Editing Main sequence (section)

== Structure ==
{{Main|Stellar structure}}
[[File:Solar internal structure.svg|right|upright=1.0|thumb|This diagram shows a cross-section of a Sun-like star, showing the internal structure.]]
Because there is a temperature difference between the core and the surface, or [[photosphere]], energy is transported outward. The two modes for transporting this energy are [[radiation]] and [[convection]]. A [[radiation zone]], where energy is transported by radiation, is stable against convection and there is very little mixing of the plasma. By contrast, in a [[convection zone]] the energy is transported by bulk movement of plasma, with hotter material rising and cooler material descending. Convection is a more efficient mode for carrying energy than radiation, but it will only occur under conditions that create a steep temperature gradient.<ref name=brainerd/><ref name=aller91/>

In massive stars (above {{solar mass|10}})<ref name=aaa102_1/> the rate of energy generation by the CNO cycle is very sensitive to temperature, so the fusion is highly concentrated at the core. Consequently, there is a high temperature gradient in the core region, which results in a convection zone for more efficient energy transport.<ref name=hannu/> This mixing of material around the core removes the helium ash from the hydrogen-burning region, allowing more of the hydrogen in the star to be consumed during the main-sequence lifetime. The outer regions of a massive star transport energy by radiation, with little or no convection.<ref name=brainerd/>

Intermediate-mass stars such as [[Sirius]] may transport energy primarily by radiation, with a small core convection region.<ref name=lockner06/> Medium-sized, low-mass stars like the Sun have a core region that is stable against convection, with a convection zone near the surface that mixes the outer layers. This results in a steady buildup of a helium-rich core, surrounded by a hydrogen-rich outer region. By contrast, cool, very low-mass stars (below {{solar mass|0.4}}) are convective throughout.<ref name=science295_5552/> Thus the helium produced at the core is distributed across the star, producing a relatively uniform atmosphere and a proportionately longer main-sequence lifespan.<ref name=brainerd/>