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A Public Outreach Module:
Sunlight and Solar Heat: How Are They Made?

The Sun is the Source of Food Energy

The Sun is the Source of Food Energy
Courtesy: McREL

The sun is the major external source of the energy, in the form of heat and light, needed to make the Earth's processes work. The sun's light provides energy for most life forms. Plants use sunlight, water, and minerals they collect from the soil to form foodstuffs for themselves and for animals. We eat plants and animals for food, ultimately tracing the food energy back to sunlight. The sun's heat on the Earth's surface and atmosphere provides the energy to move the atmosphere and oceans, producing winds, ocean currents, and the water cycle. The sun's heat and light maintain the Earth's temperature. On a global scale, climate is determined by the sun's energy affecting materials such as soil, rocks, and water at and near the Earth's surface.

water cycle

The sun's heat and light energy are constantly radiated from its surface in all directions. Early people who attempted to relate observations of the sun with everyday experiences interpreted the sun as a ball of fire or a hot object. Now, scientists often describe the sun as an "invisible fire," referring to its chemical and physical processes. How does the sun produce heat and light in quantities sufficient to be sent into space in all directions and still affect the Earth 93,000,000 miles away?

the Sun
Courtesy: NASA

The sun, like all stars, contains mostly hydrogen. It produces energy from nuclear reactions. In most common chemical reactions, the outer parts of whole atoms, the electrons, interact. Nuclear reactions are a type of chemical reaction in which the nuclei of atoms interact, rather than the electron-bearing surfaces of intact atoms. Interaction at the nuclear level requires huge amounts of energy to initiate the reaction, but at the same time releases unbelievable amounts of energy during the reaction. In stars, the nuclear reactions are primarily the fusion of hydrogen nuclei to form helium nuclei.

The temperature in the center of the sun is an incredible 15,000,000 degrees (measured on the Kelvin scale). At that temperature the hydrogen atoms are ripped apart, resulting in a mixture of hydrogen nuclei and negatively-charged electrons that are no longer part of an atom. A hydrogen nucleus is a positively-charged particle called a proton. A helium nucleus is a positively-charged group of four particles, two protons and two neutrons. Where did those neutrons in the helium nucleus come from? They are formed by the fusion of two protons. The chemical equation for this fusion reaction looks like this:

proton + proton = deuteron + positron + neutrino

formation of Deuteron

The protons are the hydrogen nuclei that make up 91% of the atoms in the sun. The deuteron is a two-part object combining a proton with a neutron. The neutron was formed from the mass of one of the original protons, which was destroyed by the reaction. The positive charge of the destroyed proton is carried on the positron. The momentum of the escaping positron is balanced by momentum of the uncharged neutrino.

It takes tremendous amounts of energy for protons (hydrogen nuclei) to combine and change into neutrons. This energy is supplied in the cores of stars by extreme heat and high pressure.

Formation of a deuteron is the first step in a star's creation of helium from hydrogen. In the next step, a proton collides with a deuteron inside the sun to form a helium-3 nucleus. This helium-3 nucleus is not normal helium, since it has only two protons and one neutron. The chemical equation for the second step in the sun's fusion reaction is:

proton + deuteron = helium-3 nucleus

Formation of Helium-3
Formation of Helium-3.       Courtesy: McREL

The third and final step is for two helium-3 nuclei to collide and form a normal helium-4 nucleus and release two extra protons. The chemical equation for this step is:

helium-3 nucleus + helium-3 nucleus = helium-4 nucleus + 2 protons

Formation of Helium-4
Formation of Helium-4.      Courtesy: McREL

Of course, given the tremendous temperatures inside the sun, the helium-4 nuclei will collide with protons and with each other, creating still other types of chemical elements. These fusion reactions and other similar processes in stars have led to the formation of all the other natural chemical elements.

The Sun Provides Heat and Light
The Sun Provides Heat and Light

In the process of undergoing these nuclear fusion reactions, the sun emits large amounts of heat and light. This energy is a result of the positively-charged positrons smashing into the loose electrons, and annihilating each other. According to Einstein's famous equation, the total mass of both positrons and electrons is destroyed, turning into energy. This is the energy the Earth receives as heat and light.

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