Question

Suppose a particular spectral line has a wavelength of 500 nm in the lab. If you look at the spectrum of star A, you see this line at 480 nm. Star B exhibits this line at 510 nm, and the line is measured to be at 530 nm in star C. Which of the following statements is true?

a. Star A is moving the fastest of the three stars.
b. Star A is the only star moving away from us, star B and star C are both moving towards us (with star C moving towards us the fastest)
c. Star C is moving away from us faster than star B, star A is moving towards us.

Answer 1

c. Star C is moving away from us faster than star B, star A is moving towards us.

Since objects moving away are redshifted, and the amount of redshift tells us the speed, you can see how to tell the speed and direction of an object just by looking at its spectrum.