The stellar atmosphere physical system I. Phenomenological definition and representation of a stellar atmosphere

This paper is the first in a series of two, whose aim is to analyse stellar atmosphere modelling both from the physical and the algorithmic standpoint. Here we will set the stage and define stellar atmospheres on physical grounds. The structure of the outer stellar layers, determined by the interactions between the constituting material and the radiation field that permeates the latter, will be analysed from an operational point of view. In the second paper we will present an operative sequential iterative method for the solution of the stellar atmosphere problem, which may be considered as a paradigm of non-linear and non-local problems. To have such an easy and reliable working tool at hand allows one to test the physical hypotheses introduced in the modelling of astrophysical objects. In other words, it makes possible to set up a veritable numerical laboratory for computational astrophysics. In The Oxford English Dictionary ’operationalism’ is defined as a theory or system which accepts only such concepts as can be described in terms of the operations necessary to determine or prove them. Modelling any physical system implies to describe the behaviour of a (generally) complex structure in terms of the laws governing each of its elementary components. To achieve that one must resort to a series of operations that includes the ideal dissection of the system into an ensemble of interacting parts, their identification and quantification by means of physical variables, the successive translation of physical magnitudes and their relevant mutual interactions into a system of equations (either continuous or discrete) and eventually their solution. This analytical procedure is possibly the most effective tool we have at hand for scientific inquiry. On the other hand, The Oxford English Dictionary defines the term ’operative’ as characterized by oper-