Chapter 16. Fusion

16.1 Introduction

Visual image of the Sun at sunset

The goals of this module: After completing this exercise, you should be able to:

1. Explain why thermonuclear fusion is the only known energy source capable of sustaining a star over its entire lifetime.
2. Describe the steps of the main hydrogen fusion reaction that occurs inside the Sun.
3. Describe the steps of the main hydrogen fusion reaction that occurs inside the Sun.

In this module you will explore:

1. How traditional energy sources fail to support the Sun over its 10 billion year lifetime.
2. What happens during the proton-proton chain.
3. The conditions needed for fusion to occur inside the Sun.

Why you are doing it: The ability to generate its own energy is the distinguishing characteristic of stars. Understanding how a star is able to create enough energy to sustain itself over its lifetime is fundamental to understanding not just our Sun, but all of the stars in the Universe.

16.2 Background

The Inner Regions of the Sun: the site of energy production. Courtesy of SOHO/EIT consortium. SOHO is a project of international cooperation between ESA and NASA.

The Sun is the only object in the Solar System which produces a significant amount of energy; the light we see from every other object in the Solar System is simply reflected sunlight. The Sun's energy both warms and lights the Earth; it is what allows life here to flourish. The Sun's high temperature (about 5800 °K at its surface) and large surface area allow it to radiate an enormous amount of energy every second - nearly 400 trillion trillion joules each second. (A Joule of energy per second is known as a watt, so the Sun is radiating as much energy as 4 trillion trillion 100 watt light bulbs each second!) The amount of energy output per second by the Sun is called its luminosity.

Our Sun has been emitting this much luminosity for approximately 4.6 billion years, and astronomers believe it will continue to do so for about another 5 billion years. The most remarkable thing is that the Sun will continuously produce light at basically the same rate over this entire 10 billion year lifetime! That constitutes an amazing amount of total energy production!

In the first part of this activity, you will explore what energy sources are capable of sustaining a star for billions of years. In the second part, you will examine that energy source in detail.

Let's first consider some possible mechanisms for the Sun's energy generation.

16.3 Chemical Energy as a Source of Energy

The first possible energy source considered is energy released due to chemical burning.

Perhaps the Sun is a ball of gases burning in the way a log in a fire does. To determine if this is a viable energy source for the Sun, we need to figure out how much energy can be extracted via chemical burning, and whether this process can sustain the Sun for its 10 billion year lifetime.

Click on START to continue.

16.4 Gravitational Contraction as a Source of Energy

A second possible solar energy source is energy released due to gravitational contraction.

Kelvin-Helmholtz contration

When any object contracts, it generates heat. This is called Kelvin-Helmholtz contraction. This process is known to occur during the early stages of star formation, so we know that this was an important process for the Sun during its early stages.

Can contraction heat the solar interior enough that it will radiate energy into space at the current rate for its entire lifetime?

Watch the animation above to see how long heat could be produced by contraction of the Sun.

16.5 Thermonuclear Fusion as a Source of Energy

During the 1920s, astronomers proposed a third possible energy source, thermonuclear fusion, also commonly called nuclear burning (but notice that this is a misnomer since nothing actually burns in fusion!). Fusion is a reaction in which the nuclei of two atoms merge together to form a larger nucleus. In fusion reactions, the mass of the larger nucleus is less than the sum of the masses of the original nuclei. In the Sun, fusion combines four hydrogen nuclei in a 3-step process to form a helium nucleus.

Albert Einstein discovered that the "missing" mass is converted into energy during the fusion reaction according to the equation E=mc². In this equation, 'm' is the mass that is converted during the reaction, 'c' is the speed of light, and 'E' is the energy produced by the reaction. Because the speed of light is a very large number, even a very small amount of mass can create a large amount of energy!

Let's do another calculation to see how much energy we're talking about. Click on "Continue".

100 billion years is a very long time. However, fusion can convert mass into energy only in the solar core, so hydrogen fusion will be able to power the Sun for about 10 billion years. Nevertheless, fusion is a very efficient means of generating energy.

16.6 Thermonuclear Fusion Process - Step 1

Now that we have established that thermonuclear fusion is the process that fuels our Sun, let's examine how that process occurs.

Fusion in the Sun occurs via the proton-proton chain, which consists of three steps. In each step, energy is released in the form of (high energy) gamma ray photons. It is this energy that heats the interior of the Sun and allows it to shine.

In step one, two hydrogen nuclei collide and combine. Click on "step 1" in the animation below to see what happens.

Fusion in the Sun

The neutrino and the positron (or positively charged electron) are both produced when a proton changes into a neutron. The proton and neutron together form a hydrogen isotope, called ²H, or "heavy hydrogen". The energy released in this step is produced when the electron and positron collide and annihilate.

16.7 Thermonuclear Fusion Process - Step 2

In step two, the ²H nucleus produced in step one collides and combines with a third proton.

Fusion in the Sun

Click on "step 2" in the animation above to watch what happens next.

The collision in step two produces a new nucleus with two protons and one neutron, an isotope of helium. It also releases more energy. The new nucleus is Helium since there are two protons present. It's referred to as Helium-3 (³He) because there are three particles in this nucleus.

16.8 Thermonuclear Fusion Process - Step 3

In step three, two ³He nuclei collide and combine.

Fusion in the Sun

Click on "step 3" in the animation above to see the end result.

The energy produced in this step is from the motions of the two protons, which are released. Overall, we started with six protons, three to make each of the two ³He nuclei. In the last step, we got two protons back, so the whole reaction that happened in three steps can be summarized as:

The energy produced in the fusion reaction is released as gamma ray photons. After many, many interactions these energetic photons are gradually transformed into longer wavelength, less energetic visible light by the time the energy works it way very slowly to the surface of the Sun. Because this energy is trapped inside the Sun for so long, the internal temperature is maintained, allowing fusion to continue in the core. Once the energy reaches the surface, it is radiated away as the light and heat that we receive from the Sun.

In the animation of the proton-proton chain, step two ended with the production of a single ³He nucleus, but step three began with two ³He nuclei colliding. Where did the second ³He nucleus come from?

The answer is that the second nucleus was the result of fusion of a different set of protons in the core. It takes two different sets of protons, each proceeding through steps one and two separately, to produce the two helium isotopes needed for step three.

16.9 Where Does the Fusion Occur?

Now that we know how fusion occurs, we can ask ourselves where fusion occurs.

Look carefully at the panel that compares the luminosity of the Sun and distance from the core.

16.10 Why Does Fusion Require Such High Densities and Temperatures?

Why does fusion require such high densities and temperatures?
To answer this, first recall the initial step of the proton-proton cycle. In the first step, you take two protons and fuse them together to form a heavy hydrogen nucleus (²H). But how do you get the protons to collide in the first place? You may remember that according to electromagnetism, like charges repel each other.

At room temperature, the protons repel each other before they can get close enough to fuse.

As the temperature increases to 10 million Kelvin, the protons get closer together and fusion is favored over repulsion. Temperature measures the energy associated with the motion of particles; this means that the higher the temperature of a gas, the faster the gas particles are moving. At higher and higher speeds, the protons can get closer together before they repel each other. At high enough temperatures, the proton speeds allow them to get extremely close. When protons are within 10^-15 meters as shown in the first step in the fusion process, a force stronger than electromagnetic repulsion binds them together. This force is appropriately known as the strong nuclear force. The strong nuclear force allows fusion to occur. Once a new nucleus forms, the strong nuclear force keeps it intact.

16.11 Quick Check Quiz

Indepth Activity: Fusion