The Main Sequence and Beyond

Abstract

Most of the stars that we observe in the night sky have all got one thing in common — they are on the main sequence. There are, of course, exceptions: Betelgeuse has left the main sequence and has become a red giant star, the hydrogen burning at the centre of its core has stopped, and now helium is burning by fusion processes; while Sirius B has evolved far from the main sequence and has become a white dwarf star, with no nuclear fusion occurring at all within it. But for the large majority, the main sequence is a stable time, with only small changes in mass and luminosity occurring. However, as a star ages, changes occur in the way energy is formed, and this in turn affects its size and thus its luminosity, and so the star leaves the main sequence to begin the next phase of its life. This chapter then will look at these periods in a star’s life, whether it is a small, low-mass and cool star, or a bright, hot and high-mass star.