Transcript: Luminosity, distance, and apparent brightness are all related by the inverse square law of light. If we measure any two of these quantities we can estimate the third. For example, if two stars have the same apparent brightness but one is known to be three times more distant, say by parallax measurement, than the more distant star must be nine times more luminous. Or, if we have two stars of equal luminosity and one appears four times fainter than we know the fainter star is two times further away. Finally, if we have two stars at an equal distance, perhaps they’re in a cluster, than the star that appears five times brighter must be five times more luminous.