Solar core - wikipedia Solar core An illustration of the structure of the Sun Granules Sunspot Photosphere Chromosphere Convection zone Radiation zone Tachocline Solar core Corona Flare Prominence Solar wind 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 / cm ( 150 times the density of liquid water ) 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 peta pascals ( PPa ) ) at the center. Due to fusion, the composition of the solar plasma drops from 68 - 70 % hydrogen by mass at the outer core, to 33 % hydrogen at the core / Sun center. The core inside 0.20 of the solar radius contains 34 % of the Sun 's mass, but only 0.8 % of the Sun 's volume. Inside 0.24 solar radius, the core 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. Contents 1 Composition 2 Energy conversion 2.1 Proton - proton chain reaction 2.2 CNO cycle 3 Equilibrium 4 Energy transfer 5 References 6 External links Composition 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 and helium 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 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, at the center of the Sun. Energy conversion Approximately 3.6 × 10 protons ( hydrogen nuclei ), or roughly 299 million metric tons of hydrogen, are converted into helium nuclei every second releasing energy at a rate of 3.86 × 10 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. The energy produced by fusion in the core, except a small part carried out by neutrinos, must travel through many successive layers to the solar photosphere before it escapes into space as sunlight, kinetic or thermal energy of particles. The energy conversion per unit time ( power ) of fusion in the core varies with distance from the solar center. At the center of the Sun, fusion power is estimated by models to be about 276.5 watts / m. Despite its intense temperature, the peak power generating density of the core overall is similar to an active compost heap, and is lower than the power density produced by the metabolism of an adult human. The Sun is much hotter than a compost heap due to the Sun 's enormous volume. The low power outputs occurring inside the fusion core of the Sun may also be surprising, considering the large power which might be predicted by a simple application of the Stefan -- Boltzmann law for temperatures of 10 to 15 million kelvins. However, layers of the Sun are radiating to outer layers only slightly lower in temperature, and it is this difference in radiation powers between layers which determines net power generation and transfer in the solar core. At 19 % of the solar radius, near the edge of the core, temperatures are about 10 million kelvins and fusion power density is 6.9 W / m, which is about 2.5 % of the maximum value at the solar center. The density here is about 40 g / cm, or about 27 % of that at the center. Some 91 % of the solar energy is produced within this radius. Within 24 % of the radius ( the outer `` core '' by some definitions ), 99 % of the Sun 's power is produced. Beyond 30 % of the solar radius, where temperature is 7 million K and density has fallen to 10 g / cm the rate of fusion is almost nil. There are two distinct reactions in which 4 H nuclei may eventually result in one He nucleus : `` proton - proton chain reaction '' and the `` CNO cycle '' ( see below ). Proton - proton chain reaction Proton - proton chain reaction Main article : Proton -- proton chain reaction The first reaction in which 4 H nuclei may eventually result in one He nucleus is known as the proton - proton chain, is : ( 1 H + 1 H → 2 D + e + + v then 2 D + 1 H → 3 H e + γ then 3 H e + 3 H e → 4 H e + 1 H + 1 H ( displaystyle left ( ( \begin ( aligned ) && ( ) ^ ( 1 ) ! H+ ^ ( 1 ) ! H& \rightarrow ( ) ^ ( 2 ) ! D + e ^ ( + ) + v ( \text ( then ) ) && ( ) ^ ( 2 ) ! D+ ( ) ^ ( 1 ) ! H& \rightarrow ( ) ^ ( 3 ) ! He+ gamma ( \text ( then ) ) && ( ) ^ ( 3 ) ! He+ ( ) ^ ( 3 ) ! He& \rightarrow ( ) ^ ( 4 ) ! He+ ( ) ^ ( 1 ) ! H+ ( ) ^ ( 1 ) ! H end ( aligned ) ) \right. ) This reaction sequence is thought to be the most important one in the solar core. The characteristic time for the first reaction is about one billion years even at the high densities and temperatures of the core, due to the necessity for the weak force to cause beta decay before the nucleons can adhere ( which rarely happens in the time they tunnel toward each other, to be close enough to do so ). The time that deuterium and helium - 3 in the next reactions last, by contrast, are only about 4 seconds and 400 years. These later reactions proceed via the nuclear force and are thus much faster. The total energy released