Reinhard Genzel, at the Max Planck Institute for Extraterrestrial Physics, and Andrea Ghez, at the University of California, Los Angeles, independently lead, from the mid-1990s onward, two teams that track for decades, using ground-based telescopes equipped with adaptive optics — a technique that corrects atmospheric distortion in real time to achieve image sharpness almost comparable to that of space — the precise orbital motion of individual stars situated extraordinarily close to the center of our own galaxy, the Milky Way, in the region known as Sagittarius A*. Both teams observe that these stars orbit at extreme speeds — thousands of kilometers per second — around a completely invisible central point, and that their orbital trajectories can only be explained if that invisible point concentrates a mass equivalent to about four million suns compressed into an extraordinarily compact region, providing the strongest observational evidence to date for the existence of a supermassive black hole at the center of the Milky Way — a theoretical concept predicted much earlier but lacking equivalent direct observational confirmation. The work of both teams, complementary and at times competitive over more than two decades of continuous observation, establishes the standard observational method for detecting supermassive black holes at the nuclei of galaxies, spectacularly confirmed in 2019 with the first direct image of a black hole (in the galaxy M87) obtained by the Event Horizon Telescope.