Figure 14-32 A Limit on the Speed of Variations in Brightness The rapidity with which the brightness of an object can vary significantly is limited by the time it takes light to travel across the object. If the object 1 ly in size emits a sudden flash of light, the flash will be observed from Earth to last a full year. If the object is 2 ly in size, brightness variations will last at least 2 years as seen from Earth, and so on.