Stellar Radiation & Stellar Types

Nuclear Fusion

A star forms out of a slowly condensing cloud of gas. As the pressure and density in the core increase so does the temperature. At a certain point a critical temperature is reached at which nuclear fusion can occur. Nuclear fusion is the process of combining nuclei to make a nucleus with a greater number of protons and neutrons.

Deep inside the Sun the temperature reaches 15 million K. At such an extreme temperature the positively charged atomic nuclei are moving at high speed. Occasionally two charged particles will collide with sufficient energy so that they stick together. This is the fundamental process that occurs in all stars.

Proton-Proton Chain

The simplest reaction to be found in stars is the conversion of hydrogen into helium - a process known as the proton-proton chain. In this process six hydrogen atoms are needed to create one helium nucleus of two protons and two neutrons. P>

Process		Result	Extras
1	proton + proton	deuterium	positron, neutrino
2	proton + deuterium	helium-3	gamma-ray
3	helium-3 + helium-3	helium-4	2 protons

In Step 1 two protons come together to form deuterium (a nucleus of one proton and one neutron).  This interaction also involves the liberation of a positron (a positively charged electron) and a neutrino.  This process occurs 10³⁸ times per second in the Sun.

In Step 2 the resulting deuterium nucleus combines with a proton to make the rare isotope helium-3 consisiting of one neutron and two protons.  This reaction also produces a gamma-ray.

In Step 3 the helium-3 nucleus becomes a helium-4 nucleus by the addition of a neutron.  This step can occur in several ways, but the combination of two helium-3 nuclei is the most common way.

Steps 1 and 2 must occur twice before step 3 can occur.  Six protons go into the cycle, but two come back out.  The overall process results in 4 protons (hydrogen nuclei) becoming a helium-4 nucleus, two positrons, two neutrinos and two gamma-rays.

Each helium-4 nucleus has a mass that is about 99.3% of the mass of four protons.  In the Sun, for example, 600 million tons of hydrogen are converted into 596 million tons of helium every second.  The missing four million tons of matter is released as energy in accordance with Einstein's equation E=mc².

Figure 2.1: Proton-Proton Chain

CNO Cycle

The other major process that occurs in more massive stars (due to the necessary higher core temperature) is the Carbon-Nitrogen-Oxygen (CNO) cycle.  This more complex process involves the combination of carbon-12 nuclei with protons to form successively heavier nuclei up to oxygen. More details on the CNO cycle can be found in booklet 5 - 'Stellar Processes and Evolution' under the chapter of Nucleosynthesis.

Hydrostatic Equilibrium

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Last Update: 07 Sep 2005