Energy Production in the Sun

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 must have produced a huge amount of energy over its lifetime -- 6 x 10⁴³Joules. Note--the book will sometimes consider another measure of energy known as the erg. An erg is related to the Joule as 1 Joule = 10⁷ ergs. So, in ergs, the Sun radiates ~ 6 x 10⁵⁰ ergs in its lifetime.

There were four ideas proposed to power the Sun:

• The Sun was formed hot and has been cooling off
• The Sun is slowly shrinking and producing gravitational energy
• The Sun is undergoing chemical burning
• The Sun is a huge nuclear reactor

As we will find out, the energy is generated in the core of the Sun, in its inner 20 % in radius. This means that in order to have thermal equilibrium we must understand how the energy gets from the center of the Sun to the surface of the Sun. All methods of energy transport act in a way that moves heat from hot regions to cool regions. There are three main ways stars transport energy:

• Radiative transport
• Convection
• Conduction

The important transport mechanisms for the Sun are radiation transport and convection.