The Sun has been shining away for 4.6 billion years.
For well over 3 billion years, according to the fossil record, life has existed
on Earth. The fact that life was not scorched or frozen out of existence during
that time span indicates that the luminosity of the Sun has been fairly steady.
What energy source has kept the Sun hot for billions of years, instead of
cooling off like an unplugged iron?
What about chemical sources, such
as the burning of hydrogen? (We know there’s plenty of hydrogen in the Sun.)
Burning one kilogram of hydrogen to form water, by the reaction
2H + O -> H2O + energy,
releases 140 million joules of energy. (The `joule’ is the standard unit of
energy in the metric system. It is the energy required to keep a 1 watt light
bulb lit up for 1 second. 140 million joules is equivalent to 33,000 Calories
or 40 kilowatt-hours.) If the Sun consisted entirely of hydrogen (which it
doesn’t), and if you could find a source of oxygen with which to burn it (never
mind where all that oxygen would come from), it could keep the Sun at its
present luminosity for a mere 20,000 years.
A more potent energy source is needed to keep the Sun shining for
billions of years. What about nuclear fusion? Four hydrogen nuclei can be fused
together into a helium nucleus, accompanied by the release of energy:
4H -> He + energy.
Fusing one kilogram of hydrogen into helium releases 630 trillion joules
of energy. That’s nearly 5 million times what you’d get by burning the same
hydrogen to get water. Thus, fusion of hydrogen will keep the Sun shining at a
constant luminosity not for 20 thousand years, but for as much as 100 billion
years.
But why is sun so important to us?
Nothing is more important
to us on Earth than the Sun. Without the Sun’s heat and light, the Earth would
be a lifeless ball of ice-coated rock. The Sun warms our seas, stirs our
atmosphere, generates our weather patterns, and gives energy to the growing
green plants that provide the food and oxygen for life on Earth.
We know the Sun through its heat and light, but other, less
obvious aspects of the Sun affect Earth and society. Energetic atomic particles
and X-rays from solar flares and other disturbances on the Sun often affect
radio waves traveling the Earth’s ionosphere, causing interference and even
blackouts of long-distance radio communications. Disturbances of the Earth’s
magnetic field by solar phenomena sometimes induce huge voltage fluctuations in
power lines, threatening to black out cities. Even such seemingly unrelated
activities as the flight of homing pigeons, transatlantic cable traffic, and
the control of oil flow in the Alaska pipeline apparently are interfered with
by magnetic disturbances caused by events on the Sun. Thus, understanding these
changes – and the solar events that cause them – is important for scientific, social,
and economic reasons.
We have long recognized the importance of the Sun and watched
it closely. Primitive people worshiped the Sun and were afraid when it would
disappear during an eclipse. Since the early seventeenth century, scientists
have studied it with telescopes, analyzing the light and heat that manage to
penetrate our absorbing, turbulent atmosphere. Finally, we have launched solar
instruments and ourselves-into space, to view the Sun and its awesome eruptions
in their every aspect.
