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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 subsequent to its RGB (red giant branch) phase, comprising a further 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 throughout the core has ceased, a helium-burning shell can form 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 as part of stellar evolution, but there are some more exotic situations, particularly involving interacting binary stars. Helium burning requires a temperature on the order of 5 × 108 K.