Portal:Stars

Four images of Sun

The Sun is the star at the center of the Solar System. The Sun has a diameter of about 1,392,000 kilometers (865,000 mi) (about 109 Earths), and by itself accounts for about 99.86% of the Solar System's mass; the remainder consists of the planets (including Earth), asteroids, meteoroids, comets, and dust in orbit. About three-quarters of the Sun's mass consists of hydrogen, while most of the rest is helium. Less than 2% consists of other elements, including iron, oxygen, carbon, neon, and others.

The Sun's color is white, although from the surface of the Earth it may appear yellow because of atmospheric scattering. Its stellar classification, based on spectral class, is G2V, and is informally designated a yellow star, because the majority of its radiation is in the yellow-green portion of the visible spectrum. In this spectral class label, G2 indicates its surface temperature of approximately 5,778 K (5,505 °C), and V (Roman five) indicates that the Sun, like most stars, is a main sequence star, and thus generates its energy by nuclear fusion of hydrogen nuclei into helium.

Main sequence star

The main sequence is a continuous and distinctive band of stars that appear 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. 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.

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.

Main sequence stars with more than ten solar masses undergo convection in the core region, which acts to stir up the newly created helium and maintain the proportion of fuel needed for fusion to occur. When core convection does not occur, a helium-rich core develops surrounded by an outer layer of hydrogen. For stars with lower masses, this convective core is progressively smaller until it disappears at about 2 solar masses. Below this mass, stars have cores that are radiative but are convective near the surface. With decreasing stellar mass the convective envelope increases, and main sequence stars below 0.4 solar masses undergo convection throughout their mass.