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The Solar Interior


Why We Study the Sun 
The Big Questions 
Magnetism - The Key 


The Interior 
The Photosphere 
The Chromosphere 
The Transition Region 
The Corona 
The Solar Wind 
The Heliosphere 


Photospheric Features 
Chromospheric Features 
Coronal Features 
Solar Wind Features 


The Sunspot Cycle 
Solar Flares 
Post Flare Loops 
Coronal Mass Ejections 
Surface and Interior Flows 
Waves and Helioseismology 


Flare Mechanisms 
3D Magnetic Fields 
The Solar Dynamo 
Sunspot Cycle Predictions 
Solar Wind Dynamics 

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The solar interior is separated into four regions by the different processes that occur there. Energy is generated in the core. This energy diffuses outward by radiation (mostly gamma-rays and x-rays) through the radiative zone and by convective fluid flows (boiling motion) through the outermost convection zone. The thin interface layer between the radiative zone and the convection zone is where the Sun's magnetic field is thought to be generated.

The Core

The Sun's core is the central region where nuclear reactions consume hydrogen to form helium. These reactions release the energy that ultimately leaves the surface as visible light. These reactions are highly sensitive to temperature and density. The individual hydrogen nuclei must collide with enough energy to give a reasonable probability of overcoming the repulsive electrical force between these two positively charged particles. The temperature at the very center of the Sun is about 15,000,000° C (27,000,000 ° F) and the density is about 150 g/cm³ (about 10 times the density of gold or lead). Both the temperature and the density decrease as one moves outward from the center of the Sun. The nuclear burning is almost completely shut off beyond the outer edge of the core (about 25% of the distance to the surface or 175,000 km from the center). At that point the temperature is only half its central value and the density drops to about 20 g/cm³.

In the process of fusing hydrogen to form helium, the nuclear reactions also produce elementary particles called neutrinos. These elusive particles pass right through the overlying layers of the Sun and, with some effort, can be detected here on Earth. The number of neutrinos we detect is but a fraction of the number we expect. This problem of the missing neutrinos is one of the great mysteries of solar astronomy.

The Radiative Zone

The radiative zone extends outward from the outer edge of the core to the interface layer at the base of the convection zone (from 25% of the distance to the surface to 70% of that distance). The radiative zone is characterized by the method of energy transport - radiation. The energy generated in the core is carried by light (photons) that bounces from particle to particle through the radiative zone. Although the photons travel at the speed of light, they bounce so many times through this dense material that an individual photon takes about a million years to finally reach the interface layer. The density drops from 20 g/cm³ (about the density of gold) down to only 0.2 g/cm³ (less than the density of water) from the bottom to the top of the radiative zone. The temperature falls from 7,000,000° C to about 2,000,000° C over the same distance.

The Interface Layer

The interface layer lies between the radiative zone and the convective zone. The fluid motions found in the convection zone slowly disappear from the top of this layer to its bottom where the conditions match those of the calm radiative zone. This thin layer has become more interesting in recent years as more details have been discovered about it. It is now believed that the Sun's magnetic field is generated by a magnetic dynamo in this layer. The changes in fluid flow velocities across the layer (shear flows) can stretch  magnetic field lines of force and make them stronger. There also appears to be sudden changes in chemical composition across this layer.

The Convection Zone

The convection zone is the outer-most layer. It extends from a depth of about 200,000 km right up to the visible surface. At the base of the convection zone the temperature is about 2,000,000° C. This is "cool" enough for the heavier ions (such as carbon, nitrogen, oxygen, calcium, and iron) to hold onto some of their electrons. This makes the material more opaque so that it is harder for radiation to get through. This traps heat that ultimately makes the fluid unstable and it starts to "boil" or convect. These convective motions carry heat quite rapidly to the surface. The fluid expands and cools as it rises. At the visible surface the temperature has dropped to 5,700° K and the density is only 0.0000002 gm/cm³ (about 1/10,000th the density of air at sea level). The convective motions themselves are visible at the surface as granules and supergranules.


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Author: David H. Hathaway,, (256) 544-7610
Mail Code SD50, NASA/Marshall Space Flight Center, Huntsville, AL 35812


Responsible Official: John M. Davis,, (256) 544-7600
Mail Code SD50, NASA/Marshall Space Flight Center, Huntsville, AL 35812


Last revised 2000 July 17 - D. H. Hathaway

Reproduced from