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At the very heart of the Milky Way is a region known as Sagittarius A. This region is known the be the home of a supermassive black hole with millions of times the mass of our own Sun. And with the discovery of this object, astronomers have turned up evidence that there are supermassive black holes at the centers most most spiral and elliptical galaxies.
The best observations of Sagittarius A*, using Very Long Baseline Interferometry radio astronomy have determined that it’s approximately 44 million km across (that’s just the distance of Mercury to the Sun). Astronomers have estimated that it contains 4.31 million solar masses.
Of course, astronomers haven’t actually seen the supermassive black hole itself. Instead, they have observed the motion of stars in the vicinity of Sagittarius A*. After 10 years of observations, astronomers detected the motion of a star that came within 17 light-hours distance from the supermassive black hole; that’s only 3 times the distance from the Sun to Pluto. Only a compact object with the mass of millions of stars would be able to make a high mass object like a star move in that trajectory.
The discovery of a supermassive black hole at the heart of the Milky Way helped astronomers puzzle out a different mystery: quasars. These are objects that shine with the brightness of millions of stars. We now know that quasars come from the radiation generated by the disks of material surrounding actively feeding supermassive black holes. Our own black hole is quiet today, but it could have been active in the past, and might be active again in the future.
Some astronomers have suggested other objects that could have the same density and gravity to explain Sagittarius A, but anything would quickly collapse down into a supermassive black hole within the lifetime of the Milky Way.
We have written many articles about Sagittarius A. Here’s an article about how the Milky Way’s black hole is sending out flares, and even more conclusive evidence after 16 years of observations.
We have recorded an episode of Astronomy Cast all about the Milky Way. Give it a listen: Episode: 99 – The Milky Way