CREATION BY EVOLUTION
planets will be broken up and another one born. This rhythm may go on forever, so far as we can tell, for there appear to be no agencies tending to put an end to it.
Each star or sun, apart from these incidents associated with the origin and evolution of planets, has a long, slow evolution of its own. Each is at first a young son, very tenuous, relatively cool, and blue-white in color. As it grows older it becomes hotter, denser, and yellowish in color. With increasing age it becomes progressively denser and cooler and changes in color from lighter to darker red. With this progressive increase in density the constitution of its atmosphere undergoes remarkable changes. In the young suns the atmosphere consists of numerous lighter elements and compounds that can exist at relatively low temperatures; in the somewhat older suns, which are intensely hot, the atmosphere contains only the lightest and simplest atoms, such as hydrogen and helium; in the old, red suns the atmosphere includes not only atoms of the heavier elements, but various compounds. The density of some of these aged suns is amazing. Astrophysicists estimate that a cupful of material from one of these old red suns, if weighed on the surface of our earth, would scale twenty-five tons.
A sun endures so long and changes so slowly that an ephemeral being like a man can observe no change in it. We know, however, that our own galaxy is made up of suns of all grades of brightness and density, and we therefore infer that suns have a regular course of existence—an evolution. Our own particular sun is middle-aged, verging on senility. To an observer living on a planet in another solar system it would appear as a reddish-yellow star, relatively a dwarf as compared with many of the giant suns in our galaxy.
A sun is at all times giving off enormous amounts of energy. This activity alone would cause it to change pro-
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