Binary stars consist of a pair of stars orbiting a common central point.
Observations with telescopes reveal that the orbital motion conforms well to the same pattern observed for planets in our solar system and can be accurately described with the same theories. From the rate at which the stars appear to be orbiting each other, and our knowledge of astronomical distance scales, we can deduce that the orbital speeds of these stars can be quite high. Suppose that the speed of light depended on source velocity. Then, since at various stages in its orbit one of these stars has a varying velocity with respect to an Earth based observer, the speed at which light is emitted towards Earth should vary according as to where the star was in its orbit. However, even if the variation in light speed was quite small, the enormous distances to the nearest stars means that there would be a significant difference in the time taken for light to reach Earth.