Transcript: In 1595 the amateur astronomer and Lutheran pastor David Fabricius noticed the bright star in a constellation Cetus fading until it became invisible and was amazed several months later to see it reappear. The star Mira, called wonderful, has a period of eleven months and is a classic example of a long period variable star. Most stable stars are in hydrostatic equilibrium. They act like a thermostat, but giants have atmospheres that trap some fraction of the energy as it’s released. The energy dams up causing the outer layers to heat up and expand. The expansion lowers the pressure and then the layer can contract again, and the process is repeated. Notice that a variable star in a long period situation is not due to any variation in the rate of energy generation in the core. It’s due to a variation in the rate of energy release. Think of a boiling pan of water with a lid on it. The energy flow through the pan is constant, but steam builds up under the lid and the lid periodically tips to release the steam. This is a similar type of mechanism.