Energy Production in the Sun

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What sort of energy generation requirement has observation placed on the Sun?

• The Luminosity of the Sun is 4 x 10²⁶ Watts

• The Earth (and Solar System and hence the Sun) is 4.6 billion years old and it appears that the Sun was roughly the same brightness over this time span.

The combination of these two things says that the Sun has produced a huge amount of energy over its lifetime -- 6 x 10⁴³Joules .

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There were four ideas proposed to power the Sun:

• The Sun was formed hot and has been cooling off for the last 4.5 billion years. This leads to a lifetime of 10s of millions of years.

• The Sun is slowly shrinking (it is not in hydrostatic and thermal equilibrium). Gravity has been slowly causing the Sun to shrink and compress producing enough energy (heat) in the process to make the Sun shine. This gravitational contraction (compression) was worked out by Lord Kelvin in the 1800s. Lord Kevin showed that the Sun could shine in this manner for 20-30 million years at its current luminosity.

• The Sun produces energy through ordinary chemical burning (combustion) . Coal has a heat of combustion of ~ 35 million Joules per kilogram ===> 7x10³⁷ Joules is available if the Sun were pure coal ===> lifetime of 6,000 years.

• The Sun is a huge nuclear reactor. Nuclear energy is efficient enough to power the Sun for up to 100 billion years.

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