Solar System -

The Sun

Two diagrams showing the internal cross-section of a Sun-type star.
The Core

Structure

Credit: NASA
WILLIAM & DEBORAH HILLYARD
The next layer is the Radiative Zone that surrounds the core out to a diameter of about 1 million km (~625,000 miles) and also weighs almost half of the Sun.  Energy travels through this zone by radiation, but the photons are frequently scattered by particles in the gas and may take up to one million years to get through.  Unlike the core, this zone is much the same as it was when the Sun formed. 
This is the outer layer from the radiative zone almost to the surface.  While it comprises only about 2% of the Sun's mass, it occupies nearly 2/3rds of the volume, so the density of this region is very small.  Heat from the radiative zone heats up the lower levels which rise to the surface, give off their heat and sink back down again. 
The temperature of the Sun's photosphere is between 4500ºK and 6000ºK.  It comprises convection cells each of which is about 1000 kilometers in diameter.  As each of these cells, or "granules" lasts for only about eight minutes, the surface has a turbulent, roiling pattern. The convection cells group into "super granules" that may be up to 30,000 kilometers in diameter, and last up to 24 hours.
The Sun's atmosphere comprises four layers; the Photosphere, the Chromosphere, the Transition Region and the the Corona.  Interestingly, temperatures rise in some of the outer layers from 10,000K to 20,000K in the Chromosphere to as much as 500,000ºK or more in the Corona. 
The original composition of the Sun was 72 percent hydrogen, 26 percent helium, and 2 percent other elements.  Because it has been converting hydrogen into helium for so long, the composition of the core has change.  Hydrogen now comprises about 35 percent of the mass at the center rising to 65 percent at the outer edge of the core.  The rest of the Sun's composition is relatively unchanged.  The core is about 350,000 km (~220,000 miles) across, but contains almost half the mass of the Sun. 
The Radiative Zone
The Convection Zone
The Photosphere
The Atmosphere
Cross-Section
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