Red Giant Branch stars experience a temporary dip in brightness during their evolution, a phenomenon called the RGB luminosity bump. The cause of this bump's onset has puzzled astrophysicists, though they understood how it ends.
New research published in The Astrophysical Journal Letters proposes the first quantitative explanation. Saskia Hekker, Professor of Theoretical Astrophysics at the Heidelberg Institute for Theoretical Studies, argues the bump begins when a thermodynamic property called specific entropy crosses a critical threshold at the Base of the Convective Zone inside the star.
As RGB stars evolve, their inert helium cores contract while their surfaces expand. This forces the boundary between the star's radiative zone and convective envelope deeper inward. Earlier mixing in the star had created a boundary between helium-enriched and hydrogen-rich gas. As this boundary moves, the entropy difference across it shrinks. When entropy drops past a specific value, the luminosity bump begins.
Previous explanations relied on mechanical processes, suggesting the hydrogen-burning shell reaching a certain boundary triggered the change. Hekker's work ties the onset instead to thermodynamics rather than layer mechanics alone.
"This succinct explanation of the onset of the bump, is the first quantitative description proposed so far," Hekker stated.
The research addresses a gap between stellar models and observations. In computer models, the bump appears at higher luminosities than seen in real stars, appearing later in their evolution. Understanding the true mechanism could refine how astrophysicists construct stellar evolution models and calibrate their predictions against observed stellar properties.