Red Giants and the Horizontal Branch
When stars of moderate mass start to exhaust the amount of hydrogen in their core, the core has to heat up to keep the fusion rate high enough to balance the pressure of the gas above it. As the core begins to exhaust all its hydrogen, burning ignites in a shell above the core of helium ash. This causes the overall luminosity, or brightness, of the star to increase, the overlying volume to expand, and the atmosphere of the star to cool - the star ascends the so called red giant branch, becoming brighter, cooler (and consequently) redder. Many find this intuitive, but there is one strange thing - why should the interior of the star becoming hotter make the exterior cooler? Our friend the virial theorem, which says that 2K+U = 0, where K is the kinetic energy and U is the gravitational potential energy, might have something to say. The interior getting hotter, and the star consequently expanding, increases both K and U, since U becomes less negative. Consequently K for the outer re...