Transcript: Apparent Brightness in astronomy is the number of photons per second collected at the Earth from an astronomical source. It depends on three things: First, the collecting area of the device used to observe the source of light. In the case of a telescope, the aperture of collecting area is much larger than the eye, so the apparent brightness is greater. It depends on the distance to the source; apparent brightness varies according to the inverse square law. If the source is two times closer, the apparent brightness is four times larger. If the source is three times closer, the apparent brightness is nine times larger, and so on. And it depends on the intrinsic brightness of the source, which is to say how many photons per second the source actually emits. Apparent brightness can be quoted as photons per second or can be given relative to the Sun or another bright star, in which case it’s a ratio with no units.