Do stars' orbits in galaxies obey Kepler's Laws? (Intermediate)

An ellipse has 2 foci around which the bulk of stars flow. Let's assume that a huge star was pointed out in one of these foci.

Would it then be possible to give proof that all stars in the system, were circling around the focus, just as it is said by Kepler's first law.

Kepler's Laws were derived for the planets orbiting the Sun in our solar system. And they work for any situation in which you have one discrete object with a mass far greater than any of the the objects that orbit it. But with galaxies you have a very different situation. The mass of a galaxy is not all concentrated at the center. It's spread out among all the constituents of the galaxy, across hundreds of thousands of light years. As explained here, even the supermassive black hole at the center of the Milky Way isn't massive enough to greatly affect the orbits of the stars in our galaxy (except for those very close to the center).

In practice, and this is especially true for galaxies' stars, the paths are not perfect ellipses. There is often some precession (a slow change in position) of the orbiting object's path, causing it to never repeat the exact same orbit about the central mass (you can search online for "apsidal precession" if you want to know more).

This page was last updated on July 18, 2015

About the Author

Christopher Springob

Christopher Springob

Chris studies the large scale structure of the universe using the peculiar velocities of galaxies.  He got his PhD from Cornell in 2005, and is now a Research Assistant Professor at the University of Western Australia.

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