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[[File:HRDiagram.png|thumb|upright=1.4|A [[Hertzsprung–Russell diagram]] plots the [[luminosity]] (or [[absolute magnitude]]) of a star against its [[color index]] (represented as B−V). The main sequence is visible as a prominent diagonal band that runs from the upper left to the lower right. This plot shows 22,000 stars from the [[Hipparcos Catalog]] together with 1,000 low-luminosity stars (red and white dwarfs) from the [[Gliese Catalogue of Nearby Stars]].]]

In [[astronomy]], the '''main sequence''' is a continuous and distinctive bandclassification of [[star]]s thatwhich appearsappear on plots of stellar [[color index|color]] versus [[absolute magnitude|brightness]]. These color-magnitude plots are known as [[Hertzsprung–Russella diagram]]s after their co-developers, [[Ejnar Hertzsprung]]continuous and [[Henrydistinctive Norris Russell]]band. Stars on this band are known as '''main-sequence stars''' or [[dwarf star]]s, and positions of stars on and off the band are believed to indicate their physical properties, as well as their progress through several types of star life-cycles. These are the most numerous true stars in the universe and include the [[Sun]]. Color-magnitude plots are known as [[Hertzsprung–Russell diagram]]s after [[Ejnar Hertzsprung]] and [[Henry Norris Russell]].

After condensation and ignition of a star, it generates [[thermal energy]] in its dense [[stellar core|core region]] through [[nuclear fusion]] of [[hydrogen]] into [[helium]]. During this stage of the star's lifetime, it is located on the main sequence at a position determined primarily by its mass but also based on its chemical composition and age. The cores of main-sequence stars are in [[hydrostatic equilibrium]], where outward thermal pressure from the hot core is balanced by the inward pressure of [[gravitational collapse]] from the overlying layers. The strong dependence of the rate of energy generation on temperature and pressure helps to sustain this balance. Energy generated at the core makes its way to the surface and is radiated away at the [[photosphere]]. The energy is carried by either [[radiation]] or [[convection]], with the latter occurring in regions with steeper temperature gradients, higher opacity, or both.