Introduction

The chemical composition of matter is often the result of factors which, at first, are not at all obvious. Consider the following examples. In certain very stable stellar atmospheres a separation of the chemical elements can take place. One might think that the heavier elements would be the first to sink with respect to the abundant hydrogen, which forms the bulk of (most) stellar matter. In the earth's upper atmosphere, for example, there is a region where the heavy species sink. The number density of a given species roughly follows the law for an isothermal atmosphere, N ∝ exp(–z/H), where z is the altitude, and H the scale height, H = ℜT/gμ. (See the Index for the meaning of symbols not explained in the text.) Thus molecules such as O2 and N2 are concentrated at low altitudes relative to atomic hydrogen and helium.

For the stars in question, the situation is not so simple. There is a competing, upward force due to radiation pressure that can overwhelm gravity. Given time, exotic heavy elements such as mercury or platinum can be pushed up from the envelope of a star and concentrated in the photosphere, where they may be revealed by spectroscopy. In these stars, the abundant, light elements have a tendency to sink! We shall discuss this counterintuitive process in some detail in Chapter 13, since much of the writer's own research has been concerned with its observational consequences.