Main sequence

In astronomy, the main sequence is a classification of stars which appear on plots of stellar color versus brightness as a continuous and distinctive band. Stars spend the majority of their lives on the main sequence, during which core hydrogen burning is dominant. These main-sequence stars, or sometimes interchangeably dwarf stars, are the most numerous true stars in the universe and include the Sun. Color-magnitude plots are known as Hertzsprung–Russell diagrams after Ejnar Hertzsprung and Henry Norris Russell. When a gaseous nebula undergoes sufficient gravitational collapse, the high pressure and temperature concentrated at the core will trigger the nuclear fusion of hydrogen into helium (see stars). The thermal energy from this process radiates out from the hot, dense core, generating a strong pressure gradient. It is this pressure gradient that counters the star's collapse under gravity, maintaining the star in a state of hydrostatic equilibrium. The star's position on the main sequence is determined primarily by the mass, but also by age and chemical composition. As a result, radiation is not the only method of energy transfer in stars. Convection plays a role in the movement of energy, particularly in the cores of stars greater than 1.3 to 1.5 times the Sun's mass, again depending on age and chemical composition. When discussing chemical composition, astronomers generally refer to the metallicity of the star. This is the abundance of heavier-than-helium elements present in the star. For example, the fraction of the Sun by mass currently composed of hydrogen (denoted X) is 74.9%. For helium (denoted Y) it is 23.8%, meaning the star's metallicity, or mass fraction of all other elements, is 1.3% (denoted Z). This is a typical range for similar-mass main sequence stars. In fact, a higher metallicity leads to a higher opacity whereby the energy production can remain concentrated...