Editing Main sequence (section)

== Energy generation ==
{{See also|Stellar nucleosynthesis}}
[[File:Nuclear energy generation.svg|right|upright=1.5|thumb|[[Logarithm]] of the relative energy output (ε) of [[proton–proton chain|proton–proton]] (PP), [[CNO cycle|CNO]] and [[triple-alpha process|triple-α]] fusion processes at different temperatures (''T''). The dashed line shows the combined energy generation of the PP and CNO processes within a star. At the Sun's core temperature, the PP process is more efficient.]]
All main-sequence stars have a core region where energy is generated by nuclear fusion. The temperature and density of this core are at the levels necessary to sustain the energy production that will support the remainder of the star. A reduction of energy production would cause the overlaying mass to compress the core, resulting in an increase in the fusion rate because of higher temperature and pressure. Likewise, an increase in energy production would cause the star to expand, lowering the pressure at the core. Thus the star forms a self-regulating system in [[hydrostatic equilibrium]] that is stable over the course of its main-sequence lifetime.<ref name=brainerd/>

Main-sequence stars employ two types of hydrogen fusion processes, and the rate of energy generation from each type depends on the temperature in the core region. Astronomers divide the main sequence into upper and lower parts, based on which of the two is the dominant fusion process. In the lower main sequence, energy is primarily generated as the result of the [[proton–proton chain]], which directly fuses hydrogen together in a series of stages to produce helium.<ref name=hannu/> Stars in the upper main sequence have sufficiently high core temperatures to efficiently use the [[CNO cycle]] (see chart). This process uses atoms of [[carbon]], [[nitrogen]], and [[oxygen]] as intermediaries in the process of fusing hydrogen into helium.

At a stellar core temperature of 18 million [[kelvin]], the PP process and CNO cycle are equally efficient, and each type generates half of the star's net luminosity. As this is the core temperature of a star with about {{solar mass|1.5}}, the upper main sequence consists of stars above this mass. Thus, roughly speaking, stars of spectral class F or cooler belong to the lower main sequence, while A-type stars or hotter are upper main-sequence stars.<ref name=clayton83/> The transition in primary energy production from one form to the other spans a range difference of less than a single solar mass. In the Sun, a one solar-mass star, only 1.5% of the energy is generated by the CNO cycle.<ref name=apj555/> By contrast, stars with {{solar mass|1.8}} or above generate almost their entire energy output through the CNO cycle.<ref name=maurizio05/>

The observed upper limit for a main-sequence star is {{solar mass|120–200}}.<ref name=apj620_1/> The theoretical explanation for this limit is that stars above this mass can not radiate energy fast enough to remain stable, so any additional mass will be ejected in a series of pulsations until the star reaches a stable limit.<ref name=apj162/> The lower limit for sustained proton-proton nuclear fusion is about {{solar mass|0.08}} or 80 times the mass of [[Jupiter]].<ref name=hannu/> Below this threshold are sub-stellar objects that can not sustain hydrogen fusion, known as [[brown dwarf]]s.<ref name=apj406_1/>