Andrea Ghez shared the 2020 Nobel Prize for discovering the compact object at the Milky Way’s center with Reinhard Genzel.
Ghez is an American astrophysicist who led a long-term observing program at UCLA, while Genzel led an independent team based at the Max Planck Institute and the University of California, Berkeley. Their groups tracked stars orbiting the radio source Sagittarius A* at the center of the Milky Way. The stars’ extremely fast, tightly bound orbits showed that an enormous mass was concentrated in a very small region.
That evidence established the presence of a supermassive black hole, although the Nobel citation carefully called it a “supermassive compact object.” The 2020 Physics Prize was divided: Roger Penrose received half for theoretical work on black-hole formation, while Ghez and Genzel shared the other half.
A common mix-up is assuming Ghez discovered the object alone. Her work was independent of Genzel’s, and both teams contributed decisive observational evidence. The object is usually called Sagittarius A*, abbreviated Sgr A*.