Measuring the proper motion of a galaxy
The motion of a star relative to us can be determined by measuring two quantities, radial motion and proper motion. Radial motion is the motion of a star along our line of sight. That is, motion directly toward us or away from us. Proper motion is the change in angular position of the star, from which we can calculate the motion perpendicular to the line of sight (known as transverse motion. You can see how this works by imagining someone walking through a room. If we know the rate at which the person is walking toward us or away from us (radial motion) and their movement relative to the far wall (proper motion), then we can use a bit of simple geometry to calculate their path through the room.With stars, it turns out that the radial motion is easiest to measure. Stars don't give off a continuous spectrum of light,...