The Sun Help (page 2)

By — McGraw-Hill Professional
Updated on Sep 16, 2011

How It “burns”

The Sun “shines” by constantly “burning” hydrogen, the most abundant element in the universe. You know, if you have taken chemistry classes, that hydrogen is flammable and that it burns clean and hot. (It might someday replace natural gas for heating if a method can be found to cheaply and abundantly produce it and safely distribute it.) However, the Sun “burns” hydrogen in a far more efficient and torrid fashion: by means of nuclear fusion . The enormous pressure deep in the Sun, caused by gravity, drives hydrogen atoms into one another. Hydrogen atoms combine to form helium atoms, and in the process, some of the original mass is converted directly into energy according to Einstein’s famous equation E = mc 2 (energy equals mass times the speed of light squared).

The earliest theories concerning the Sun involved ordinary combustion, the only question being what, exactly, was burning. Coal was suggested as a fuel for the Sun, but if this were the case, the Sun would have burned out long ago. Besides this, there was the little problem of how all that coal got up there into space. Another idea involved the direct combination of matter with antimatter, resulting in total annihilation. However, if this theory were true, the Sun would be far brighter and hotter than it is. The hydrogen-fusion theory accounts for what we see and is consistent with theories concerning the age of the Universe and the age of Earth.

About The Sun’s Life

How long will the Sun’s supply of hydrogen fuel last? Should we worry about the possibility that the supply will run out soon and Earth will cool off and freeze over?

Eventually, the Sun will burn out, but it will not happen for quite awhile. In fact, most scientists believe that the Sun will continue to shine for at least 1 billion more years at about the same level of brilliance as it does today. There are a lot of hydrogen atoms in that globe. It has a radius, remember, of 695,000 kilometers, or 69,500,000,000 centimeters. Scientists would write that as 6.95 × 10 10 cm. Remember the formula for calculating the volume of a sphere from your middle school geometry class: V = 4/3π r 3 , where V is the volume and r is the radius. Using your calculator, you can figure out, using 3.14 as the value of π, that there are about 1.4×10 33 , or 1.4 decillion, cubic centimeters of matter in the Sun. Written out in full, that number looks like this:


With this number in mind—or out of mind, because it’s incomprehensibly large—you might be willing to accept practically any claim as to the Sun’s longevity, except, of course, life everlasting. The Sun will perish. The symptoms of aging will begin in 1000 or 2000 million years.

As the supply of hydrogen runs out, the Sun will expand, and its surface will cool off. However, Earth will heat up because the bloated Sun will appear much larger in the sky and will send far more energy to Earth’s surface than is the case now. The climate will become intolerably hot; the polar ice caps will melt; wildfires will reduce all plant life to ashes. Sometime during this process, any remaining humans and other mammals will die off. The oceans, lakes, and rivers will boil dry. All living things, even the hardiest bacteria and viruses, will die. The atmosphere will be blown off into space. Some astronomers think that the Sun’s radius will grow until it exceeds the radius of Earth’s orbit so that the Sun will swallow Earth up and vaporize it.

Don’t let this scenario depress you. By then humanity will have colonized a couple of dozen other planets and will grieve no more about the fate of Earth than we do today about the buried houses of ancient cities. If our descendants remember us at all, it will be with fascination. Knowledge of our present society might be conveyed by legend, by stories told to children at bedtime, by tales about a place called Terra that sank beneath the surface of a stormy star after its inhabitants had fled, a place where people burned the decomposed by-products of dead plants and animals in order to propel surface transport vehicles. And the children will laugh and say that such a ridiculous place couldn’t have existed.

After the red-giant phase, the Sun will fuse helium into carbon, iron, and other elements, and will shrink as gravity once again gains dominance over the pressure of nuclear heat. However, this process cannot continue forever. A point will be reached at which no further nuclear reactions can take place, and then gravitation will assert its ultimate power. The Sun will be crushed into an orb of planetary size and, as the last of its heat dissipates, will fade away and spend the rest of cosmic time as an incredibly dense, dark ball.

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