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16.2 Mass, energy, and the theory of relativity  (Page 6/13)

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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 labeled 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 labeled “Neutrino”. At the point of the central arrow is a deuterium nucleus drawn as a blue dot (proton) and a red dot (neutron) and labeled “2H”. At the point of the lower arrow is a positron drawn in purple and labeled “Positron”.
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 center 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 (recall what you learned in The Electromagnetic Spectrum ). Later, as the photons lose still more energy through collisions in the crowded center 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 center 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 labeled “2H”. At lower left is a proton drawn in blue, and labeled 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 labeled “3He”. A single wavy arrow is drawn moving away from the deuterium nucleus and is labeled “Gamma ray”.
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.
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Source:  OpenStax, Astronomy. OpenStax CNX. Apr 12, 2017 Download for free at http://cnx.org/content/col11992/1.13
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