from the red-giant branch (RGB) onto the horizontal branch of the Hertzsprung–Russell diagram. Stars initially between about and have larger helium cores that do not become degenerate. Instead their cores reach the Schönberg–Chandrasekhar mass at which they are no longer in hydrostatic or thermal equilibrium. They then contract and heat up, which triggers helium fusion before the core becomes degenerate. These stars also become hotter during core helium fusion, but they have different core masses and hence different luminosities from HB stars. They vary in temperature during core helium fusion and perform a blue loop before moving to the asymptotic giant branch. Stars more massive than about also ignite their core helium smoothly, and also go on to burn heavier elements as a red supergiant. Stars remain on the horizontal branch for around 100 million years, becoming slowly more luminous in the same way that main sequence stars increase luminosity as the virial theorem shows. When their core helium is eventually exhausted, they progress to helium shell burning on the asymptotic giant branch (AGB). On the AGB they become cooler and much more luminous. Horizontal branch morphology Stars on the horizontal branch all have very similar core masses, following the helium flash. This means that they have very similar luminosities, and on a Hertzsprung–Russell diagram plotted by visual magnitude the branch is horizontal. The size and temperature of an HB star depends on the mass of the hydrogen envelope remaining around the helium core. Stars with larger hydrogen envelopes are cooler. This creates the spread of stars along the horizontal branch at constant luminosity. The temperature variation effect is much stronger at lower metallicity, so old clusters usually have more pronounced horizontal branches. Although the horizontal branch is named because it consists largely of stars with approximately the same absolute magnitude across a range of temperatures, lying in a horizontal bar on a color–magnitude diagrams, the branch is far from horizontal at the blue end. The horizontal branch ends in a "blue tail" with hotter stars having lower luminosity, occasionally with a "blue hook" of extremely hot stars. It is also not horizontal when plotted by bolometric luminosity, with hotter horizontal branch stars being less luminous than cooler ones. The hottest horizontal-branch stars, referred to as extreme horizontal branch, have temperatures of 20,000–30,000 K. This is far beyond what would be expected for a normal core helium burning star. Theories