Fusion in Stars

The Sun, an average star in the Milky Way Galaxy, is 4.3 light minutes away from Earth.

The Alpha or Proxima Centauri is 4.3 light years from the Sun.
The Procyon is 8.6 and 11.4 light years from the Sun
The universe consists of millions of stars.

Fusion in the Sun

The mass of the Sun is 332,000 times that of the Earth. The density at the centre of the Sun is 100 time that of water. The temperature at the centre is at least 15 million K, and 6000 K at the surface. The Sun is a ball of plasma, held together by gravity.

These conditions of high temperature and high density are ideal for thermal nuclear fusion reaction. There are two detailed schemes for the fusion reactions: the hydrogen cycle and the carbon cycle

The hydrogen cycle

H + H ® D + b+ (+ b-) + n
D + H ® 3He + g
3He + 3He ® 4He + 2 H

The overall reaction:

4 H ® 4He + 2 (b + (+ b-) + g + n)

The fact that carbon, nitrogen, and oxygen atoms are present in the Sun and the high probability of the following reactions suggest another scheme catalyzed by carbon for the fusion of hydrogen in the Sun. The following scheme is called the carbon cycle

12C + H ® 13N + g
13N ® 13C + b+ (+ b-) + n
13C + H ® 14N + g
14N + H ® 15O + g
15O ® 15C + b+ (+ b-) + n
15N + H ® 12C + 4He + g

The overall reaction:

4 H ® 4He + 2 (b+ (+ b-) + g + n)

Fusion takes place mainly in the interior of the Sun, and energy released becomes kinetic energy of particles. Energy is absorbed and re-radiated by particles. Emission of energy from the surface of the Sun is rather constant.






Lives of Stars

Depending on the masses, the lives of stars go through different stages. The real massive ones explode into supernova after the white dwarf stage, but the massive ones explode into a nova.

© cchieh@uwaterloo.ca