Cepheid variable stars are high-luminosity stars that undergo very regular variations in brightness. A typical Cepheid will dim to a fraction of its maximum brightness and then grow brighter again with a period ranging from a few days to several months. During this cycle, the star is moving between two different states. At maximum brightness, the star is more compact and hotter, and pressure within the star causes it to expand. As the star expands, the pressure is released and the star cools. Eventually, the force of gravity is stronger than the outward pressure on within the star, and it collapses in on itself, heating, becoming brighter, and starting the cycle over again. The absolute luminosity of Cepheids, which are 5 to 20 times more massive than the Sun, is related in a precise way to the period of the brightness oscillation, which allows them to be used as standard candles. See: luminosity, standard candle.