Asymptotic giant branch forming helium (known as hydrogen burning), and a very large envelope of material of composition similar to main-sequence stars. When a star exhausts the supply of hydrogen by nuclear fusion processes in its core, the core contracts and its temperature increases, causing the outer layers of the star to expand and cool. The star becomes a red giant, following a track towards the upper-right hand corner of the HR diagram. Eventually, once the temperature in the core has reached approximately , helium burning (fusion of helium nuclei) begins. The onset of helium burning in the core halts the star's cooling
Giant star giant or supergiant spectral features while still burning hydrogen in their cores, due to mixing of heavy elements to the surface and high luminosity which produces a powerful stellar wind and causes the star's atmosphere to expand. A star whose initial mass is less than approximately will not become a giant star at all. For most of their lifetimes, such stars have their interior thoroughly mixed by convection and so they can continue fusing hydrogen for a time in excess of 10 years, much longer than the current age of the Universe. They steadily become hotter and more luminous throughout