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Stellar energy
It was soon realised that the energy of the stars was produced by nuclear fusion, the
temperatures required in the stars being lower than those needed in the laboratory because
of the enormous pressures in the stellar interiors.
One such reaction that occurs in the Sun is the so called PP chain shown below.
1
1
2
+
1H + 1H
1H + e + energy
2
1H
3
2He
+
1
1H
3
2He
+ energy
3
4
2He
+ 2 1H + energy
+ 2He
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Part of the energy in the first reaction is high-speed
neutrinos and these can be detected on the Earth.
Note: the apparent imbalance between particles in these
equations can be understood if we look at the first of these. On
the left we have two protons and no neutrons. On the right we
have one proton and one neutron and a positron. One of the
protons has been converted into a neutron and a positron. Similar
reasoning will explain the other equations.
Altair and surrounding star field
Other reactions may then occur such as:
3
4
2He + 2He
7
4Be + 
Heavier and heavier nuclei can be produced by successive fusion reactions. The types of
reaction taking place in the Sun are shown below:
12
6C
+ 1H
1
13
7N
+ energy
13
7N
13
6C
+ e+
1
14
7N
+ energy
1
15
8O
+ energy
15
8O
15
7N
+ e+
1
12
6C
+ 2He
13
6C
+ 1H
14
7N
+ 1H
15
7N
+ 1H
4
The net result of this series of reactions is:
1
4 1H
4
4 2He + 2e+ + energy
In other words, 564 million tonnes of hydrogen are being converted into 560 million tonnes of
helium every second, producing a staggering output of 3.90x1020 MW - equivalent to some
million million million large power stations!
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Although the Sun is converting four million tonnes of mass into energy every second it is
thought that it will continue to last for a further 109 years.
At higher temperatures (around 108 K) the fusion of helium starts the building of heavier
elements.
The reaction is:
4
2He
8
4Be
+
4
2He
4
2He
8
4Be
12
6C
+ 
 


+
+ 

This is known as the triple alpha process and is the first stage of ‘helium burning’.
Beryllium 8 is very unstable and so can revert to two alpha particles but eventually an
equilibrium is set up and maintained.
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