Almost every galaxy, including our Milky Way, has a supermassive black hole (an invisible object of great density) at its heart, with masses billions of times the mass of the Sun. Detecting these central black holes, however, takes some detective work. For one thing, they are invisible. This is because a black hole is a place in space where gravity is so strong that neither particles or light can escape from it. 

Precise measurements of the orbits of these stars allowed astronomers to confirm the existence of this supermassive black hole and to measure its mass.  Astronomers Reinhard Genzel and Andrea Ghez have been done that for over 30 years. Genzel who works at the Max Planck Institute for Extraterrestrial Physics in Germany  and Ghez who works at UCLA, have been observing stars at the center of our galaxy with the aim of studying what goes on there. They tracked around thirty of the brightest stars in that region and found that stars that are within a radius of one light-month from the centre move rapidly in a dance-like motion like a swarm of bees, while those that are outside this area follow elliptical orbits in a more orderly manner. The rapid movements of the central stars led them to conclude that an extremely massive, invisible object must be dictating the central stars’ frantic movements. This object – known as Sagittarius A* (Sgr A*) – is the most convincing evidence yet of a supermassive black hole at the centre of our Milky Way.

For their work in discovering the universe’s most famous black hole (which also provide empirical support for the theoretical existence of black holes implicit in Einstein’s theory of General Relativity), Genzel and Ghez shared the 2020 Nobel Prize for Physics with the theoretical physicist Roger Penrose.  Meanwhile, astronomers are now busy studying why the heart of galaxies often hosts a supermassive black hole or perhaps more than one.

ESO Video: Animation of S2’s precession effect