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

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.