The Milky Way is a disk of hundreds of billions of stars, and for decades astronomers suspected a strong gravity source at its center. Using infrared telescopes and adaptive optics, Genzel and Ghez measured the positions and velocities of central stars with milliarcsecond precision. Elliptical orbits revealed a dark mass about four million times that of the Sun, making a black hole the only plausible explanation. Their work provided the most convincing dynamical evidence for a galactic-center black hole. Today, relativistic redshift and other subtle effects are being measured to test gravity even further.

Genzel used ESO’s VLT while Ghez employed the Keck telescopes, combining laser-guide-star adaptive optics and interferometric techniques to correct atmospheric turbulence. They tracked Keplerian orbits of fast-moving stars like S2 and S38 for more than two decades, integrating proper motions with radial velocities to pinpoint the central mass and position at sub-AU precision. The inferred mass concentration has a radius smaller than the expected event horizon and a density exceeding 10^17 kg m⁻³, ruling out star clusters or dark-matter clumps. During pericenter passage, relativistic redshift was detected, confirming post-Newtonian corrections. These observations suggest that supermassive black holes grew early in galaxy formation and provide a bridge to active galactic nuclei studies.

Using speckle holography and laser adaptive optics, both teams achieved positional accuracies better than 0.1 mas for Galactic-Centre stars. At S2’s 2018 pericenter they measured a GR redshift of +200 km s⁻¹, constraining terms up to 1.05 PN order. Bayesian MCMC yielded M• = (4.10 ± 0.03) × 10^6 M☉ and a distance R₀ = 8.178 ± 0.035 kpc. Potential models including non-Keplerian terms were rejected with Bayes factors > 10^5 in favor of a black hole. Ongoing observations have found new short-period stars like S29, with detection of Lense-Thirring precession set as the next milestone.