The change in a star's spectrum when it moves away from Earth is called redshift.

Here's why:

* Doppler Effect: This principle applies to light waves just like sound waves. When a source of light is moving away from an observer, the wavelengths of light appear stretched out. This means the light shifts towards the red end of the electromagnetic spectrum (longer wavelengths).

* Spectral Lines: Stars emit light at specific wavelengths, creating dark lines (absorption lines) in their spectra. Redshift causes these lines to shift towards longer wavelengths (redder).

So, when a star is moving away from us, its spectral lines are "redshifted", indicating that the star is receding.