How to Explain Nuclear Fusion in Our Sun

In the late 19th century, a debate broke out among scientists over the age of the sun and its expected lifetime. Physicists of the day could not imagine any process besides gravitational contraction or chemical reactions that would give rise to the sun's energy --- and yet those processes would give a very short age for the sun, somewhere in the tens of millions of years. Geologists already knew the Earth must be older than that. It wasn't until the discovery of nuclear reactions in the 20th century that the puzzle was solved. Here's how to explain the process of nuclear fusion in the sun's core to an audience.

Instructions

1

Remind your audience that all matter is formed of subunits called atoms. An atom consists of a tiny, extremely dense nucleus containing protons and neutrons; the protons have positive charge and the neutrons have no charge. The nucleus is encircled by a large electron cloud.

2

Use a metaphor to explain the relative sizes of particles in an atom. If an atom were the size of a baseball stadium, the nucleus would be about the size of a fly landing on the pitcher's head. Most of the space in an atom is occupied by the electron cloud.

3

Explain what happens as material heats up. First a solid will melt into a liquid. At still higher temperatures, the liquid will boil and become a gas. If you continue to heat the gas to extremely high temperatures --- temperatures like those inside the sun --- atoms in it will become ionized and lose their electrons, forming a plasma.

4

Point out that nuclei have positive charge and therefore ordinarily repel each other. In order for them to overcome this repulsion and collide, very high temperature and pressure is needed. During the early years of our sun's life, gravitational contraction provided the heat needed to kickstart a nuclear fusion process.

5

Explain that at very high temperatures, the hydrogen nuclei in a plasma are traveling so fast they can sometimes collide despite the repulsion they experience since they are both positively charged. Once they collide, the strong interaction or strong nuclear force (the force that holds nuclei together) keeps them stuck together to form a new and larger nucleus. The new nucleus has a slightly smaller mass than the old, however, meaning that some of the mass has been converted into energy.

6

Remind your audience of Einstein's famous equation, E = mc-squared, where c is the speed of light, which is a very large number. Since m is multiplied by c squared, this equation implies that mass can be converted to energy, and when this conversion takes place, a very small amount of mass becomes a very large amount of energy.

7

Pause briefly for dramatic effect, then break down the process of fusion in the sun into the actual steps that take place. The sun is more than 71 percent hydrogen by mass, with another 27 percent being helium. The fusion reactions in the sun's core use hydrogen as fuel. A hydrogen nucleus is just a lone proton with no neutrons attached.

8

Tell your audience that the process begins when two hydrogen nuclei fuse and produce a nucleus of deuterium, a heavier isotope of hydrogen that has one neutron and one proton. As one of the two protons changes into a neutron, a neutrino and a positron are ejected in the process. A positron is the antimatter counterpart of an electron, and when it collides with an electron they annihilate each other, converting mass into energy. A neutrino is an elementary particle that has no charge, interacts weakly with other matter and has a very tiny mass.

9

Explain that a deuterium nucleus can in turn fuse with another hydrogen to form a light helium nucleus with one neutron and two protons. The slight difference in mass between the independent deuterium and hydrogen nuclei and the resulting helium nucleus is converted to energy in the form of electromagnetic radiation, more specifically a gamma ray.

10

Explain that two of the light helium nuclei formed through this process can now fuse to become a nucleus of helium-4, which has two protons and two neutrons, ejecting two protons (two hydrogen nuclei) in the process. The whole series of events is called the "p-p chain reaction," and it's the process that heats the core of the sun so it shines and provides energy for us here on Earth.