by these reactions in turning 4 hydrogen atoms into 1 helium atom is 26.7 MeV. CNO cycle Main article : CNO cycle CNO cycle. The second reaction sequence, in which 4 H nuclei may eventually result in one He nucleus is called the CNO cycle generates less than 10 % of the total solar energy. This involves carbon atoms which are not consumed in the overall process. The details of this CNO cycle are as follows : ( 12 C + 1 H → 13 N + γ then 13 N → 13 C + e + + ν then 13 C + 1 H → 14 N + γ then 14 N + 1 H → 15 O + γ then 15 O → 15 N + e + + ν then 15 N + 1 H → 12 C + 4 H e + γ ( displaystyle left ( ( \begin ( aligned ) && ( ) ^ ( 12 ) ! C+ ( ) ^ ( 1 ) ! H& \rightarrow ( ) ^ ( 13 ) ! N+ gamma ( \text ( then ) ) && ( ) ^ ( 13 ) ! N& \rightarrow ( ) ^ ( 13 ) ! C + e ^ ( + ) + \nu ( \text ( then ) ) && ( ) ^ ( 13 ) ! C+ ( ) ^ ( 1 ) ! H& \rightarrow ( ) ^ ( 14 ) ! N+ gamma ( \text ( then ) ) && ( ) ^ ( 14 ) ! N+ ( ) ^ ( 1 ) ! H& \rightarrow ( ) ^ ( 15 ) ! O+ gamma ( \text ( then ) ) && ( ) ^ ( 15 ) ! O& \rightarrow ( ) ^ ( 15 ) ! N + e ^ ( + ) + \nu ( \text ( then ) ) && ( ) ^ ( 15 ) ! N+ ( ) ^ ( 1 ) ! H& \rightarrow ( ) ^ ( 12 ) ! C+ ( ) ^ ( 4 ) ! He+ gamma end ( aligned ) ) \right. ) This process can be further understood by the picture on the right, starting from the top in clockwise direction. Equilibrium The rate of nuclear fusion depends strongly on density. Therefore, 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 weight of the outer layers. This would reduce the fusion rate and correct the perturbation ; 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. However the Sun gradually becomes hotter during its time on the main sequence, because the helium atoms in the core are denser than the hydrogen atoms they were fused from. This increases the gravitational pressure on the core which is resisted by a gradual increase in the rate at which fusion occurs. This process speeds up over time as the core gradually becomes denser. It is estimated that the sun has become 30 % brighter in the last four and a half billion years and will continue to increase in brightness by 1 % every 100 million years. Energy transfer The high - energy photons ( gamma rays ) released in fusion reactions take indirect paths to the Sun 's surface. According to current models, random scattering from free electrons in the solar radiative zone ( the zone within 75 % of the solar radius, where heat transfer is by radiation ) sets the photon diffusion time scale ( or `` photon travel time '' ) from the core to the outer edge of the radiative zone at about 170,000 years. From there they cross into the convective zone ( the remaining 25 % of distance from the Sun 's center ), where the dominant transfer process changes to convection, and the speed at which heat moves outward becomes considerably faster. In the process of heat transfer from core to photosphere, each gamma ray in the Sun 's core is converted during scattering 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 neutrino oscillation. References Jump up ^ García, Ra ; Turck - Chièze, S ; Jiménez - Reyes, Sj ; Ballot, J ; et al. ( Jun 2007 ). `` Tracking solar gravity modes : the dynamics of the solar core ''. Science. 316 ( 5831 ) : 1591 -- 3. Bibcode : 2007Sci... 316.1591 G. doi : 10.1126 / science. 1140598. ISSN 0036 - 8075. PMID 17478682. Jump up ^ http://solarscience.msfc.nasa.gov/interior.shtml Jump up ^ composition ^ Jump up to : McDonald, Andrew ; Kennewell, John ( 2014 ). `` The Source of Solar Energy ''. Bureau of Meteorology. Commonwealth of Australia. Missing or empty url = ( help ) Jump up ^ Table of temperatures, power densities, luminosities by radius in the sun Archived 2001 - 11 - 29 at the Library of Congress Web Archives Jump up ^ Karl S. Kruszelnicki ( 17 April 2012 ). `` Dr Karl 's Great Moments In Science : Lazy Sun is less energetic than compost ''. Australian Broadcasting Corporation. Retrieved 25 February 2014. Jump up ^ see p 54 and 55 Jump up ^ See Archived 2001 - 11 - 29 at the Library of Congress Web Archives Jump up ^ Pascale Ehrenfreund ; et al., eds. ( 2004 ). Astrobiology : future perspectives. Dordrecht ( u.a. ) : Kluwer Academic. ISBN 1402023049. Retrieved 28 August 2014. Jump up ^ These times come from : Byrne, J. Neutrons, Nuclei, and Matter, Dover Publications, Mineola, New York, 2011, ISBN 0486482383, p 8. Jump up ^ The Sun 's evolution Jump up ^ Earth Wo n't Die as Soon as Thought Jump up ^ Mitalas, R. & Sills, K.R. `` On the photon diffusion time scale for the sun '' http://adsabs.harvard.edu/full/1992ApJ...401...759M External links Animated explanation of the core of the Sun ( University of South Wales ). Animated explanation of the temperature and density of the core of the Sun ( University of South Wales ). The Sun Internal structure Core Radiation zone Tachocline Convection zone Atmosphere Photosphere Supergranulation Granule Faculae Sunspot Chromosphere Plage Spicule Moreton wave Corona Transition region Coronal hole Coronal loop Coronal mass ejection Prominence Helmet streamer Supra - arcade downflows Variation Solar cycle List of solar cycles Solar maximum Solar minimum Wolf number Solar wind Flare Helioseismology Heliosphere Current sheet Termination shock Heliosheath Heliopause Bow shock Related topics Heliophysics Solar activity Solar astronomy Solar dynamo Eclipse Solar energy Solar neutrino Solar observation Solar phenomena Solar physics Solar System Solar telescope Solar time Space weather Standard solar model Radiation Spectral class : G2V Star Formation Accretion Molecular cloud Bok globule Young stellar object Protostar Pre-main - sequence star Herbig Ae / Be Orion T Tauri FU Orionis Herbig -- Haro object Hayashi track Henyey track Evolution Main sequence Red - giant branch Horizontal branch Red clump Asymptotic giant branch Protoplanetary nebula Planetary nebula PG1159 star Dredge - up Instability strip Luminous blue variable Blue straggler Stellar population Supernova Supernova impostor Hypernova Hertzsprung -- Russell diagram Color -- color diagram Luminosity class Subdwarf Dwarf Blue Red White Yellow Brown Subgiant Giant Blue Red Yellow Bright giant Supergiant Blue Red Yellow Hypergiant Yellow Spectral classification O WR Be OB Subdwarf O Subdwarf B Late - type Chemically peculiar Am Ap / Bp Oscillating Barium Carbon CH CN Extreme helium Lambda Boötis Lead HgMn Technetium Shell Remnants White dwarf Helium planet Neutron star Radio - quiet Pulsar Binary X-ray Magnetar Stellar black hole X-ray binary Burster Hypothetical stars Boson star Black dwarf Exotic Electroweak star Quark Strange Preon Planck Dark matter star Dark - energy star Black star Gravastar Frozen star Q star Quasi-star Thorne -- Żytkow object Iron star Blitzar Nucleosynthesis Deuterium burning Lithium burning Proton -- proton chain CNO cycle Helium flash Triple - alpha process Alpha process Carbon burning Neon burning Oxygen burning Silicon burning S - process R - process Fusor Nova Symbiotic Remnant Luminous red nova Structure Core Convection zone Microturbulence Oscillations Radiation zone Atmosphere Photosphere Starspot Chromosphere Corona Stellar wind Bubble Bipolar outflow Accretion disk Asteroseismology Helioseismology Eddington luminosity Kelvin -- Helmholtz mechanism Properties Designation Dynamics Effective temperature Kinematics Magnetic field Absolute magnitude Mass Metallicity Rotation UBV color Variable star Mira variable Star systems Binary Contact Common envelope Eclipsing Symbiotic Multiple Star cluster Open cluster Globular cluster Super star cluster Planetary system Earth 's Solar System Earth - centric observation of Pole star Circumpolar star Constellation Asterism Magnitude Apparent Extinction Photographic Radial velocity Proper motion Parallax Photometric - standard star Lists Star names Arabic Chinese Extremes Most massive Highest temperature Largest volume Smallest volume Brightest Historical Most luminous Nearest Nearest bright Stars with exoplanets Brown dwarfs White dwarfs Milky Way novae Notable supernovae Supernova candidates Supernova remnants Planetary nebulae Timeline of stellar astronomy Related articles Substellar object Brown dwarf Sub-brown dwarf Planet Galactic year Galaxy Supercluster Guest star Gravity Icarus ( most distant individual star ) Intergalactic star Infrared dark cloud Starfield Category : Stars Star portal Retrieved from `` https://en.wikipedia.org/w/index.php?title=Solar_core&oldid=855049232 '' Categories : Sun Hidden categories : Pages using web citations with no URL Webarchive template other archives All articles with unsourced statements Articles with unsourced statements from November 2015 Talk Contents About Wikipedia Беларуская Български Bosanski Català Čeština Dansk Español فارسی Français 한국어 Hrvatski Italiano עברית Lietuvių Македонски മലയാളം Nederlands 日本 語 Norsk Polski Português Română Русский Slovenčina Српски / srpski Srpskohrvatski / српскохрватски Українська 中文 20 more Edit links This page was last edited on 15 August 2018, at 15 : 41 ( UTC ). 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Question: what is the approximate temperature at the center of the sun where nuclear fusion occurs
Answer: 15 million kelvins (15 million degrees Celsius, 27 million degrees Fahrenheit).