A) visually we can only see one star
B) some of the lines in the spectrum are double, with the spacing changing over time
C) an analysis of the ways the lines in the spectrum change allows us to calculate the star's distance directly
D) we can use the spectrum to determine the sum of the masses of the two stars
E) we can often use the changes in the positions of the spectral lines to measure the radial velocity of the stars in the system
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