Suppose that you measure the light of a star between 640 and 680 nm and you get a very strong peak around 650 nm. what can you say about the star's motion?

Answer: The star is moving away from Earth

Explanation:

Let's begin by explaining what the Doppler shift is:

This is related to the Doppler effect and refers to the change in a wave perceived frequency (or wavelength=color) when the emitter of the waves, and the observer move relative to each other.

From there, it is deduced that the farther the object is, the more redshifted it is in its spectrum. For example, as a star moves away from the Earth, its espectrum turns towards the red and as the star moves toward the Earth, its espectrum turns towards the blue.

In this sense, since the peak in wavelength is around 650 nm, which corresponds to red, it can be said the star is moving away from Earth.