Sagittarius A* is a massive black hole at the center of the Milky Way. Now that astronomers have imaged it, they can begin to learn more about black holes within other galaxies across the universe.
In the 1980s, two teams of astronomers started tracking the motions of stars near this mysterious source of radio waves. They saw stars whirling around a dark object at speeds up to a third of the speed of light. Their motions suggested that at the center of the Milky Way was a black hole 4 million times the mass of the Sun. Reinhard Genzel and Andrea Ghez later shared the Nobel Prize in Physics for this discovery.