Solar Core

The energy from the Sun comes from nuclear reactions in its core, generating a luminosity of 4 x 1026 W, or 2 x 104 W/kg. The Sun has been producing approximately this vast amount of energy for the past 4.5 billion years. For the entire lifetime of the Sun, the total amount of energy generated has been 3 x 1013 joule/kg. The generation of this energy has been remarkably steady and is expected to continue for another 5 billion years, through the process of nuclear fusion.

Fusion works by combining two atomic nuclei to form one and releases energy according to the law of conservation of mass and energy. During fusion reactions, mass is lost but is converted to energy according to Einstein's equation

where E = energy, m = mass, and c = the speed of light.

During nuclear fusion the total amount of mass and energy is conserved, but the mass is gradually converted to energy, which is the energy that has been emitted from the Sun for the past 4.5 billion years. Nuclear energy is generated by fusion along a reaction series called the proton-proton chain. Positively charged atomic nuclei naturally repel each other, but if they collide at very high speeds (such as generated by high temperatures in the core of the Sun) then they can overcome the repulsive forces between the nuclei and then be influenced by a different force called the strong nuclear force. This force, which acts at distances smaller than 10-15 m, can bind the two positively charged nuclei together, releasing energy in the process. Temperatures in excess of 107 K are needed to generate the speeds that cause this nuclear fusion.

When the two nuclei of hydrogen (protons) interact in the fusion reaction, they produce a new proton, a neutron, a positron, and a neutrino. The positron has the same properties as an electron except that it has a positive charge, and it is classified as an anti-particle of an electron, or as antimatter. When the fusion reaction occurs and the positron is formed, it is immediately released into a sea of free electrons in the solar core. When matter and antimatter particles meet they violently annihilate each other and release energy in the form of gamma rays.

The other particle released in the fusion reaction is a neutrino, a particle with no charge and a mass so low that it is approximately equal to about 1/10,000 that of an electron. They move at nearly the speed of light and are nearly (but not quite) impossible to detect since they can penetrate anything, even a wall of lead several light-years thick.

The proton and neutron produced in the fusion reaction merge to form a deuteron, which is the nucleus of deuterium or heavy hydrogen. It is called "heavy" since it has an extra neutron relative to the most common form of hydrogen, which lacks a neutron. Nuclei that have the same number of protons but different numbers of neutrons are known as isotopes of the same element, and deuterium is an isotope of hydrogen. The isotopic number is written as a prefix to the element symbol in standard notation. Thus, the hydrogen fusion reaction that powers the sun can be written as

This reaction is only the first in the proton-proton chain that powers the sun, followed quickly by the formation of helium (He) by the interaction of deuterium with an isotope of helium according to the following equation:

Next, helium 4 is produced by the fusion of two helium 3 isotopes according to the following equation:

3He + 3He ^ 4He + 1H + 1H + energy (gamma ray photons)

The net effect of the proton-proton reaction chain is that four hydrogen nuclei are fused into one helium 4 isotope plus two neutrinos, releasing a large amount of energy in the form of gamma rays. As the gamma ray photons move through the sun they slowly lose energy as it is absorbed by ions and electrons, getting converted to heat by the convecting layer and then is emitted at the photosphere in the form of visible light that is observed from Earth. The helium remains in the core of the sun, and the neutrinos escape at close to the speed of light.

Calculations of the mass to energy conversion show that about 600 million tons of hydrogen have been fused into helium in the core of the sun every second for the past several billion years.

See also astronomy; astrophysics; aurora, Aurora Borealis, Aurora Australis; cosmic rays; greenhouse effect; origin and evolution of the Earth and solar system; solar system; star formation; stellar evolution; sun halos, sundogs, and sun pillars.


Chaisson, Eric, and steve McMillan. Astronomy Today. 6th ed. upper saddle River, N.J.: Addison-Wesley, 2007.

Comins, Neil F. Discovering the Universe. 8th ed. New

York: W. H. Freeman, 2008. NAsA. Imagine the universe! The sun. Available online. uRL: l1/sun.html. modified August 22, 2008.

NAsA. Worldbook, sun. Available online. uRL: http:// modified November 29, 2007. scienceDaily: Astrophysics News. scienceDaily LLC. Available online. uRL: space_time/astrophysics/. Accessed october 24, 2008. snow, Theodore P. Essentials of the Dynamic Universe: An Introduction to Astronomy. 4th ed. st. Paul, minn.: West Publishing Company, 1991.

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