Stars - P-P-Chain

There is a series of steps in this reaction. Each step releases energy, part of which we see as visible light

and feel as heat.

³Helium Production

Two protons (hydrogen nuclei) combine to produce one deuterium nucleus (²H) with one proton becoming

a neutron releasing a positron (anti-electron) and an electron neutrino. The positron annihilates with an

electron producing two gamma-ray photons. The deuterium combines with another proton to produce

³helium (³He), which is a light isotope of helium, plus another gamma-ray. While the temperature remains

around 10 M°K, not too much more happens. It is interesting to note that, on average, a hydrogen

nucleus has to wait for one billion years before it combines with another hydrogen nucleus. A star just

contains so many hydrogen nuclei, it is enough to keep it going.

PP I Branch

Once the temperature rises above 10 M°K, up to about 14 M°K, the PP I reaction combines two ³He

nuclei to produce one ⁴He nucleus and two hydrogen nuclei.

PP II Branch

Once the temperature rises above 14 M°K, up to about 23 M°K, the PP II reaction comes into play. In

addition to an abundance of hydrogen, there are ³He and ⁴He nuclei. A ³He and a ⁴He nucleus combine

to form ⁷Berylium plus a gamma-ray. The ⁷Be captures an electron and decays to ⁷Lithium plus an

electron neutrino. Finally, the ⁷Li combines with a hydrogen nucleus to form two ⁴He nuclei.

PP III Branch

Once the temperature rises above 23 M°K, the much rarer PP III reaction starts. Again, a ³He and a ⁴He

nucleus combine to form ⁷Be plus a gamma-ray. Now the ⁷Be combines with a proton to produce ⁸Boron

plus a gamma-ray. The ⁸Boron now decays into a ⁸Be nucleus and releases a positron, an electron

neutrino and a gamma-ray. Finally, the ⁸Be forms two ⁴He nuclei.

The energy production comes from the fact that the final ⁴Helium nucleus contains 0.7% less mass than

the original four protons. This mass is converted into pure energy, and is what powers stars like the Sun.

Astronomy & Cosmology -