Main Sequence Star
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. Stars spend the majority of their lives on the main sequence, during which core hydrogen burning is dominant. These main-sequence stars, or sometimes interchangeably dwarf stars, are the most numerous true stars in the universe and include the Sun. Color-magnitude plots are known as Hertzsprung–Russell diagrams after Ejnar Hertzsprung and Henry Norris Russell.
When a gaseous nebula undergoes sufficient gravitational collapse, the high pressure and temperature concentrated at the core will trigger the nuclear fusion of hydrogen into helium (see stars). The thermal energy from this process radiates out from the hot, dense core, generating a strong pressure gradient. It is this pressure gradient that counters the star's collapse under gravity, maintaining the star in a state of hydrostatic equilibrium. The star's position on the main sequence is determined primarily by the mass, but also by age and chemical composition. As a result, radiation is not the only method of energy transfer in stars. Convection plays a role in the movement of energy, particularly in the cores of stars greater than 1.3 to 1.5 times the Sun's mass, again depending on age and chemical composition.
When discussing chemical composition, astronomers generally refer to the metallicity of the star. This is the abundance of heavier-than-helium elements present in the star. For example, the fraction of the Sun by mass currently composed of hydrogen (denoted X) is 74.9%. For helium (denoted Y) it is 23.8%, meaning the star's metallicity, or mass fraction of all other elements, is 1.3% (denoted Z). This is a typical range for similar-mass main sequence stars. In fact, a higher metallicity leads to a higher opacity whereby the energy production can remain concentrated in the core without being radiated or transferred away to the star's outer layers. This hotter environment speeds up nuclear fusion and decreases the amount of time the star will spend on the main sequence.
The main sequence is divided into upper and lower parts, based on the dominant process that a star uses to generate energy. The Sun, along with main sequence stars below about 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. The proton-proton chain is still occurring, but it produces less energy than the CNO cycle. Main-sequence stars where the CNO cycle is the dominant energy production process 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. The 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.
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, into a supergiant, red giant, or directly to a white dwarf.
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- 2013-05-04T00:00:00.000000Z
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