![]() ![]() The more massive a star is, the shorter its lifespan on the main sequence. When core convection does not occur, a helium-rich core develops surrounded by an outer layer of hydrogen. The Main-sequence stars below 0.4 M ☉ undergo convection throughout their mass. With decreasing stellar mass, the proportion of the star forming a convective envelope steadily increases. Below this mass, stars have cores that are entirely radiative with convective zones near the surface. 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. 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. The Sun, along with main sequence 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. The main sequence is sometimes divided into upper and lower parts, based on the dominant process that a star uses to generate energy. The energy is carried by either radiation or convection, with the latter occurring in regions with steeper temperature gradients, higher opacity, or both. ![]() Energy generated at the core makes its way to the surface and is radiated away at the photosphere. The strong dependence of the rate of energy generation on temperature and pressure helps to sustain this balance. 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. 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. Color-magnitude plots are known as Hertzsprung–Russell diagrams after Ejnar Hertzsprung and Henry Norris Russell.Īfter condensation and ignition of a star, it generates thermal energy in its dense core region through nuclear fusion of hydrogen into helium. These are the most numerous true stars in the universe and include the Sun. Stars on this band are known as main-sequence stars or dwarf stars, 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. In astronomy, the main sequence is a classification of stars which appear on plots of stellar color versus brightness as a continuous and distinctive band. 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. The main sequence is visible as a prominent diagonal band from upper left to lower right. A Hertzsprung–Russell diagram plots the luminosity (or absolute magnitude) of a star against its color index (represented as B−V). ![]() For the racehorse, see Main Sequence (horse).
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