As late as the 1990s, it was theorized that a supermassive black hole was at the center of the Milky Way—but unproven.
To find evidence, astronomers began by observing the Milky Way center regularly, identifying individual stars and their orbits.
As the stars’ orbits became more defined, researchers began theorizing about the Milky Way center: What do the stars point to?
Researchers determined that something 4 million times the mass of the Sun is inside these orbits.
The stars’ orbits provide overwhelming evidence of a supermassive black hole at the center of our galaxy.
A Story Of Black Holes: Sagittarius A*
Identifying our galaxy’s supermassive black hole by tracking stars’ orbits.
A black hole is a region of space packed with so much mass that its own gravity prevents anything from escaping—even a ray of light. Although we can’t see a black hole, telescopes can observe the material around it. Matter swirling around a black hole, which can be made up of gas and dust, heats up and emits radiation that can be detected. In some cases, telescopes can observe the gravitational influence of a black hole on the motions of nearby individual stars.
Although a point-like radio source, known as Sagittarius A* (pronounced Sagittarius A-star), was detected at the center of our Milky Way galaxy in the 1970s, researchers could not accurately determine enough of the characteristics of this compact object to precisely describe it. Beginning in the 1990s, astronomers began using new techniques to measure the orbits of the fast-moving stars orbiting this area. By 2008, they not only tracked complete orbits, but also gained enough information to determine the mass of and distances to the stars. At this point, they could confirm that the mass of the object at the center of those orbits is 4.6 million times the mass of our sun and its size can be no bigger than the orbit of Pluto. That meant it could be nothing other than a supermassive black hole. Researchers still track these stars, but for new reasons: to test fundamental physics, including Einstein’s general theory of relativity, which continues to hold up.
Quick Facts: Sagittarius A*
Also known as:
Distance from Earth:
4.6 million times the mass of the Sun
Type of object:
Supermassive black hole
Location in the sky:
Location in the universe:
Center of our Milky Way galaxy
Did you know:
In 2018, researchers found evidence for thousands of stellar-mass black holes located within 3 light-years of Sagittarius A* at the center of our Milky Way galaxy.
Explore More About Black Holes
Find out more with these additional resources from NASA’s Universe of Learning
Credits: Sagittarius A*
Infrared light images and star orbits from the W. M. Keck Telescopes: A. Ghez and the UCLA Galactic Center Group
Subject-matter expertise provided by Dr. Tuan Do and Dr. Shoko Sakai
Produced by the Space Telescope Science Institute’s Office of Public Outreach