Читать книгу Exploring the Solar System - Peter Bond - Страница 41

Until the early 20^th century, the source of the Sun's heat and light was unknown. Although the extreme conditions in the Sun's core were recognized by 1870, there was still no known physical process that could account for the solar furnace. For example, heat from the gravitational collapse of the Sun would only be sufficient to keep it shining for some 15 million years.

The explanation eventually came after a series of breakthroughs at the Cavendish Laboratory in Cambridge, England, and elsewhere. In 1905, Albert Einstein published his famous equation E = mc², which basically states that a small quantity of mass is equivalent to a very large amount of energy. The release of energy is always associated with a reduction in mass.

By 1911, Ernest Rutherford had shown that the atom comprised a tiny nucleus which was surrounded by particles called electrons. The nucleus itself was eventually found to be composed of other particles – protons and neutrons.

Nuclear reactions not only involved shifts in proton–neutron combinations, which released much greater amounts of energy than any chemical reactions, but they were also the cause of radioactivity – particles which are emitted from nuclei as a result of nuclear instability.

By the 1920s, Francis Aston had discovered isotopes, elements with the same chemical properties but different atomic masses. The isotopes resulted from the presence of different numbers of neutrons in an element's nucleus.

By now, it was known from laboratory experiments that some elements could be transformed into other elements. Arthur Eddington put together the pieces of the puzzle by proposing that stars such as the Sun were crucibles inside which hydrogen was transformed into helium. This transformation, he suggested, was the source of their energy.

The subsequent confirmation that hydrogen is the most abundant element in the Sun supported the idea that hydrogen nuclei (protons) are fused together inside the Sun's core, under conditions of extreme temperature and pressure. With the development of the atomic (fission) bomb in 1945, and then the hydrogen (fusion) bomb in 1952, there could be no doubt that such nuclear reactions produce vast amounts of radiation and energy.

According to the most generally accepted current theory, the Sun's energy output is the result of a series of nuclear fusion reactions, the most dominant of which is known as the proton–proton or p‐p chain. These reactions take place in the central core (Figure 2.10).

In the first stage of the most common p‐p chain reaction (known as P‐P I), two positively charged protons collide with enough energy to overcome the repulsive electrical force between them, uniting to form a deuteron.⁵

However, since a deuteron comprises a single proton and a single neutron, one of the original protons must be transformed into a neutron by emitting a positively charged particle, known as a positron, along with a low‐energy neutrino. Some of the energy liberated during this reaction is converted into radiation when the positron collides with an electron.

Figure 2.10 The P‐P I proton‐proton chain reaction has three stages. During this nuclear process, two pairs of hydrogen nuclei (protons) are united to create a helium nucleus. During this process, two of the protons are changed into neutrons, so the helium nucleus comprises two protons and two neutrons. This proton transformation is made possible by the release of a positively charged anti‐particle, called a positron, and a neutrino (v), leaving behind a neutral (chargeless) neutron. Gamma rays (γ) are also released during this collision process.

(Wikimedia)

The second stage occurs less than a second later, when the deuteron collides with another proton to form a nucleus of light helium (helium‐3), while releasing a gamma ray. Finally, two helium‐3 nuclei meet and fuse to form a nucleus of normal helium. When this occurs, two protons are returned to the surrounding gas. This leaves a helium nucleus comprising two protons and two neutrons.

The entire process may take more than a million years, on average, but the sheer number of reactions that occur each second inside the Sun has a remarkable multiplier effect. Roughly 10 trillion trillion trillion helium nuclei are created every second, resulting in a reduction in the Sun's mass of about 4.3 million tonnes per second. This seems an incredible amount, but the loss is insignificant compared with the Sun's total mass of two thousand trillion trillion tons. Since the helium nucleus is only 0.7% lighter than its original components, the Sun has actually lost only a few hundredths of one percent of its mass during its 5‐billion‐year history.

More serious is the rate of conversion of hydrogen into helium. About 600 million tons of hydrogen are transformed into helium each second in the Sun's core. Over its lifetime, approximately 37% of the hydrogen has already been converted into helium. After another 5 billion years of chain reactions, our star will run out of hydrogen and require a new source of fuel.