The facts discussed above are the products of one of the great achievements of twentieth-century astronomy: the construction of stellar models that describe the internal structure of a star.
The mechanisms at work in a stellar interior can be described by four mathematical formulae known collectively as the laws of stellar structure. These equations describe how important quantities such as the temperature and pressure change with varying distances from the star's center. A stellar model calculation involves choosing starting values for the important stellar parameters and running the model to see if a self-consistent solution emerges. If one does not, the parameters are repeatedly adjusted and the model rerun until a consistent solution is achieved. A successful model must reproduce the observed quantities at the stellar surface-i.e., the surface temperature of the model star should be the same as the temperature actually observed for a real star of the same mass and size.
In the first part of this century, astronomers calculated stellar models laboriously by hand. More recently, computers have enabled astronomers to construct increasingly detailed models. The work of stellar astronomers has, in just the past several decades, given mankind the essential answer to the ancient question, "Why does a star shine?"
Science EncyclopediaScience & Philosophy: Spectroscopy to Stoma (pl. stomata)Star - Energy Generation, Stellar Models, Mass: The Fundamental Stellar Property, Four Stars, Variable Stars - The nature of the stars