Galactic Black Holes and Consuming Rest of Galaxy
Date: Fall 2012
Why has the black hole at the center of the Milky Way not consumed the entire galaxy?
For the same reason the planets have not fallen into the Sun. Gravity is attractive, but is not equivalent to a vacuum cleaner, meaning gravity does not "suck." The gravity of a single object alone, such as that due to the Sun and the central black hole, cannot disrupt a stabile orbit of, e.g., a planet around a star or a star around the center of the Galaxy.
Joseph P. Bernstein
This is an over-simplified analogy, but you might ask why does a vacuum cleaner not suck up all the dirt, dust and air in a room?
For the vacuum cleaner to capture dirt, dust and air, those components have to get close enough for the suction to overcome all the other forces operating on the various components. In the case of a black hole, gravity is still operative. So stellar dust, stars, etc. must get close enough for the gravitational force to overcome all of the other forces operating on surrounding bodies. The black hole can “sweep out” a part of a galaxy, but when all the matter within that part of the Universe has been “cleaned out”, there is nothing left for the black hole to drag into its event horizon.
Good question. The black hole at the center of our galaxy, though massive, is nowhere near strong enough to consume more than few nearby stars. Hope this helps.
David H. Levy
Click here to return to the Astronomy Archives
Update: December 2011