A correlation between the size of elliptical galaxies, their average surface brightness, and the velocity dispersion of the stars within them. When these three properties are plotted on a graph with three axes, they describe a narrow plane, which is the fundamental plane. Any theory of the origin of elliptical galaxies must explain why these three quantities are so closely related. Astronomers use the fundamental-plane relationship to estimate the distances of ellipticals from a combination of their size and surface brightness.
Subjects: Astronomy and Astrophysics.