Scale of the Sun's Energy
   In just one second the Sun emits more energy than humans have used in the last 10,000 years. The Sun has been shining relatively steadily for 4.6 billion years. Until the early 20th century, humans did not know of any process that could explain the energy production of the Sun. Even if a fire, such as those that occur on Earth, were as large as the Sun, the fire would consume the mass of the Sun in a few thousand years.

   Scientists now know that the Sun is mainly composed of hydrogen, the lightest and most abundant element in the universe. The Sun contains an enormous amount of hydrogen, however, which makes the Sun very massive. All matter inside the Sun is gravitationally attracted to all the other matter in the Sun, and this attraction tends to pull the Sun's mass together. This inward pull creates high pressures and temperatures inside the Sun.

   The center is so violent and hot that collisions between atoms break the hydrogen atoms apart into their subatomic ingredients. A hydrogen atom is made up of a nucleus that contains a positively charged proton, and a negatively charged electron that orbits the nucleus. In the Sun, collisions separate the electron from the nucleus, freeing each to move about the solar interior. The positively charged nuclei, or protons, are called ions. A gas in which particles are ionized, or have electric charges, is called plasma. Scientists often consider plasma, such as the material inside the Sun, to be a fourth state of matter—the three more familiar states of matter are gas, liquid, and solid.

Nuclear Fusion in the Core
   The separation of hydrogen nuclei from their electrons makes nuclear fusion possible at the Sun's core, producing the Sun's light and heat. With their electrons gone, hydrogen nuclei (protons) can be packed much more tightly than complete atoms. At great depths inside the Sun, the pressure of overlying material is enormous, the protons are squeezed tightly together, and the material is very hot and densely concentrated. At the Sun's center, the temperature is 15.6 million degrees C (28.1 million degrees F), and the density is more than 13 times that of solid lead. This is hot and dense enough to make the nuclei fuse together. Outside the solar core, where the overlying weight and compression are less, the gas is cooler and thinner, and nuclear fusion cannot occur.

   The nuclear fusion reaction that powers the Sun involves four protons that fuse together to make one nucleus of helium. Two of the original protons become neutrons (electrically neutral particles about the same size as protons). The result is a helium nucleus, containing two protons and two neutrons. The helium nucleus is slightly less massive (by a mere 0.7 percent) than the four protons that combine to make it. The fusion reaction turns the missing mass into energy, and this energy powers the Sun.

   The relationship between energy and the missing matter was explained in 1905 by a German-born American physicist Albert Einstein. The mass loss, m, during the transformation of four protons into one helium nucleus, supplies an energy, E, according to the relation E = mc², where c is the speed of light. The speed of light is a constant number equal 3 × 10⁸ m/s (1 × 10⁹ ft/s).

   Every second, fusion reactions convert about 700 million metric tons of hydrogen into helium within the Sun's energy-generating core. In doing so, about 5 million metric tons of this matter become energy. This energy leaves the Sun as radiation, and the part of this radiation that constitutes visible light is what makes the Sun shine.

   The rate of nuclear reactions in the Sun is relatively low, because protons repel each other. This repulsion often prevents them from getting close enough to each other to fuse. Protons push each other away because they have the same electrical charge. The particles must overcome this repulsion in order to fuse together. Only a tiny fraction of the protons inside the Sun are moving fast enough to overpower this repulsive electrical force. The nuclei that are moving fast enough can get very close together, and a force called the strong nuclear force takes over. The strong nuclear force is, as its name implies, very powerful, but only over very short distances. It pulls the nuclei together and holds them together. In this way, nuclear reactions proceed at a relatively slow pace inside the Sun. If the pace were much quicker, the Sun would explode like a giant hydrogen bomb.

The Proton-Proton Chain

   Four protons do not combine directly to form a helium nucleus, since the protons are constantly moving and are almost never in the same place at the same time. Moreover, the electrical repulsion between four protons is too great to overcome, even if the four protons happen to come together at an appropriate speed at the same time. Instead, the protons come together in a series of steps to form a helium nucleus, and these steps are called the proton-proton chain.

