Main Sequence

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The main sequence is a continuous and distinctive band of stars that appears on plots of stellar color versus brightness. These color-magnitude plots are known as Hertzsprung–Russell diagrams after their co-developers, Ejnar Hertzsprung and Henry Norris Russell. Stars on this band are known as main-sequence stars or "dwarf" stars.

After a star has formed, it creates energy at the hot, dense core region through the 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 gravitational pressure 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.

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 (or 1.5 solar masses) 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, while main-sequence stars below 0.4 solar masses undergo convection throughout their mass. When core convection does not occur, a helium-rich core develops surrounded by an outer layer of hydrogen.

In general, the more massive the star 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 solar masses becoming white dwarfs directly, while 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.

Read more about Main Sequence:  History, Formation, Properties, Dwarf Terminology, Parameters, Energy Generation, Structure, Luminosity-color Variation, Lifetime, Evolutionary Tracks

Other articles related to "main sequence, main":

Main Sequence - Evolutionary Tracks
... See also Stellar evolution Once a main-sequence star consumes the hydrogen at its core, the loss of energy generation causes gravitational collapse to resume ... At this point the star is evolving off the main sequence and entering the giant branch ... across the HR diagram, to the upper right of the main sequence, is called an evolutionary track ...
Algol Paradox
... this evolution, and the more quickly it leaves the main-sequence, entering either a subgiant or giant phase ... a subgiant, and the star with much greater mass is still on the main-sequence ... Thus the more massive star, rather than the less massive one, should have left the main sequence ...
Stars With No Turnoff Point
... and the low temperatures and pressures mean the lifetimes of these stars on the main sequence from zero point to turn off point is measured in trillions of years ... universe, therefore all red dwarfs are main sequence stars ... The cooling star is now off the main sequence and is known as a helium white dwarf ...
HD 13189
... a spectral classification of K1II-III, making it a giant star that has evolved away from the main sequence ... This mass range is typical of a B-type main sequence star, suggesting the star belong to stellar class B when it was on the main sequence ...
Semidetached Binaries - Evolution - Mass Transfer and Accretion
... As a main-sequence star increases in size during its evolution, it may at some point exceed its Roche lobe, meaning that some of its matter ventures into a ... ones, it was observed that the more massive component Algol A is still in the main sequence, while the less massive Algol B is a subgiant star at a later evolutionary stage ... lobe, and most of the mass was transferred to the other star, which is still in the main sequence ...

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