from the book, "Our Fascinating Earth"
Although earth has existed for at least four and a half billion years, it is, by astronomical standards, still relatively young. Doubtless life on earth will go on as long as the sun continues to burn, thereby supplying the planet with life—giving energy. The sun has already been burning for over six billion years and shows no indication of ever slowing its rate of fuel consumption. Scientists believe that the sun will be long—lived and has enough fuel for several billion years to come.
Many leading astronomers believe our sun's ultimate destiny is to become what is known as a red giant. Assuming that this will eventually happen, the following events will probably occur.
For eons the sun will continue to burn as it does now, but eventually, as the hydrogen fuel is depleted, certain dynamic processes will cause the sun to expand. At first the rate of expansion will be relatively slow, but it will accelerate rapidly until the sun becomes a full—fledged red giant. At this stage the greatly expanded sun will fill the entire orbit of the planet Mercury and may expand even farther.
The effect of the earth will be dramatic even though it will take a billion or more years to unfold. First the polar caps will melt and the water will cover most of the land. Temperatures on the surface of this planet will soon rise above 1,300 degrees Fahrenheit. This will cause the oceans to boil away, and the atmosphere will be driven into outer space. The long—awaited judgment day will at last have come and gone. Life will no longer exist on the surface of the earth. But for many more years the lifeless, scorched planet will continue to rotate on the grill of the dying sun.
However, this intense celestial drama does not end there. Eventually, as the sun cools, it will shrink, growing smaller and smaller and shining more and more faintly. Approximately thirty billion years from the present the last of its fires will gently flicker out. The darkness of night will then reign in our solar system for all eternity.
Perhaps the ingenuity of earth's inhabitants will prevail, and they will find ways to escape the holocaust by surviving underground or making mass migrations to other worlds. This should be of little concern to modern humans, since these catastrophic events will not even begin to happen for at least another billion years.