Introduction

This chapter deals with the physics of the Sun and the constituents of the solar system. It draws heavily on the material developed in Chaps. 2 and 3 and on Vol. I, Chaps. 2, 8, and 9.

The Standard Solar Model

Given the mass of the Sun, its initial composition, and its current age, we should be able to develop a model for the Sun by using the equations described in Chaps. 2 and 3. Such an evolutionary calculation will predict all other structural properties of the Sun at the present time, which may then be compared with observations. Among the input variables, the mass of the Sun, M⊙ = (1.9891 ± 0.0004) × 1033 gm, is known quite accurately. The age of the Sun has to be estimated indirectly and is expected to be approximately (4.5 ± 0.1) × 109 yr. The initial composition of the Sun is not well known but the ratio Z/X = 0.02739–0.02765 is thought to be well determined. Because X + Y + Z = 1 and Z/X are given, the initial composition can be parameterised by a single variable, say, the value of helium fraction Y. By varying the value of Y, we can construct a class of solar models and choose the one that fits best with the observations. In reality, there arises (at least) one more parameter in modelling the solar structure because of theoretical uncertainty in the description of convection.