Here's why:

* Doppler Effect: Just like the pitch of a siren changes as it moves towards or away from you, light from a moving star also experiences a Doppler shift.

* Blueshift: When a star moves towards Earth, the light waves it emits are compressed, making the wavelengths shorter. This shifts the light towards the blue end of the electromagnetic spectrum, hence the term "blueshift."

* Redshift: Conversely, if a star is moving away from Earth, the light waves are stretched, making the wavelengths longer and shifting the light towards the red end of the spectrum.

By analyzing the amount of blueshift or redshift in a star's light, astronomers can determine its velocity relative to Earth.