In the light from a distant star, a particular spectral line is observed. The wavelength of this line varies between being about 2% shorter than the same line as observed in an earth based laboratory, and about 2% longer. Which of the following best describes the star.
(a) The star is moving away from the earth.
(b) The star is moving towards the earth.
(c) The star is very, very massive as compared to the sun.
(d) The star is orbiting about some massive hidden object.