 Here's a recent photograph of the Alpha Centauri binary system taken by Hubble. Turns out that half of all stars are actually binary systems like this one. And it is the orbital motion of these kinds of stars that enabled us to measure stellar mass, just like we calculated the mass of the Sun by the motion of the planets around it. These stars orbit the system's center of gravity called the barycenter. We can observe the distance between the stars and locate the barycenter as the center of their elliptical motion. We can also observe the length of time it takes to make a full orbit, its period. This, along with Kepler's laws and Newton's gravitational equations, is all we need. All observations of Alpha Centauri show that the distance between the two stars is just under 24 times the distance from the Earth to the Sun, with A's distance to the barycenter being a little less than half of that. In addition, we see that the orbital period is almost 80 years. This gives us the mass of A at just over the mass of the Sun and the mass of B at just under the mass of the Sun.