Transcript: Apparent brightness does not express a star or other source of light’s true energy output. Astronomers are more interested in absolute brightness or equivalently absolute magnitude or luminosity. For example, we can consider a situation where the apparent brightness of a 100 watt light bulb at a distance of 100 meters is actually the same as the apparent brightness of a dim, 1 watt nightlight at a distance of 10 meters, and both of those are the same at apparent brightness as an arc lamp of 10,000 watts at a distance of a kilometer. The three are obviously hugely different situations in terms of the intrinsic emission of light, and yet the apparent brightness of all three is the same. This is the situation astronomers find themselves in. Stars are not all 100 watt light bulbs, nor do they have their brightness written on them. And so astronomers try to use absolute brightness but find that it’s a very poor reflection of true brightness or distance.