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Concept
Solar core
Summary
The core of the Sun is considered to extend from the center to about 0.2 to 0.25 of
solar radius (). It is the hottest part of the Sun and of the Solar System. It has a density of 150 g/cm3 at the center, and a temperature of 15 million kelvins (15 million degrees Celsius, 27 million degrees Fahrenheit).
The core is made of hot, dense plasma (ions and electrons), at a pressure estimated at 265 billion bar (3.84 trillion psi or 26.5 petapascals (PPa)) at the center. Due to fusion, the composition of the solar plasma drops from 68 to 70% hydrogen by mass at the outer core, to 34% hydrogen at the core/Sun center.
The core inside 20% of the solar radius contains 34% of the Sun's mass, but only 0.8% of the Sun's volume. Inside 24% of the solar radius is the core which generates 99% of the fusion power of the Sun. There are two distinct reactions in which four hydrogen nuclei may eventually result in one helium nucleus: the proton–proton chain reaction – which is responsible for most of the Sun's released energy – and the CNO cycle.
The Sun at the photosphere is about 73–74% by mass hydrogen, which is the same composition as the atmosphere of Jupiter, and the primordial composition of hydrogen at the earliest star formation after the Big Bang. However, as depth into the Sun increases, fusion decreases the fraction of hydrogen. Traveling inward, hydrogen mass fraction starts to decrease rapidly after the core radius has been reached (it is still about 70% at a radius equal to 25% of the Sun's radius) and inside this, the hydrogen fraction drops rapidly as the core is traversed, until it reaches a low of about 33% hydrogen, at the Sun's center (radius zero). All but 2% of the remaining plasma mass (i.e., 65%) is helium.
Approximately 3.7 protons (hydrogen nuclei), or roughly 600 million tonnes of hydrogen, are converted into helium nuclei every second releasing energy at a rate of 3.86 joules per second.
The core produces almost all of the Sun's heat via fusion: the rest of the star is heated by the outward transfer of heat from the core.
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Solar neutrino problem
The solar neutrino problem concerned a large discrepancy between the flux of solar neutrinos as predicted from the Sun's luminosity and as measured directly. The discrepancy was first observed in the mid-1960s and was resolved around 2002. The flux of neutrinos at Earth is several tens of billions per square centimetre per second, mostly from the Sun's core. They are nevertheless hard to detect, because they interact very weakly with matter, traversing the whole Earth.
Solar radius
Solar radius is a unit of distance used to express the size of stars in astronomy relative to the Sun. The solar radius is usually defined as the radius to the layer in the Sun's photosphere where the optical depth equals 2/3: is approximately 10 times the average radius of Jupiter, 109 times the radius of the Earth, and 1/215th of an astronomical unit, the approximate distance between Earth and the Sun. The solar radius to either pole and that to the equator differ slightly due to the Sun's rotation, which induces an oblateness in the order of 10 parts per million.
Proton–proton chain
The proton–proton chain, also commonly referred to as the p–p chain, is one of two known sets of nuclear fusion reactions by which stars convert hydrogen to helium. It dominates in stars with masses less than or equal to that of the Sun, whereas the CNO cycle, the other known reaction, is suggested by theoretical models to dominate in stars with masses greater than about 1.3 times that of the Sun. In general, proton–proton fusion can occur only if the kinetic energy (i.e.