Evolution

Abstract

The fusion of hydrogen into helium in the solar core implies that the H mass fraction decreases with time while the He mass fraction increases. This progressive change of the chemical composition affects the mean molecular weight, opacity and nuclear energy production rate. Thus, the solar structure changes with time, but the Sun evolves on a very long time scale. The oldest fossils found on Earth date back to about 10⁹ years ago. Since life is extremely sensitive to the amount of solar energy that the Earth receives, the thermodynamic structure of the Sun 10⁹ years ago was not too different from that of today. Actually, 10⁹ y = 1 Gy is an appropriate unit for the evolutionary times of the Sun and many stars. Turning back in time, the hydrogen and helium number densities in the core approach the ratio of 10 to 1, which is presently measured in the atmosphere. The period of the solar life during which hydrogen is gradually transformed into helium is called the main sequence phase. In this section we describe the whole evolution of the Sun including the pre and post main sequence phases. We will start with the solar formation, which started as the result of a gravitational collapse of interstellar material.