Gavin is looking at stars with his farther and notices some of the stars are brighter than others. Gavin's father tells him that the brightness of a star is one way that scientists determine the star's magnitude. What other property, which Gavin and his father might observe, is used by scientists to classify stars?
A. The constellation the star is in
B. The gas that makes up the star
C. The star's rotation
D. The star's color