<P> After a star has formed, it generates thermal energy in the dense core region through nuclear fusion of hydrogen atoms into helium . During this stage of the star's lifetime, it is located along the main sequence at a position determined primarily by its mass, but also based upon its chemical composition and other factors . All 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 in the core on the 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 . </P> <P> The main sequence is sometimes divided into upper and lower parts, based on the dominant process that a star uses to generate energy . Stars below about 1.5 times the mass of the Sun (1.5 M) 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, whereas main - sequence stars below 0.4 M undergo convection throughout their mass . When core convection does not occur, a helium - rich core develops surrounded by an outer layer of hydrogen . </P> <P> In general, 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 evolves away from the main sequence on the HR diagram . The behavior of a star now depends on its mass, with stars below 0.23 M becoming white dwarfs directly, whereas stars with up to ten solar masses pass through a red giant stage . More massive stars can explode as a supernova, or collapse directly into a black hole . </P> <P> In the early part of the 20th century, information about the types and distances of stars became more readily available . The spectra of stars were shown to have distinctive features, which allowed them to be categorized . Annie Jump Cannon and Edward C. Pickering at Harvard College Observatory developed a method of categorization that became known as the Harvard Classification Scheme, published in the Harvard Annals in 1901 . </P>

When does a star leave the main sequence
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