What is Sagittarius a*?

Michael Anissimov
Michael Anissimov

Sagittarius A* (pronounced "A-star") is a region in the center of our galaxy, approximately as wide as the orbit of Pluto, containing 3.7 million solar masses of material. Located near the galactic center, Sagittarius A* is suspected by astronomers to be a supermassive black hole, serving as the center of gravity for the entire galaxy. Sagittarius A* is closely orbited by at least a dozen stars, the trajectories of which have been used to estimate its mass. It may even be orbited by the first observed intermediate-mass black hole, GCIRS 13E, which is estimated at 1,300 solar masses.

Sagittarius A* is suspected by astronomers to be a supermassive black hole.
Sagittarius A* is suspected by astronomers to be a supermassive black hole.

As the mass of a black hole increases, the radius of its event horizon increases at a linear rate, but the density decreases as the cube of the radius. So, while black holes like Sagittarius A* are very massive, when you count the huge area of the event horizon, estimated at 6.25 light-hours (45 AU) or about 4.2 billion miles, the average density of the hole is no greater than that of air! Stellar-mass black holes have much greater densities behind their event horizon.

Sagittarius A* is a region near the center of the Milky Way with 3.7 million solar masses of material.
Sagittarius A* is a region near the center of the Milky Way with 3.7 million solar masses of material.

Sagittarius A* is located approximately 25,000 light years away, or half a galactic radius, at the galaxy's center. It probably formed early on in the galaxy's history. We observe supermassive black holes like Sagittarius A* in the process of being formed in other, very distant galaxies. These phenomena are called quasars and blazars.

Because the central singularity in a supermassive black hole is located so far from the event horizon, an astronaut falling into it would not experience spaghettification until deep inside the hole. The inside of a black hole would be a strange place -- with light orbiting the hole at a rapid rate, you would be constantly treated to a repetitive blur of objects in its grasp. Light from the outside would first look like only a hemisphere, with darkness all behind, then the hemisphere would get progressively smaller, becoming a little circle and eventually a point. Falling into a black hole would not be fun!

Michael Anissimov
Michael Anissimov

Michael is a longtime wiseGEEK contributor who specializes in topics relating to paleontology, physics, biology, astronomy, chemistry, and futurism. In addition to being an avid blogger, Michael is particularly passionate about stem cell research, regenerative medicine, and life extension therapies. He has also worked for the Methuselah Foundation, the Singularity Institute for Artificial Intelligence, and the Lifeboat Foundation.

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