Following exhaustion of hydrogen in the core, the overall stellar structure drastically changes. Hydrogen burning now takes place in a narrow shell surrounding the newly processed helium core. While the helium core quickly contracts and heats up, the layers above the hydrogen-burning shell undergo important expansion and cooling. The star becomes a red giant whose radius and luminosity increase in time. These stars are now located on the so-called red giant branch of the Hertzsprung–Russell diagram; they are commonly named "RGB stars". Once their central temperature reaches 100 106 K, helium starts burning in the core. For stellar masses smaller than about 2 Mʘ, this new combustion takes place in a highly degenerate matter and proceeds through a helium flash. The readjustment following the flash brings the red giant to the so-called red clump (RC) in the Hertzsprung-Russell diagram.