Astronomers have directly measured the mass of a single star; the first time such a feat has been accomplished for any solitary star other than our own Sun. The measurement has been done on a small red star located some 1,800 light-years from Earth. Knowing the masses of stars is important in understanding stellar evolution. Until now, scientists could only determine the masses of stars that are members of binary-star systems by applying Newton's laws of gravity to measurements of the stars' orbits around their center of gravity. The new measurement used Einstein's theory of relativity, combined with a large-scale program using ground-based telescopes, and the exquisite resolution of NASA's Hubble Space Telescope.