Inside The Sun
The Sun condensed about 5 000 million years ago from a pocket of interstellar gas that contracted under the attractive forces of its own gravity. An important property of the Sun is its mass (330000 times the Earth’s mass), which results in a pressure and temperature at the centre of the Sun sufficiently high to cause the nuclear reactions that sustain the prodigious output of energy. A rather small proportion of the total volume of the Sun, known as the core, contains most of the mass and is responsible for the entire luminosity: within the central sphere of one-quarter the solar radius (1.5 per cent of the volume) is concentrated half the mass, and it is here that 99 per cent of the energy is generated. Originally the core consisted of about 75 per cent hydrogen, almost 25 per cent helium and around 1 per cent of heavier elements. Although the outer part of the Sun still has this original composition, nuclear burning has altered the make-up of the core.

The source of solar energy is the proton-proton cycle, in which hydrogen nuclei are converted to helium nuclei. Today, after more than 4 500 million years of fusion in the core, the concentration by mass of hydrogen has been reduced from 75 per cent to about 35 per cent; consequently the helium abundance’ has risen to around 65 per cent. Fusion is accompanied by a mass loss, which appears as energy. To generate the observed solar luminosity of 4 x 1026 watts demands the destruction of mass at the rate of 5 x 109kgs~x. Even at this rate the change in the Sun’s mass due to fusion while it is a main-sequence star will be below 0.1 per cent. The Sun can maintain essentially its present output for about 5 000 million years. After this the exhaustion of hydrogen at the centre will induce structural changes that turn the Sun into a red-giant star. Ultimately the Sun will become a white dwarf.

Illustrate the variations of the solar temperature and density with increasing distance from the centre. The central temperature is about 1.5 x 107 K, and it decreases steadily, reaching a surface value of 5800 K. At the centre, the density is nearly 1.6 x 105kgm-3, or 12 times the density of lead. The density falls rapidly with increasing distance; for example, it reaches the value for water half-way from the centre. These extreme physical conditions in the core cause the complete ionization of matter. EVEN nuclei of the heavy elements are unable to retain orbital electrons. Therefore the solar core consists primarily of hydrogen nuclei (protons), helium nuclei (alpha particles), and free electrons. Most of the energy released from the nuclear fusion reactions is in the form of gamma-ray photons, X-ray photons, and weird particles called neutrinos. The neutrinos have such a small prob¬ability of interacting with matter that they stream straight out of the Sun. For the photons a different situation prevails because the free electrons readily scatter photons, and the nuclei can participate in non-elastic collision with photons. These properties of the electrons and nuclei make the core essentially opaque to electromagnetic radiation. Consequently it takes about 106 years for electromagnetic energy to diffuse from the core to the surface of the Sun. This long diffusion time is a major contributor to the stability of the Sun.

Out to a distance of around 0.85 solar radii, energy is transported primarily by radiation. The absence of convection in this part of the Sun prevents the helium made in the nuclear reactions being carried out of the core. At the distance 0.85 solar radii and beyond, the temperature has fallen sufficiently to enable the heavier nuclei to recapture outer orbital electrons and so form partially ionized atoms. These outer electrons of the atoms can easily absorb photons, and this leads to a sharp increase in the opacity of the solar material to radiation. Convective instability is then triggered because the radiation streaming out of the core is suddenly blocked; the transport of energy is primarily by turbulent circulating currents of gas and each element of rising gas takes energy directly to the surface. This zone of convection extends from a depth of 150 000 km or so up to the visible surface itself. At the surface, radiation again predominates as the means of energy transport.

Currents in the convection zone are thought to arrange them¬selves in three major tiers,. Deepest are the GIANT CELLS, each encompassing possibly 200000km. At an intermediate layer, SUPER-GRANULAR CELLS about 30000km in diameter are located. Finally, a layer of small currents roughly 1 000 km across and up to 2 000 km deep reaches to the surface. The tops of this upper layer make up the Sun’s visible surface.

An important check on our knowledge of the solar interior has come from attempts to detect the abundant flux of neutrinos, which is released in the core to escape into space easily on account of the almost negligible interaction of neutrinos with matter. An experiment started in the late-1960s utilized a neutrino detector that contained 600 tonnes of liquid tetrachlorethylene (C2C14). This substance is relatively effective in recording certain of the neutrinos thought to be emerging from the Sun because they have the appropriate energy to interact with the chlorine atoms. The neutrino telescope was positioned deep in a gold mine to reduce spurious responses caused by cosmic rays. After the experiment had run for several years it consistently recorded the number of solar neutrinos to be less than the theoretical predictions. Subsequently, experimental and theoretical refinements reduced the discrepancy somewhat, but the reasons for the lack of agreement are not clear. The experiment has demonstrated that our under¬standing of solar physics is perhaps not as complete as the simplified outline given here would suggest. One possibility, among several, is that the Sun’s luminosity may vary slightly over a time scale of 2 X 108 years; this might also account for some of the major ice epochs experienced on Earth.

Comments are closed.