blue loop

A blue loop is an episode within post-main-sequence stellar evolution in which a star grows hotter, then cools again, the episode taking on the order of a million years. Its brightness generally also increases and decreases such that the episode forms a looping track on the H-R diagram that is bluer than the star's track before and after. It is an episode of helium burning, occurring in early stars, e.g., more than a few solar masses, during their giant or supergiant phases. Such an episode can occur:

• during the red-giant branch (RGB) (generally only once), in more massive stars in which helium in the core gradually ignites (rather than a helium flash) and burns off.
• between the RGB and the asymptotic giant branch (AGB), in place of the horizontal branch.
• during the AGB, possibly more than once.

A star can reside within the instability strip during a portion of such a blue loop episode, during which they are pulsating stars such as W Virginis variables.