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in volume when cooled. The contraction of a gaseous body, such as we believe the sun to be, is greater than that of a solid or liquid. The heat of the sun is radiated from streams of matter constantly rising from the interior, which radiate their heat when they reach the surface. Being cooled they fall back again, and the heat caused by this fall is what keeps the sun hot.

It may seem almost impossible that heat sufficient to last for millions of years could be generated in this way; but the known force of gravity at the surface of the sun enables us to make exact computations on the subject. It is thus found that in order to keep up the supply of heat it is only necessary that the diameter of the sun should contract about a mile in twenty-five years—or four miles in a century. This amount would not be perceptible until after thousands of years. Yet the process of contraction must come to an end some time. Therefore, if this view is correct, the life of the sun must have a limit. What its limit may be we cannot say with exactness, we only know that it is several millions of years, but not many millions.

The same theory implies that the sun was larger in former times than it is now, and must have been larger and larger every year that we go back into its history. There was a time when it must have been as large as the whole solar system. In this case it could have been nothing but a nebula. We thus have the theory that the sun and solar system have resulted from the contraction of a nebula—through millions of years. This view is familiarly known as the nebular hypothesis.