Red clump
The red clump is a clustering of red giants in the Hertzsprung–Russell diagram at around 5,000 K and absolute magnitude (MV) +0.5, slightly hotter than most red-giant-branch stars of the same luminosity. It is visible as a more dense region of the red giant branch or a bulge towards hotter temperatures. It is most distinct in many, but not all, galactic open clusters, but it is also noticeable in many intermediate-age globular clusters and in general nearby field stars (e.g. the Hipparcos stars).
The red clump giants are cool horizontal branch stars, stars originally similar to the sun which have undergone a helium flash and are now fusing helium in their cores.
Red clump stellar properties vary depending on their origin, most notably on the metallicity of the stars, but typically they have early K spectral types and temperatures around 5,000 K. The absolute visual magnitude of red clump giants near the sun has been measured at an average of +0.81 with metallicities between −0.6 and +0.4 dex.
There is a considerable spread in the properties of red clump stars even within a single population of similar stars such as an open cluster. This is partly due to the natural variation in temperatures and luminosities of horizontal branch stars when they form and as they evolve, and partly due to the presence of other stars with similar properties. Although red clump stars are generally hotter than red-giant-branch stars, the two regions overlap and the status of individual stars can only be assigned with a detailed chemical abundance study.
Modelling of the horizontal branch has shown that stars have a strong tendency to cluster at the cool end of the zero age horizontal branch (ZAHB). This tendency is weaker in low metallicity stars, so the red clump is usually more prominent metal-rich clusters. However, there are other effects, and there are well-populated red clumps in some metal-poor globular clusters.
Stars with a similar mass to the sun evolve towards the tip of the red giant branch with a degenerate helium core. More massive stars leave the red giant branch early and perform a blue loop, but all stars with a degenerate core reach the tip with very similar core masses, temperatures, and luminosities. After the helium flash they lie along the ZAHB, all with helium cores just under 0.5 M☉ and their properties determined mostly by the size of the hydrogen envelope outside the core. Lower envelope masses result in weaker hydrogen shell fusion and give hotter and slightly less luminous stars strung along the horizontal branch. Different initial masses and natural variations in mass loss rates on the red giant branch cause the variations in the envelope masses even though the helium cores are all the same size. Low metallicity stars are more sensitive to the size of the hydrogen envelope, so with the same envelope masses they are spread further along the horizontal branch and fewer fall in the red clump.
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