Astronomers calculate the mass of the Milky Way galaxy by measuring the speed of stars orbiting the galaxy's centre. To escape from the galaxy's gravitational pull, an object would have to overcome the gravitational energy that binds it to the galaxy, which is proportional to the galaxy's mass. The galaxy's mass can also be estimated by measuring the motion of stars in the galaxy's outer regions, which are less tightly bound to the galaxy than stars closer to the centre.