Physics of the Sun

The first part of this paper reviews the fundamental observed data bearing on the physics of the Sun - in particular, its size, mass, and luminosity. In the second part, an outline of the equilibrium model of solar interior is given. It is pointed out that the forces determining its structure are primarily gravitational, and that most of its interior is likely to be in radiative equilibrium. High internal temperature (attaining some 14 x 106° K at the centre) brings about far-reaching ionization which, in turn, permits solar matter to behave like a perfect gas even at a central density of 90 g/cm3, and causes part of the radiant energy leaking slowly outwards to have the nature of soft X-rays. The third part reviews the principal nuclear reactions capable of transforming hydrogen into helium under conditions which prevail in the solar interior, and whose exothermic nature provides the source of energy which causes the Sun to shine. It is pointed out that a high temperature-dependence of such reactions restricts the actual energy production essentially to central portions of the Sun; and high opacity of its internal matter is bound to keep this central thermonuclear reactor of the solar system burning at an exceedingly steady rate. The concluding section of this paper reviews the physics and chemistry of the solar visible surface and of its atmosphere. It is pointed out that most phenomena observed in these regions involve but minute transport of mass; but, unlike in the interior, the forces governing them are electromagnetic rather than gravitational. The survey ends with a brief description of the solar corona, whose physical structure is dominated by a temperature of the order of 106° K. Dynamically the corona is a fleeting, unstable phenomenon constantly dissolving and being replenished; but the source of the energy necessary for the maintenance of so high a temperature remains one of the great unsolved puzzles of solar physics.