# The Sun A Nuclear PowerhousePage 1

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Happy Sun

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## Why Does the Sun Shine?

The Sun gives off energy (duh)!

The energy must come from somewhere - there’s no free lunch.

Conservation of energy is a fundamental tenet of physics.

Where does the energy come from?

Until the 20th century only 2 possibilities were known:

Chemical reactions

Gravity

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## The Sun’s Energy Output

How bright is the Sun?

The Sun produces 4x1026 watts

Watt is the unit for the rate of energy use, commonly seen on light bulbs and appliances.

Our largest power plants produce around 5 x 109 watts of power (5,000 megawatts or 5 gigawatts)

Sun = 8 x 1016 of these power plants (80,000 trillion)

Anyway you look at it, the Sun gives off a lot of energy.

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What are chemical reactions?

Rearrange the atoms in molecules, as in 2H2 + O2  2H2O + energy. This reaction combines hydrogen and oxygen gas to produce water plus energy.

Reverse the process: 2H2O + energy  2H2 + O2. By adding energy we can dissociate water into hydrogen and oxygen.

The energy factor is often left out of chemical reaction formulas, for convenience.

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If the Sun is powered by burning coal or oil, how long could its fuel last?

Only a few thousand years!

A process that uses fuel more efficiently is needed - something that gets more energy out of every kilogram of material.

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## Gravity Squeeze?

Gravitational contraction: falling layers of the Sun's material compress the Sun  heat energy

drop a book  noise! Gravitational potential energy.

A contraction of 40m per day would account for the Sun’s energy output.

Efficiency ~ 1/10000 %

Gravity could power the Sun for about 100 million years  but the Sun is at least 4 billion years old!

Gravity can't be the Sun's main energy source

But it did help ignite the Sun when it formed

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15.2 Mass, Energy, and the Special Theory of Relativity

To understand the way the Sun produces energy, we need to learn a little about nuclear physics and the special theory of relativity.

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