2. Energizing the Sun

Nuclear fusion reactions in the Sun

The ultimate source of the Sunís energy is nuclear fusion in its core. The intense pressures and searing temperatures at the Sunís core are fusing together the nuclei of the most abundant element in the Universe, hydrogen, to form nuclei of the second most abundant element, helium. The mass lost in the nuclear fusion reactions supplies the energy that makes the Sun shine. The nuclear fusion begins when two of the fastest moving protons collide head on, and very occasionally tunnel through the electrical barrier that almost always keeps them apart. When two protons merge and come into each other, they initiate a chain of reactions that ends when four protons have joined together to make one helium nucleus consisting of two protons and two neutrons. This sequence of nuclear fusion reactions is called the proton-proton chain. Outside the solar core, where the overlying weight and compression are less, the gas is cooler and thinner and nuclear fusion cannot occur. In main-sequence stars more massive than the Sun, carbon acts as a catalyst in hydrogen burning by the CNO cycle; less massive main-sequence stars burn hydrogen by the proton-proton chain.

Mass Lost is Energy Gained

Energy can only be derived from energy, and the source of energy in nuclear fusion is mass loss. The foundation was provided by Albert Einsteinís special theory of relativity, which included the famous formula E = mc2 for the equivalence of mass, m, and energy, E. Because the velocity of light c = 299,792,458 meters per second is a very large number, only a tiny amount of mass is needed to produce a huge amount of energy. Even the smallest grain of sand holds an enormous quantity of energy locked up inside its atoms.

The next clue to explaining the Sunís awesome energy came from measurements in the laboratory on Earth. They showed that the helium nucleus is slightly less massive (by a mere 0.7 percent) than the sum of the mass of the four hydrogen nuclei, or four protons, that combine to make it. So, what you get out in making helium is less than what you put into it, like the usual outcome of a slot machine or other type of gambling. The part that disappears goes into energizing the Sun and other stars. The mass difference, ?m, is converted into energy, ?E, to power the Sun, all in accordance with Einsteinís equation ?E = ?m c2.

Hydrogen Burning

The Sun and most other stars are mainly composed of hydrogen, and that is the material that must energize them. If these stars were only made of heavy elements, instead of hydrogen, they could not shine. To understand how hydrogen is burned within the central furnace of stars, we need to know about the sub-atomic constituents of matter. The most familiar sub-atomic particles, the particles that make up atoms, are protons, neutrons and electrons. The nucleus of a hydrogen atom consists of a single proton, and the nucleus of any other atom contains protons and neutrons, collectively called nucleons. The nucleus of the helium atom, for example, has four nucleons - two neutrons and two protons.

Our understanding of nuclear reactions additionally required knowledge of the positron and the neutrino. The positron is the positive electron, or the anti-matter version of the electron, with the same mass and a reversed charge. The insubstantial neutrino has no charge and very little mass; it moves very fast at nearly the velocity of light. There are three types of neutrinos, and the kind of neutrino that is made inside the Sun is called an electron neutrino.

Fig. .. 

It wasnít until the late 1930s that Hans A. Bethe and Charles L. Critchfield demonstrated how a sequence of nuclear reactions makes the Sun shine. Bethe was awarded the Nobel Prize in Physics in 1967 for this and other discoveries concerning energy production in stars. The hydrogen fusion reactions that fuel the Sun are collectively called the proton-proton chain (Fig. 2.2). They are also known as hydrogen-burning reactions, for it is hydrogen nuclei, the protons, that are being consumed to make helium; but it is a chain of nuclear reactions and not combustion in the ordinary chemical sense.

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Copyright 2010, Professor Kenneth R. Lang, Tufts University