Scientists determined long ago that the sun is not merely burning like a great ball of coal. Indeed, if it were merely coal, it would have burned for only a few thousand years, and would have turned to cold cinders billion years ago. Some other explanation was required. It is atomic, or nuclear energy that fires the stars. This energy the same as that of the hydrogen bomb comes from the process we call nuclear fusion, in which the nuclei, or cores, of hydrogen atoms collide, uniting to form helium nuclei and giving off burst of energy. No other process we know could possibly pour out such sustained quantities of energy. Moreover, we know that for the sun to stabilize at its present size, it must have a temperature and pressure at its core sufficient to support nuclear reactions. Thus, deep within the sun, each second, 564 million tons of hydrogen are converted to 560 million tons of helium. The remaining four million tons each second radiates away as heat and light.
If the sun has been shinning at its present brightness since the earth was formed nearly five billion years ago, each pound of solar matter must have yielded already at least 4,000,000 kilowatt-hour of energy. At that rate, a pound of sun would keep a kitchen stove going with all burners on for several hundred years. Fantastic as the sun's overflow of energy must appear, the nuclear fusion actually goes on at a slow pace, atomically speaking. The sun may be considered as a very slow burning hydrogen bomb, since it takes, on the average, about a million years for two hydrogen nuclei to collide and fuse. These tiny particles, even in the sun's dense interior, are on the average almost as far apart, in proportion to their size, as the earth and Venus. Moreover, they require a head on crash at extraordinary high speeds in order to fuse.
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