20. If a star's spectrum is shifted to the hlue end of the spectrum this means the star is moving A. away from Earth B. toward Earth C. at the same speed as Earth D. much slower than Enrth

B. toward Earth<br /><br />A blueshift in a star's spectrum indicates that the star is moving towards Earth. This is due to the Doppler effect, which compresses the wavelengths of light from an approaching object, shifting them towards the blue end of the spectrum.<br />