A narrow band running from the upper left to lower right on the Hertzsprung-Russell diagram, on which stellar luminosity is plotted as a function of temperature, with temperature decreasing towards the right. The temperature and luminosity of most stars places them on the main sequence. This reflects the fact that these two fundamental quantities are largely determined by the mass of the star, some variation being introduced by differences in chemical composition. The main sequence is thus a mass sequence. The points for the most massive stars lie at the upper left and those for the least massive at the lower right. In stars on the main sequence, the fusion of hydrogen into helium in the stellar core is the source of energy. Stars in stages of stellar evolution before and after this phase are represented by points elsewhere on the HR diagram. For example, after hydrogen has been exhausted in the core, adjustments taking place internally cause a star to evolve in a way that carries it away from the main sequence towards the upper right of the HR diagram. Most stars spend about 90 per cent of their observable life on the main sequence.