Transcript: The motions of stars and gas within the disk of the galaxy can be used to estimate the mass of the Milky Way galaxy, but the Sun is one of billions of stars, some of which are interior to the Sun’s orbit and some of which are far beyond the Sun. So how is it possible to do this? Isaac Newton had a fundamental insight calculating that the motion of an orbiting object is controlled only by the mass within its orbit. This is obvious within the solar system where the planets’ motions are controlled by the Sun which sits at the center of the system. In the Milky Way it means that the Sun’s motions are only governed by the mass within the orbit of the Milky Way. All the regions outside the orbit of the Sun, the Sun does not feel those masses. Applying Newton’s calculation to the situation of the Milky Way disk yields a mass interior to the orbit of the Milky Way of two times ten to the forty-one kilograms, a hundred billion times the mass of the Sun.