Global statistical entropy and its implications for the main sequences of stars and galaxies

Université Paris-Saclay, Université Paris Cité, CEA, CNRS, AIM 91191 Gif-sur-Yvette, France

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 6 July 2025
Accepted: 6 February 2026

Abstract

In a dissipative system such as star or a galaxy, the emitted photons are decoupled from matter particles and may therefore be considered as part of a closed system to which the second law of thermodynamics applies. In the present work, I defined a global entropy using a statistical approach that accounts for the contributions of both matter particles and photons. The statistical contribution of radiation is described as a photon gas in the definition of this global entropy. The increase in global entropy can foster structure formation –rather than disorder– because structures such as stars and galaxies are efficient in dissipating energy in the form of photons, and thus in producing entropy. I show that stars generate a nearly equal amount of specific entropy and, therefore, a comparable number of photons per unit mass over their lifetime on the main sequence of the Hertzsprung–Russell (HR) diagram. This suggests that the main sequence of the HR diagram constitutes a locus of convergence toward a universal specific entropy production by stars. I then examined the implications of this approach for the star-formation main sequence in galaxies and found a similar result. The emergence of organized structures in cosmic history reflects the second law, as organized matter is efficient in generating entropy through the slicing of energy into lower frequency photons. This is also reflected in the dominant contribution of low-frequency photons to the extragalactic background light. Finally, in this paper I briefly discuss how this perspective may provide insight into the possibility of the existence of life elsewhere in the Universe.