Horizontal branch

[3] If the star has more than about 0.5 solar masses,[4] the core eventually reaches the temperature necessary for the fusion of helium into carbon through the triple-alpha process.

Within a few seconds the core becomes non-degenerate and quickly expands, producing an event called helium flash.

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 8 M☉ also ignite their core helium smoothly, and also go on to burn heavier elements as a red supergiant.

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.

The temperature variation effect is much stronger at lower metallicity, so old clusters usually have more pronounced horizontal branches.

[9] Globular cluster CMDs (Color-Magnitude diagrams) generally show horizontal branches that have a prominent gap in the HB.

[10] It requires an extended observing program to establish the star's true (that is, averaged over a full period) apparent magnitude and color.

Chemical composition is one factor (usually in the sense that more metal-poor clusters have bluer HBs), but other stellar properties like age, rotation and helium content have also been suggested as affecting HB morphology.

The label "second parameter" acknowledges that some unknown physical effect is responsible for HB morphology differences in clusters that seem otherwise identical.