Why does everything in our galaxy orbit the supermassive black hole at the center?
Category: Space Published: March 7, 2014
By: Christopher S. Baird, author of The Top 50 Science Questions with Surprising Answers and Assistant Professor of Physics at West Texas A&M University
Strictly speaking, everything in our galaxy does not orbit the supermassive black hole at the center. Everything in the galaxy orbits the center of mass of the galaxy. The supermassive black hole just happens to be at the center. If the black hole at the center were removed, the galactic orbits of almost all objects in the galaxy would not change (except for the few stars that are very close to the black hole). Our galaxy contains a lot of mass, which includes stars, gas, planets, and dark matter. The black hole in the center is only about one millionth of the total mass of our galaxy. Because mass causes gravity, and gravity causes orbits, the galactic orbital paths of all objects in the galaxy are caused by the total mass of the galaxy and not the mass of the black hole at the center.
Consider this analogy. Three girls form a circle and all lock their hands at the center of the circle. These girls now run quickly and steadily around their circle so that they can feel the strain in their arms. Even though they can feel the centrifugal force pushing them outwards, they do not fly off because their linked arms pull inward. The girls are all, in a sense, in orbit around their combined center of mass. They are not orbiting the gold ring on the finger of one of the girls, which happens to be at the center of their circle. If she took off her ring, their motion would not change much. The girls are like the objects in our galaxy, their linked arms are like the gravitational force linking everything together, and the gold ring is like the black hole. Every object in the galaxy is in orbit around the center of the combined mass of the galaxy. The center of mass is often called the "barycenter".
In general, small bodies do not orbit large bodies. Instead, large and small bodies together orbit their combined center of mass. The textbook "Orbital Mechanics" by Tom Logsdon notes, "Newton also modified Kepler's first law by noting that if both of the two bodies in question have appreciable mass, the smaller body will not orbit about the center of the larger body. Instead, both of them will orbit around their common barycenter. A similar phenomenon can be observed at a football game. When a majorette tosses her baton into the air, it does not rotate around the heavy end. Instead, the entire baton rotates about its center of mass."