Introduction

The energy radiated by the sun is a result of thermonuclear fusion, in which hydrogen is transformed into helium. This transformation involves a loss of mass, which is converted into energy. Box 1.1 lists some facts about the sun. Box 1.1 Facts about the sun

The sun is our nearest star, and with its immense size governs the movements of the planets by its force of gravity. The sun is estimated to be 4.6 × 10⁹ years old. Its radius is 696,000 km, and it consists of 71% hydrogen (H), 27% helium (He) and 2% other elements. Its weight can be estimated as 1989 × 10³⁰ metric tonnes (333,000 times as heavy as the earth).

The energy-producing process in the sun is the transformation of hydrogen atoms into helium atoms. The transformation involves a loss of mass, which is converted into large amounts of energy emitted in the form of electromagnetic radiation. The total radiation from the sun is 3.8× 10²⁶ W. Of this amount about 1.7 × 10¹⁷ W reaches the earth, and this is (1990) 15,000 times the energy that is consumed on earth.