Change in Velocity: If a star appears to move toward us for awhile and then away from us, there might be a planet nearby pulling on the star
- Every star has a unique "fingerprint" of light
- When the stars moves away from us, its "fingerprint" get redder
- When the star moves toward us, its "fingerprint" gets bluer
- Measurement of the shift from red to blue is actually a measure of the shift in the star's radial velocity.