Page 298 - NS-2 Textbook

The Sun

Earth has been warmed by the light of the Sun for 4.6 bil- lion tons of helium by nuclear fusion, with 4 million tons
lion years. Allille is maintained by the solar energy that being converted into energy. About 40 percent of this en-
is converted into chemical energy by plants. Moreover, ergy escapes in the form of visible light, with most of the
the power from all fossil fuels, water, and winds can be rest radiated as ultraviolet light. Even with this fantastic
traced back to the Sun. The Sun, therefore, is the source rate of fuel constunption, the Stm is so massive that it is
of most of the world's energy. (The only exceptions are estimated it will take another 5 billion years or more be-
nuclear energy, lunar tidal energy, and the heat produced fore its supply of hydrogen is exhausted.
in the interior of the Earth and released by volcanoes and The energy generated in the core of the Sun travels
hot springs.) outward tmtil it reaches the three layers of the Sun's at-
The Sun actually contains 99.S6 percent of all the mosphere, called the photosphere, the chromosphere,
matter in our solar system. An Llaverage" starf it is con- and the corona. The photosphere is about SOD km (500
sidered by astronomers to be medium-sized. It is com-
posed of luminous gases. The Sun's weight is about 1
million times that of Earth. The Sun's gravitational at-
traction is 270 times that of Earth; consequently, a 100-
pound keg of nails would weigh 27,000 pounds on the
Sun!
The average distance from the Sun to Earth (that is,
Earth's average orbital radius) has been calculated to be
92,870,000 miles-nearly 93 million. This average dis-
tance is known as an astrollomical lIllit, a huge tmit of
measure often used in describing distances in outer
space. The Sun has a diameter of about 865,000 miles-
about 109 times that of Earth.
It is not possible for us to look directly at the Sun
without first protecting our eyes. Any attempt to do so
will cause temporary blindness, unless some sort of filter
or special fogged lens is used. The best way to see the
Sun is tlu'ough a telescope--but oilly if using special pre-
cautions. Use this method: hold a piece of white card-
board a foot or so behind the eyepiece, and focus the
scope until the Sun's edge appears sharp. Never look di-
rectly at the Sun through a telescope or binoculars. The
Sun's rays will burn the retina of your eye, causing per-
manently impaired vision, or even blindness.

COMPOSITION OF THE SUN The atmosphere of the Sun has three main layers. Innermost is the
photosphere, an 800-km-thick layer from which visible light is emit-
Spectrographic evidence shows that the Sun consists of ted. Next is the chromosphere, extending about 10,000 km above
gases at very high temperatures. Its composition is the photosphere. Solar flares and prominences are often seen
roughly 92 percent hydrogen, 7.8 percent helium, and .2 shooting from the chromosphere. Beyond the chromosphere is the
Sun's corona, a vast region of low-density gas extending millions of
percent other elements. Each second, about 600 million miles into space. The inset shows the size of Earth relative to a typ-
tons of this hydrogen are converted into about 596 mil- ical solar flare.

293