‘Hypervelocity’ stars move fast. So fast they can escape from the gravitational pull of the Milky Way. A few tens of hypervelocity stars are known but the Gaia space telescope is expected to find hundreds more as it maps the positions and velocities of stars in the sky. A star’s velocity is split into its tangential velocity (how fast it moves across the sky, a.k.a proper motion) and radial velocity (how fast it moves towards or away from us). Combining them together using trigonometry gives the star’s total velocity.