent of light speed. Its closest approach gradually shifts around the black hole in a rosette pattern, directly confirming the relativistic precession Albert Einstein predicted a century earlier.
An Extreme Laboratory at the Galactic Center
Roughly 26,000 light-years from Earth, hidden behind thick curtains of interstellar dust, lies the dynamic core of the Milky Way. At the center sits Sagittarius A*, a supermassive black hole packing approximately four million times the mass of our Sun into a remarkably compact region. Because visible light cannot penetrate the dense gas and dust filling the plane of the galaxy, the galactic center was long shrouded in mystery, detectable primarily through radio waves and infrared emissions.
Surrounding this immense gravitational anchor is a dense cluster of stars known to astronomers as the S-cluster. Among these celestial bodies, a luminous, hot star designated S2 stands out. S2 is a massive main-sequence star of spectral type B, shining thousands of times brighter than the Sun. Its proximity to Sagittarius A* makes it a natural probe of gravity in an environment far more violent and compressed than anything found in our own solar system.
The Harrowing Trajectory of Star S2
S2 travels along one of the most extreme orbits ever mapped. Every 16 years, the star completes an elongated, highly eccentric loop around Sagittarius A*. For most of its orbit, S2 swings out to vast distances, but as it nears pericenter—the point of closest approach—the black hole's gravitational pull accelerates the star to staggering velocities.
At its closest pass, S2 skims within roughly 20 billion kilometers of the black hole's center, which corresponds to approximately 120 astronomical units, or about 17 light-hours. At this distance, the star hurtles through space at nearly 7,700 kilometers per second, reaching almost three percent of the speed of light. This intense brush with Sagittarius A* exposes S2 to extreme gravitational forces, turning the star into an ideal test particle for verifying the laws of physics under unprecedented conditions.