![]() 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. The spectra of stars were shown to have distinctive features, which allowed them to be categorized. In the early part of the 20th century, information about the types and distances of stars became more readily available. ![]() ![]() After the hydrogen fuel at the core has been consumed, the star evolves away from the main sequence on the HR diagram, into a supergiant, red giant, or directly to a white dwarf. 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. 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. These are the most numerous true stars in the universe and include the Sun.Īfter condensation and ignition of a star, it generates thermal energy in its dense core region through nuclear fusion of hydrogen into helium. Stars on this band are known as main-sequence stars or dwarf stars. These color-magnitude plots are known as Hertzsprung–Russell diagrams after their co-developers, Ejnar Hertzsprung and Henry Norris Russell. In astronomy, the main sequence is a continuous and distinctive band of stars that appears on plots of stellar color versus brightness. ![]() This plot shows 22,000 stars from the Hipparcos Catalogue 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 that runs from the upper left to the lower right. A Hertzsprung–Russell diagram plots the luminosity (or absolute magnitude) of a star against its color index (represented as B−V).
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