Page 22 - Curriculum Visions Dynamic Book

The convective zone
In the outermost shell of the Sun the temperature drops below 2,000,000°C, and the plasma in the Sun’s atmosphere becomes too cool and opaque to allow radiation to get through it. So, for the last 20% of the way to the surface the energy is carried more by convection than by radiation.
The process in this zone can be likened to heating a pot of water on a stove. The water is heated at the bottom; and as it
is warmed, it becomes less dense than the water above it. As a result, it rises, while neighboring, cooler, more dense water takes its place. This overturning is called convection, and it happens in just the same way near the surface of the Sun—but with gas at a temperature of thousands of degrees. The convection currents carry outward to the surface faster than the radiative transfer that occurs in the core and radiative zone.
Why the Sun burns but does not change
The Sun is, at present, about 75% hydrogen and 25% helium by mass (92.1% hydrogen and 7.8% helium by number of atoms).
Although this represents a huge amount of matter
being processed and a vast amount of energy released, it is insignificant compared with the mass of the Sun. At the end of its life the Sun will have turned into energy only a tenth of 1% of the matter it began with—most of it will have been converted into hydrogen. The Sun therefore does not burn up and fade away. Its mass remains almost unchanged. Its density, however, can change dramatically, as we have seen in the Sun’s life cycle on pages 10–11.
The solar atmosphere
The Sun’s atmosphere is made up of several shells of very thin gases surrounding the star. They are the photosphere, the chromosphere, and the corona.
The photosphere has properties that mean it is the part of the Sun we see. It can therefore be thought of as the skin of the Sun’s main body just as much as the lowest part of its atmosphere. The other two shells of gases are virtually invisible to us.
 A false-colour image of coronal mass ejections of plasma from the Sun into the surrounding corona (see page 35). (False colour is used to make the appearance of the ejections more obvious.)
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