Editing Main sequence (section)

{{short description|Continuous band of stars that appears on plots of stellar color versus brightness}}
{{For|the racehorse|Main Sequence (horse)}}
[[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 from upper left to 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 classification of [[star]]s which appear on plots of stellar [[color index|color]] versus [[absolute magnitude|brightness]] as a continuous and distinctive 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.

The main sequence is sometimes divided into upper and lower parts, based on the dominant process that a star uses to generate energy. The Sun, along with main sequence stars below about 1.5 times the [[solar mass|mass
 of the Sun]] ({{solar mass|1.5}}), primarily fuse hydrogen atoms together in a series of stages to form helium, a sequence called the [[proton–proton chain]]. Above this mass, in the upper main sequence, the nuclear fusion process mainly uses atoms of [[carbon]], [[nitrogen]], and [[oxygen]] as intermediaries in the [[CNO cycle]] that produces helium from hydrogen atoms. Main-sequence stars with more than two solar masses undergo convection in their core regions, which acts to stir up the newly created helium and maintain the proportion of fuel needed for fusion to occur. Below this mass, stars have cores that are entirely radiative with convective zones near the surface. With decreasing stellar mass, the proportion of the star forming a convective envelope steadily increases. The main-sequence stars below {{solar mass|0.4}} undergo convection throughout their mass. When core convection does not occur, a helium-rich core develops surrounded by an outer layer of hydrogen.

The more massive a star is, the shorter its lifespan on the main sequence. After the hydrogen fuel at the core has been consumed, the star [[stellar evolution|evolves]] away from the main sequence on the HR diagram, into a [[supergiant]], [[red giant]], or directly to a [[white dwarf]].