When compared with some other stars, the Sun is quite
small, but compared with the Earth it is enormous - having a diameter of
1,392,530 km and a mass about 330,000 times that of the Earth. The Sun's
central regions are under great pressure and reach the tremendously high
temperature of approximately 15 million *C.
In these conditions, nuclear reactions, which hydrogen
is converted into helium are self-sustaining and form the source of the
Sun's enormous output of energy. This energy is transported outward by
radiation and convection until it reaches the visible surface of the Sun
- the photosphere - where the temperature is about 5,800*C. From the photosphere,
the energy is radiated out into space.
Although the temperatures throughout the Sun are so high that all matter is in gaseous form, its disc shows a distinct edge, known as the limb. Layers of the solar atmosphere lie outside this, but they are not normally visible, except during a total solar eclipse. At other times, the observer sees light from the lower, hotter layers at the centre of the disc and from higher, cooler layers at the limbs, which thus appear darker.
