- Solar core
The core of the
Sunis considered to extend from the center to about 0.2 solar radius. It is the hottest part of the Solar System. It has a density of up to 150,000 kg/m³ (154 times the density of wateron Earth) and a temperature of close to 15,000,000 kelvin(by contrast, the surface of the Sun is close to 6,000 kelvin).
Energy is produced by
exothermicthermonuclear reactions ( nuclear fusion) that mainly convert hydrogeninto helium. The core is the only location in the Sun that produces an appreciable amount of heatvia fusion: the rest of the star is heated by energy that is transferred outward from the core. All of the energy produced by fusion in the core must travel through many successive layers to the solar photosphere before it escapes into space as sunlightor kinetic energyof particles.
About 3.6 e|38
protons (hydrogen nuclei) are converted into helium nuclei every second, releasing energy at the matter-energy conversion rate of 4.3 million tonnes per second, 380 yottawatts (3.8 e|26 watts), equivalent to 9.1 e|10 megatons of TNT per second. The rate of nuclear fusion depends strongly on density, so the fusion rate in the core is in a self-correcting equilibrium: a slightly higher rate of fusion would cause the core to heat up more and expand slightly against the weightof the outer layers, reducing the fusion rate and correcting the ; and a slightly lower rate would cause the core to cool and shrink slightly, increasing the fusion rate and again reverting it to its present level.
photons ( gamma rays and x-rays) released in fusion reactions take a long time to reach the Sun's surface, slowed down by the indirect path taken, as well as by constant absorption and reemission at lower energies in the solar mantle. Estimates of the "photon travel time" range from as much as 50 million yearscite book| last=Lewis| first=Richard| year=1983| title=The Illustrated Encyclopedia of the Universe| publisher=Harmony Books, New York| pages=65] to as little as 17,000 years.cite web| url=http://www.badastronomy.com/bitesize/solar_system/sun.html| first=Phil| last=Plait| publisher=Bad Astronomy| title=Bitesize Tour of the Solar System: The Long Climb from the Sun's Core| year=1997| accessdate=2006-03-22] After a final trip through the convective outer layer to the transparent "surface" of the photosphere, the photons escape as visible light. Each gamma ray in the Sun's core is converted into several million visible light photons before escaping into space. Neutrinos are also released by the fusion reactions in the core, but unlike photons they very rarely interact with matter, so almost all are able to escape the Sun immediately. For many years measurements of the number of neutrinos produced in the Sun were much lower than theories predicted, a problem which was recently resolved through a better understanding of the effects of neutrino oscillation.
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