10.2 What Powers the Sun

The Sun

A photograph shows the sun low in the sky at sunset.
Figure 10.2. It takes an incredible amount of energy for the Sun to shine, as it has and will continue to do for billions of years.
Sun and Fog by Ed Dunens, CC-BY-4.0

The Sun puts out an incomprehensible amount of energy—so much that its ultraviolet radiation can cause sunburns from 93 million miles away. It is also very old. As you learned earlier, evidence shows that the Sun formed about 4.5 billion years ago and has been shining ever since. How can the Sun produce so much energy for so long?

The Sun’s energy output is about 4 × 1026 watts. This is unimaginably bright: brighter than a trillion cities together each with a trillion 100-watt light bulbs. Most known methods of generating energy fall far short of the capacity of the Sun. The total amount of energy produced over the entire life of the Sun is staggering, since the Sun has been shining for billions of years. Scientists were unable to explain the seemingly unlimited energy of stars like the Sun prior to the twentieth century.

When striving to understand how the Sun can put out so much energy for so long, scientists considered many different types of energy. Nineteenth-century scientists knew of two possible sources for the Sun’s energy: chemical and gravitational energy. The source of chemical energy most familiar to them was the burning (the chemical term is oxidation) of wood, coal, gasoline, or other fuel. We know exactly how much energy the burning of these materials can produce. We can thus calculate that even if the immense mass of the Sun consisted of a burnable material like coal or wood, our star could not produce energy at its present rate for more than few thousand years. However, we know from geologic evidence that water was present on Earth’s surface nearly 4 billion years ago, so the Sun must have been shining brightly (and making Earth warm) at least as long as that. Today, we also know that at the temperatures found in the Sun, nothing like solid wood or coal could survive.

It was only in the twentieth century that the true source of the Sun’s energy was identified. The two key pieces of information required to solve the puzzle were the structure of the nucleus of the atom and the fact that mass can be converted into energy.

The Sun, then, taps the energy contained in the nuclei of atoms through nuclear fusion. Let’s look at what happens in more detail. Deep inside the Sun, a three-step process takes four hydrogen nuclei and fuses them together to form a single helium nucleus. The helium nucleus is slightly less massive than the four hydrogen nuclei that combine to form it, and that mass is converted into energy.

The initial step required to form one helium nucleus from four hydrogen nuclei is shown in Figure 10.3. At the high temperatures inside the Sun’s core, two protons combine to make a deuterium nucleus, which is an isotope (or version) of hydrogen that contains one proton and one neutron. In effect, one of the original protons has been converted into a neutron in the fusion reaction. Electric charge has to be conserved in nuclear reactions, and it is conserved in this one. A positron (antimatter electron) emerges from the reaction and carries away the positive charge originally associated with one of the protons.

Proton-Proton Chain, Step 1

Diagram of the First Step in the Proton-Proton Chain. At left are shown two protons drawn in blue, and labelled as “1H”. An arrow is drawn from each proton toward the right and converge at an illustration of a small explosion. This explosion represents the release of energy and mass from the collision of the high energy protons. Three arrows are drawn moving away from the explosion toward the right. At the point of the topmost arrow is a neutrino drawn in light blue and labelled “Neutrino”. At the point of the central arrow is a deuterium nucleus drawn as a blue dot (proton) and a red dot (neutron) and labelled “2H”. At the point of the lower arrow is a positron drawn in purple and labelled “Positron”.
Figure 10.3. This is the first step in the process of fusing hydrogen into helium in the Sun. High temperatures are required because this reaction starts with two hydrogen nuclei, which are protons (shown in blue at left) that must overcome electrical repulsion to combine, forming a hydrogen nucleus with a proton and a neutron (shown in red). Note that hydrogen containing one proton and one neutron is given its own name: deuterium. Also produced in this reaction are a positron, which is an antielectron, and an elusive particle named the neutrino.

Since it is antimatter, this positron will instantly collide with a nearby electron, and both will be annihilated, producing electromagnetic energy in the form of gamma-ray photons. This gamma ray, which has been created in the centre of the Sun, finds itself in a world crammed full of fast-moving nuclei and electrons. The gamma ray collides with particles of matter and transfers its energy to one of them. The particle later emits another gamma-ray photon, but often the emitted photon has a bit less energy than the one that was absorbed.

Such interactions happen to gamma rays again and again and again as they make their way slowly toward the outer layers of the Sun, until their energy becomes so reduced that they are no longer gamma rays but X-rays. Later, as the photons lose still more energy through collisions in the crowded centre of the Sun, they become ultraviolet photons.

