An astronomer wishes to measure on a photograph the distance between the image of a certain star and three other star images nearby. If eight images are nearby, how many choices of the three stars does he have?

8 C 3 = 8! / (3!(8 - 3)!) = 8! / (3! * 5!) = (8 * 7 * 6 * 5!) / (3! * 5!) = (8 * 7 * 6) / (6) = 56 

56 choices.