The Sun is the star about which Earth revolves. A typical star, Earth's sun is composed of gases and heavier elements compressed to enormous density and heated to levels that sustain nuclear fusion (the transformation of hydrogen into helium and heavier elements). The Sun consists of an inner core surrounded by a radiative zone and then a convective zone. The surface of the Sun is termed the photosphere. Surrounding the Sun is a solar corona—an atmosphere of hot plasma, gases, and outflowing particles.

Nuclear fusion take place in the Sun's core and it is in this region that the bulk of the Sun's production of energy, heat, and gamma rays takes place. The radiative zone surrounding the core is of such high density that photons generated in the

X-ray photograph of the solar corona. U.S. National Aeronautics and Space Administration (NASA).

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