By the time they reach the Sun’s surface, most of the photons have given up enough energy to be ordinary light—and they are the sunlight we see coming from our star. (To be precise, each gamma-ray photon is ultimately converted into many separate lower-energy photons of sunlight.) So, the sunlight given off by the Sun today had its origin as a gamma ray produced by nuclear reactions deep in the Sun’s core. The length of time that photons require to reach the surface depends on how far a photon on average travels between collisions, and the travel time depends on what model of the complicated solar interior we accept. Estimates are somewhat uncertain but indicate that the emission of energy from the surface of the Sun can lag its production in the interior by 100,000 years to as much as 1,000,000 years.

In addition to the positron, the fusion of two hydrogen atoms to form deuterium results in the emission of a neutrino. Because neutrinos interact so little with ordinary matter, those produced by fusion reactions near the centre of the Sun travel directly to the Sun’s surface and then out into space, in all directions. Neutrinos move at nearly the speed of light, and they escape the Sun about two seconds after they are created.

Proton-Proton Chain, Step 2

Diagram of the Second Step in the Proton-Proton Chain. At upper left is the deuterium nucleus from the first step drawn as a blue dot (proton) and a red dot (neutron) and labelled “2H”. At lower left is a proton drawn in blue, and labelled as “1H”. An arrow is drawn from each object toward the right and converge at an illustration of a small explosion. This explosion represents the release of energy and mass from the collision of the deuterium and proton. A single arrow is drawn emerging from the explosion pointing toward the right. At the point of the arrow is an isotope of helium, known as deuterium, drawn as 2 blue dots (protons) and 1 red dot (neutron) and labelled “3He”. A single wavy arrow is drawn moving away from the deuterium nucleus and is labelled “Gamma ray”.
Figure 10.4. This is the second step of the proton-proton chain, the fusion reaction that converts hydrogen into helium in the Sun. This step combines one hydrogen nucleus, which is a proton (shown in blue), with the deuterium nucleus from the previous step (shown as a red and blue particle). The product of this is an isotope of helium with two protons (blue) and one neutron (red) and energy in the form of gamma-ray radiation.

The second step in forming helium from hydrogen is to add another proton to the deuterium nucleus to create a helium nucleus that contains two protons and one neutron as illustrated in Figure 10.4. In the process, some mass is again lost and more gamma radiation is emitted. Such a nucleus is helium because an element is defined by its number of protons; any nucleus with two protons is called helium. But this form of helium, which we call helium-3 (and write in shorthand as 3He) is not the isotope we see in the Sun’s atmosphere or on Earth. That helium has two neutrons and two protons and hence is called helium-4 (4He).

To get to helium-4 in the Sun, helium-3 must combine with another helium-3 in the third step of fusion as illustrated in Figure 10.5. Note that two energetic protons are left over from this step; each of them comes out of the reaction ready to collide with other protons and to start step 1 in the chain of reactions all over again.

Proton-Proton Chain, Step 3

Diagram of the Third Step in the Proton-Proton Chain. At left are two deuterium nuclei each drawn as 2 blue dots (protons) and 1 red dot (neutron), and labelled “3He”. An arrow is drawn from each nucleus toward the right and converge at an illustration of a small explosion. This explosion represents the release of energy and mass from the collision of the deuterium nuclei. Three arrows are drawn moving away from the explosion toward the right. At the point of the topmost arrow is a proton drawn in blue and labelled “1H”. At the point of the center arrow a helium nucleus is drawn as two blue dots (protons) and two red dots (neutrons) and labelled “4He”. At the point of the lower arrow is a proton drawn in blue and labelled “1H”.
Figure 10.5. This is the third step in the fusion of hydrogen into helium in the Sun. Note that the two helium-3 nuclei from the second step must combine before the third step becomes possible. The two protons that come out of this step have the energy to collide with other protons in the Sun and start step one again.

Attribution

“16.0 Thinking Ahead“, “16.1 Sources of Sunshine: Thermal and Gravitational Energy“, and  “16.2 Mass, Energy, and the Theory of Relativity” from Douglas College Astronomy 1105 by Douglas College Department of Physics and Astronomy, is licensed under a Creative Commons Attribution 4.0 International License, except where otherwise noted. Adapted from Astronomy 2e.

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Fanshawe College Astronomy Copyright © 2023 by Dr. Iftekhar Haque is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License, except where otherwise noted.