How does the Doppler effect help astronomers know in which direction a star is moving?

The Doppler effect explains how frequency shifts occur in waves traveling between two objects.

If you have ever heard an increasingly strident police siren approaching you, then shifting to a lower pitch as it passes and moves further away, you have experienced the Doppler effect.

In 1842, the Austrian astronomer Christian Doppler published a hypothesis attributing the color differences of starlight to the movement of the stars toward or away from Earth. Using the analogy of an advancing ship meeting sea waves more rapidly than a ship at rest, Doppler predicted that his hypothesis could apply to all types of waves.

The British Naval architect, John Scott Russell, proved Doppler's hypothesis in an 1848 experiment applied to sound waves.

The Doppler effect describes how successive crests of waves (such as light) from an approaching emitter (such as a glowing star) arrive faster, or at a higher frequency, causing the waves to "bunch together".

The "bunching" of light waves corresponds to an increase of frequency on the color spectrum, toward the blue and violet tones. Thus a star moving toward Earth will have bluish light as seen from Earth - this phenomenon is termed "blueshift". Conversely, a star moving away from Earth will be seen as reddish in color, also known as "redshift", because light waves are "stretched" to the lower red frequencies in the spectrum.