Here's why:

* The Doppler Effect: The Doppler Effect explains how the frequency of waves (like light) changes when the source of the waves is moving relative to the observer.

* Moving Towards: When a star moves towards us, the light waves it emits are compressed, making the wavelength shorter. Shorter wavelengths of visible light appear bluer.

* Moving Away: Conversely, if a star moves away from us, the light waves are stretched, making the wavelength longer. This results in a redshift, where the light appears more red.

In summary: A blue shift indicates the star is approaching, while a redshift indicates it's moving away. This information helps astronomers understand the movement of stars and galaxies in the universe.