<P> 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 . Stars in the upper main sequence have sufficiently high core temperatures to efficiently use the CNO cycle . (See the chart .) This process uses atoms of carbon, nitrogen and oxygen as intermediaries in the process of fusing hydrogen into helium . </P> <P> 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 1.5 M, 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 . 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 . By contrast, stars with 1.8 M or above generate almost their entire energy output through the CNO cycle . </P> <P> The observed upper limit for a main - sequence star is 120--200 M . The theoretical explanation for this limit is that stars above this mass cannot 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 . The lower limit for sustained proton--proton nuclear fusion is about 0.08 M or 80 times the mass of Jupiter . Below this threshold are sub-stellar objects that cannot sustain hydrogen fusion, known as brown dwarfs . </P> <P> 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 . </P>

Why are there upper and lower limits to the masses luminosities and sizes of stars
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