Why did Andrea Ghez and Reinhard Genzel receive the 2020 Nobel Prize in Physics?

American astronomer Andrea Ghez and German astronomer Reinhard Genzel were awarded the 2020 Nobel Prize in Physics for their discovery of a supermassive black hole at the centre of the Milky Way Galaxy.21

Working independently, both researchers and their respective teams utilized advanced observational techniques—such as infrared speckle imaging and adaptive optics—to track the orbits of individual stars near the galactic centre. By measuring the velocities and paths of these stars, they demonstrated that the centre of the galaxy is a compact, massive object rather than an extended stellar cluster. Their data confirmed that this central source, known as Sagittarius A*, possesses a mass approximately four million times that of the Sun, providing compelling evidence for the existence of a supermassive black hole.2134