2 × 10⁹ years ago, the sun had 1·00013 times its present weight 1,000 × 10⁹ ” ” 1·07 ” ” 2,000 × 10⁹ ” ” 1·16 ” ” 5,700 × 10⁹ ” ” double ” 7,100 × 10⁹ ” ” 4 times ” 7,400 × 10⁹ ” ” 8 ” ” 7,500 × 10⁹ ” ” 20 ” ” 7,600 × 10⁹ ” ” 100 ” ” The first entry represents roughly the time since the earth was born. It shews that, during the whole existence of the earth, the sun’s weight has changed by only an inappreciable fraction of the whole. Consequently, it seems likely, although naturally we cannot be certain, that when the earth was born the sun was much the same as it now is, and that it has been the same, in all essential respects, throughout the whole life of the earth. To come to appreciably different conditions we have to go back to remote aeons far beyond the time of the earth’s birth. We are free to do this, for we have seen that the earth’s whole life is only a moment in the lives of the stars. We have estimated the latter as being something of the order of 5 to 10 million million years, and it is only when we go back an appreciable fraction of these long periods that we find the sun’s weight differing appreciably from its present weight. We have, for instance, to go back more than 5 million million years to find the sun with double its present weight. When we go back much further than this a new phenomenon appears; the weight of our hypothetical past sun begins to go up by leaps and bounds. In time it begins to double and more than double every 100,000 million years, and we cannot go back as far as 8 million million years without postulating a sun of quite impossibly high weight. The sun must, then, have been born some time within the last 8 million million years. The exact figures of our table may be open to suspicion, but as a general fact of observation there is no doubt that very massive stars radiate away their energy, and therefore also their weight, with extraordinary rapidity. Indeed the process is so rapid that we may disregard all that part of a star’s life in which it has more than about 10 times the weight of the sun—this is lived at lightning speed. Apart from all detailed calculations, this general principle fixes a definite limit to the ages, not only of the sun, but also of every other star. The upper limit to the age of the sun is certainly somewhere in the neighbourhood of 8 million million years.