What is called “the Doppler effect” in astronomy?

The variation in light from moving stars

The Doppler effect, a phenomenon observed in light and sound waves, occurs when the source of a wave moves relative to an observer. In astronomy, this effect explains the redshift or blueshift in the light emitted by stars and galaxies, indicating whether they are moving away from or towards us. This discovery was crucial for understanding that the universe is expanding, a revelation that has profoundly changed our conception of the universe. Thus, the Doppler effect is an essential tool for measuring the speed and direction of celestial bodies.