Astronomers often want to know whether objects in space are moving toward Earth or away from it.?

Question

How can the Doppler effect be used to identify whether a star is approaching or moving away from Earth?

Answer 1

The doppler shift applies to light as well as sound (at least that is what the theory says). The talk about the "red shift" because, if the star is moving away from us, it will be shifted towards the red end of the visual spectrum (red is longer wavelength than blue). Google "red shift" and astronomy

Answer 2

Because, like sounds, light is a wave, when a light source is moving away from you the wave is stretched in the same way a sound wave does when its sources moves away from you. (Changing tone of a police siren passing you for instance)

With a sound source the stretching or contraction (if moving towards you) changes the pitch of the sound, with light it changes the colour. Different colours are in fact different wave lengths.

Red is a long wave length so objects moving away from you expereince "red shift" as the wave is spread out. This is how they can tell how the universe is expanding because star light displays this red shift. Stars moving towards us would appear blue.