National Geographic : 1965 Nov
EKTACHROME(OPPOSITE) BY LOWELLJ. GEORGIA;KODACHROMEBY YVES DEBRAINE ) N.G.S. believed that the solar system originated in a near collision between the sun and another star, and that the material of the planets was torn loose from the sun by the tremendous gravitational pull of the passing star. Today's view, however, holds that the sun and the planets condensed from an enormous turbulent cloud of gas and dust (pages 737-9). The sun grew steadily warmer because of its immense gravitational energy. In time the protostar began to glow brightly, and its core temperature rose millions of degrees. Hydrogen nuclei, impelled by the tremen dous heat, collided with such violence that thermonuclear fusion could occur, and nu clear energy, rather than gravitational energy, began to keep the star hot. Some theoretical calculations indicate that the proportion of hydrogen in the core of the sun has decreased from about two-thirds to about one-third in the past five billion years. Temperatures have risen somewhat, and the sun has grown about five percent larger in diameter and about 25 percent more lumi nous. The great majority of stars follow this gradual trend of evolution. The sun today is a very ordinary star-a yellow dwarf midway between the largest and the smallest, and between the hottest blue stars and the coolest red stars. To earth-based observers, it is a hundred billion times bright er than any other star, though it would appear puny if it were matched at the same distance against the more brilliant stars. Rigel, for example, is 15,000 times more luminous, and 36 million suns could be fitted into Antares, a red supergiant. What of the future? Will the sun burn out? In time the core will deplete its hydrogen. With the core spent, the thermonuclear re actions will spread to outer portions where unused hydrogen still exists.