Helium burning is fusion of helium nuclei, specifically, the triple alpha process, which takes place in early stars, producing carbon and some and oxygen. In a sufficiently-massive star, helium burning occurs within the stellar core after its red-giant-branch phase, comprising a new phase termed the horizontal branch (or for some types of stars, the red clump). When the carbon and oxygen have sunk to the center and helium burning has ceased there, a helium-burning shell remains surrounding the core. In sufficiently-massive stars, additional types of fusion begin within and surrounding the core as the density and temperature increase. The above is the normal course of helium burning in stellar evolution, but there are some more exotic situations, particularly including interacting binary stars. Helium burning requires a temperature on the order of 5 × 108 K.