On the Blue Loops of Intermediate-Mass Stars

TL;DR

This paper investigates the physical mechanisms behind blue loops in intermediate-mass stars, demonstrating that the mean molecular weight increase is the key factor influencing their evolution in the Hertzsprung-Russell diagram.

Contribution

It provides a detailed analysis showing that the mean molecular weight increase, rather than opacity or fuel supply, primarily causes the blue loops in stellar evolution.

Findings

Mean molecular weight increase is the main driver of blue loops.

Opacity effects are minor in influencing the loops.

Reduced fuel supply favors blueward motion.

Abstract

We consider the blue loops in the Hertzsprung-Russell diagram that occur when intermediate-mass stars begin core helium burning. It has long been known that the excess of helium above the burning shell, the result of the contraction of the convective core during core hydrogen burning, has the effect of making such stars redder and larger than they would be otherwise. The outward motion of the burning shell in mass removes this excess and triggers the loop. Hitherto nobody has attempted to demonstrate why the excess helium has this effect. We consider the effect of the local opacity, which is reduced by excess helium, the shell fuel supply, which is also reduced, and the local mean molecular weight, which is increased. We demonstrate that the mean molecular weight is the decisive reddening factor. The opacity has a much smaller effect and a reduced fuel supply actually favours blueward motion.