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

TL;DR

This paper introduces a global entropy concept for stars and galaxies, linking entropy production to structure formation and suggesting a universal entropy production pattern in cosmic evolution.

Contribution

It defines a new global entropy framework incorporating matter and photon contributions, revealing universal entropy production patterns in stellar and galactic evolution.

Findings

Stars produce similar specific entropy over their main sequence lifetime.

Main sequence stars and galaxies show convergence towards universal entropy production.

Organized cosmic structures efficiently generate entropy through photon emission.

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 paper, we define 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 at dissipating energy in the form of photons, and thus at producing entropy. We 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. We then examine the implications of this approach for the star-formation main sequence in galaxies, and find a similar result. The emergence of organized structures in cosmic history reflects the second law, as organized matter is efficient at 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, we briefly discuss how this perspective may inform us on the possibility of the existence of life elsewhere in the universe.