   In the first step of the proton-proton chain, two exceptionally fast protons meet head on and merge into each other, tunneling through the electrical barrier between them. The two protons combine, with most of their mass forming a deuteron, the nucleus of a heavy form of hydrogen known as deuterium. A deuteron contains one proton and one neutron, so one of the protons must become a neutron in this step. The conversion of a proton to a neutron releases a much smaller particle called a neutrino. There are several types of neutrinos—the type that the proton-proton chain produces is called an electron neutrino. The reaction also creates a positron, a positively charged particle the size of an electron. The symbolic representation of the first step of the proton-proton chain is

p + p ²D + e⁺ + _e

where p represents the protons, ²D represents deuterium, e⁺ represents the positron, and _e represents the electron neutrino.

   In the second step of the chain, the deuteron collides with another proton to form a nucleus of light helium, which has two protons and one neutron. Less energy is needed to maintain a light helium nucleus than is needed to maintain a deuteron and a proton separately. The extra energy is released as a photon, or a packet of light energy. In symbolic terms, the second step is

²D + p ³He +

where ³He is light helium and represents a photon.

   In the final step of the proton-proton chain, two light helium nuclei meet and fuse together to form a nucleus of normal heavy helium, which has two protons and two neutrons. This reaction also releases two unattached hydrogen nuclei that return to the solar gas. In symbolic terms, the third step is

³He + ³He ⁴He + 2p

where ⁴He represents a normal helium nucleus with two protons and two neutrons.

   The positron created in the first step of the chain eventually collides with a free electron. The positron and the electron are opposite particles—the positron is the antimatter equivalent of the electron. When the positron and the electron collide, they annihilate each other, releasing energy. The electron and the positron disappear, their mass transformed into two photons:

e⁺ + e- 2

where e- represents the electron. The net result of the proton-proton chain is the transformation of four hydrogen nuclei into a helium nucleus (with two protons and two neutrons), two neutrinos, and six photons:

4p ⁴He + 2_e + 6.

Solar Neutrinos

   The conversion of two protons into two neutrons in the proton-proton chain produces two tiny, elusive, fast-moving neutral particles called neutrinos. Nuclear reactions in the Sun's central furnace create prodigious quantities of neutrinos. Every second the Sun releases 2 × 10³⁸ neutrinos, and every second an estimated 70 billion of these solar neutrinos pass through every square centimeter of Earth that is facing the Sun.

   Neutrinos move at the velocity of light, have no electrical charge, and have so little mass that scientists are not sure that neutrinos have any mass at all. The ghostlike neutrinos therefore travel almost unimpeded through the Sun, Earth, and nearly any amount of matter. Scientists can snag small numbers of neutrinos in massive underground detectors called neutrino telescopes. These telescopes are placed so deep underground that only neutrinos can reach them. Scientists using these telescopes have detected solar neutrinos, confirming that the Sun is indeed powered by nuclear fusion.

   The number of neutrinos detected by these telescopes, however, is only one-third to one-half of the total number of neutrinos predicted to exist by the theory of solar neutrino production. This discrepancy between the number of detected neutrinos and the number predicted is known as the solar neutrino problem. There are two possible explanations—scientists might not understand exactly how the Sun produces its energy, or they could have an incomplete knowledge of neutrinos.

   Astronomers are convinced that their models of the Sun are correct and that their predictions for the expected amount of solar neutrinos are therefore correct. Studies of the interior of the Sun substantiate the current models of how the Sun produces its energy, so most scientists agree that the problem lies in their understanding of neutrinos.

   One theory posed by scientists to explain the problem is that neutrinos from the Sun may change on the way to Earth. Scientists know of at least three types of neutrinos. Nuclear fusion reactions in the Sun produce a type of neutrino called an electron neutrino. The other two proven types of neutrinos are called muon neutrinos and tau neutrinos. Most neutrino telescopes, especially those devoted to solar research, can only detect electron neutrinos. In the 1990s studies of muon neutrinos (produced by reactions between particles called cosmic rays and Earth's atmosphere) showed that muon neutrinos might change into tau neutrinos. Scientists believe that electron neutrinos from the Sun may also change into another type of neutrino. This change would mean the electron neutrino detectors miss many of the Sun's neutrinos.

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