Discontinuity and disorder: Explaining the luminosity bump in evolved stars

A close-up of the bump with the luminosity maximum and minimum (Lmax and Lmin) indicated. The specific entropy at the surface is shown by the dashed light blue line and the axis on the right. Credit: The Astrophysical Journal Letters (2026). DOI: 10.3847/2041-8213/ae9ff0

In her research, Saskia Hekker finds that entropy is the key driver of the luminosity bump in evolved stars. The bump occurs when low-mass stars on the red-giant branch experience a temporary decrease in brightness. This phenomenon, known as the red-giant-branch bump, is caused by a discontinuity in the star's chemical composition, which moves to deeper layers during evolution. The change in entropy due to this discontinuity causes the star's luminosity to decrease, marking the onset of the bump. Hekker's work provides the first quantitative description of the bump's onset, offering new insights into stellar evolution theory.

• The bump is a well-understood endpoint but its onset remains elusive.
• Entropy is the key factor in the bump's onset, as it measures disorder.
• The discontinuity in the star's chemical composition causes the bump.
• Hekker's research is the first of its kind and addresses a long-standing question in astrophysics.

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