A star’s light is alternately shifted from the blue part of the light spectrum to the red part.

What does this mean about the length of light waves and movement of the star?

A.
The wavelengths are getting shorter then longer, meaning the star is moving away from the observer.

B.
The wavelengths are getting shorter then longer, meaning the star is moving toward the observer.

C.
The wavelengths are getting shorter then longer, meaning the star is revolving around another unseen object.

D.
The wavelengths are getting shorter then longer, meaning the star is rotating rapidly.