Red clump

Did You Know?

• The red clump should not be confused with the 'red bump' or red-giant-branch bump, a less noticeable clustering of giants partway along the red-giant branch caused by internal convection.
• Red clump giants are generally hotter than red-giant-branch stars, but the two regions overlap and individual stars can only be assigned with a detailed chemical abundance study.
• The distance to the Galactic Center measured via red clump stars gives 7.52 kpc, in agreement with other methods.

Properties

Red clump stellar properties vary depending on their origin, most notably on metallicity. They typically have early K spectral types and effective temperatures around 5,000 K. The absolute visual magnitude of red clump giants near the sun averages +0.81 for metallicities between −0.6 and +0.4 dex. Even within a single population like an open cluster, there is considerable spread in properties, partly due to natural variation in horizontal branch stars and partly due to overlap with red-giant-branch stars.

Evolution

Stars tend to gather at the cool end of the zero age horizontal branch, a pattern that shows up strongly in stellar modeling. This clustering is less pronounced in stars with low metallicity, so the red clump is generally more visible in metal-rich star clusters. Still, some metal-poor globular clusters do have well-populated red clumps. Sun-like stars move toward the tip of the red-giant branch with a degenerate helium core. Heavier stars leave the red-giant branch early and go through a blue loop, but any star with a degenerate core arrives at the tip with nearly identical core masses, temperatures, and brightness. After the helium flash, these stars settle on the zero age horizontal branch with helium cores just under a certain size, and their properties are mostly set by the hydrogen envelope around the core. A smaller envelope means weaker hydrogen shell fusion, producing hotter and slightly dimmer stars spread along the horizontal branch. Differences in initial mass and natural variation in mass loss on the red-giant branch create these envelope size differences, even though the helium cores are all the same size. Low-metallicity stars are more affected by the hydrogen envelope’s size, so for the same envelope mass they spread out more along the horizontal branch, and fewer end up in the red clump. Red clump stars are always hotter than the red-giant branch they came from, but red clump and red-giant-branch stars from different populations can overlap. In omega Centauri, for example, metal-poor red-giant-branch stars can be as hot as or hotter than more metal-rich red clump giants. Some stars that are not true horizontal branch stars also appear in the same part of the H-R diagram. Stars too massive to form a degenerate helium core on the red-giant branch ignite helium before reaching the tip and perform a blue loop. For stars only a bit more massive than the sun, this blue loop is very short and happens at a brightness similar to red clump giants. These stars are about ten times rarer than sun-like stars, even rarer compared to the sub-solar stars that can become red clump giants, and the blue loop lasts far less time than a red clump giant spends on the horizontal branch. That makes these imposters much less common in the H-R diagram, but they can still be detected.

Standard candles

In theory, the absolute luminosities of red clump stars are fairly independent of stellar composition or age, making them good standard candles for estimating distances within our galaxy and to nearby galaxies and clusters. Variations due to metallicity, mass, age, and extinctions affect visual observations too much for usefulness, but the effects are much smaller in the infrared. Near infrared I band observations have been used to establish red clump distances, with absolute magnitudes at solar metallicity measured at −0.22 in the I band and −1.54 in the K band.