Final answer:
The star's radial velocity is measured using the Doppler effect, which detects the tiny variations in the star's speed caused by the gravitational pull of planets in orbit. By studying the radial velocity, astronomers can identify the presence of planets.
Step-by-step explanation:
The star's radial velocity refers to the speed of the star towards or away from the observer. Astronomers measure this using the Doppler effect, which causes the star's spectral lines to shift towards the blue or red end of the spectrum. When a massive planet is in orbit around the star, its gravitational pull causes the star to wobble, resulting in tiny variations in its radial velocity. Therefore, when studying the star's radial velocity, astronomers are looking for these small variations that indicate the presence of one or more planets.