This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: Toward the end of their lives, low-mass stars undergo an abrupt dip in brightness, also known as the luminosity bump. While the endpoint of this bump is well understood, its onset has remained elusive.
Now, in a paper published by The Astrophysical Journal Letters, Saskia Hekker, leader of the Theory and Observations of Stars group at the Heidelberg Institute for Theoretical Studies (HITS), finds that entropy is the key driver of the bump. In observations and stellar evolution models alike, low-mass stars moving along the red-giant branch undergo a phenomenon known as the red-giant-branch bump, during which they temporarily experience a decrease in luminosity when an increase would be expected. This confounding phenomenon has been investigated in Hekker's latest research, in which she demonstrates that the answer might be entropy, a measure of disorder.
This process is usually precipitated by a discontinuity in a star's chemical composition, emerging at the bottom of the convective stellar envelope, i.e., a star's outer layers. During evolution, the discontinuity moves to deeper layers with different temperatures and pressures, changing the entropy. This causes the star's luminosity to decrease, thus defining the onset of the bump.
"This succinct explanation of the onset of the bump is the first quantitative description proposed so far," Hekker says. Hekker's research is the first of its kind and begins to answer a long-standing question in astrophysics. According to Hekker, describing the mechanism behind the bump in this new way will provide key insights into the use of the bump as an anchor point in stellar evolution theory.
Hekker, The Role of Specific Entropy in the Onset of the Luminosity Bump, The Astrophysical Journal Letters (2026). DOI: 10.3847/2041-8213/ae9ff0 Journal information: Astrophysical Journal Letters Provided by Heidelberger Institut für Theoretische Studien gGmbH MA in English, copy editor since 2021 with experience in higher education and health content. Dedicated to trustworthy science news.
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