The Sun and Solar Energy
During its first 50 million years, the sun contracted to approximately its present size.
Gravitational energy released by the collapsing gas heated the interior, and when the core was
hot enough, the contraction ceased and the nuclear burning of hydrogen into helium began in the
core. The sun has been in this stage of its life for about 4.5 billion years.
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The sun, by the gravitational effects of its mass, dominates the planetary system that includes
the earth. By the radiation of its electromagnetic energy, the sun furnishes all of the energy
supporting all life on earth, because all foods and all fuels are derived ultimately from plants
using the energy of sunlight: photosynthesis. Because of its proximity to the earth, and because
it is such a typical star, the sun is a unique resource for the study of stellar phenomena. No
other star can be studied in such detail. The star closest to the sun is located approximately
4.3 lightyears away. To observe features on it's surface of comparable size to those that can
be seen routinely on the sun would require a telescope more than 18 miles in diameter. Such a
telescope, moreover, would have to be put into space,like the Hubble telescope orbiting the
earth currently, to avoid distortions caused by the earth's atmosphere.
The total amount of energy emitted by the sun in the form of radiation is remarkably constant,
varying by no more than a few tenths of 1 percent over several days. This energy output is
generated deep within the sun. Like most stars, the sun is made up primarily of hydrogen
(specifically, 71% hydrogen, 27% helium, and 2% other elements). Near the center of the sun the
temperature is almost 29,000,000 degrees Fahrenheit and the density is 150 times that of water.
Under these conditions the nuclei of individual hydrogen atoms interact, undergoing nuclear
fusion. In this process two hydrogen nuclei combine to make one helium nucleus, and energy is
released in the form of gamma radiation. This energy is equivalent to that which would
be released from the explosion of 100 billion one-megaton hydrogen bombs per second. The
nuclear burning of hydrogen in the core of thesun extends out to about 25 percent of the sun's