CONSTITUTION OF THE SUN Modern View of the Sun's Constitution.—The chief facts con cerning the sun which have so far been brought to light are now before us, and the question arises: What idea of the sun do they lead us to form? Following the independent investigations of Arthur Stanley Eddington and James Hopwood Jeans, we think of a vast concourse of broken fragments of atoms in violent move ment, held together by gravitational attraction. An atom, it should be remarked, is now pictured as a sort of miniature solar system, in which a number of electrons, or units of negative electricity, revolve round a positive nucleus. The net amount of positive electricity in the nucleus is, in the normal atom, exactly equal to the sum of the negative charges constituting the revolving electrons, and one element is distinguished from another by the amount of this charge. Thus, the hydrogen atom consists of a nucleus with one unit of positive charge and a single satellite electron; the helium nucleus has two units of positive charge and two satellite electrons; and so on, up to uranium, the heaviest known element, which has 92 units of charge in the nucleus and 92 revolving elec trons. By certain methods some of the electrons can be detached, one by one, from their orbits and set free. The remainder of the atom is then said to be ionised. Atoms can thus be singly, doubly, and, in general, multiply ionised, according to the number of electrons which are so detached. In such states, however, they are unstable and recapture electrons to make up their deficiency at the earliest opportunity.
Positive nuclei and electrons, then, are the material units of which the sun is believed to be composed. Whether the nuclei are entirely of the kinds existing on the earth, or whether in the interior of the sun there are also heavier ones, is a question on which opinions differ; the direct evidence of the spectroscope, of course, reveals the composition of the sun's atmosphere only. Since the sun is radiating energy, conditions of stability require that it shall get hotter towards the centre, so that the effective tem perature of 6,000° represents approximately the minimum solar temperature. Now from laboratory experiments and deductions therefrom, we know that high temperature is one of the agencies through which atoms are ionised : the interior of the sun must therefore be pictured as a swarming crowd of electrons and ionised atoms, the degree of ionisation of the atoms and the speed of their motions increasing as the centre is approached. Near
the centre, in fact, the nuclei are almost, if not quite, stripped bare of their satellite electrons.
In the ceaseless and inconceivably rapid motions electrons are captured by nuclei and set free again millions of times per second. Both the cause and effect of this process is radiation. Every time an electron is captured the energy of its former motion is liberated as a unit beam, or quantum, of radiation; every time a quantum falls on an atom tuned to receive it a satellite electron absorbs its energy and flies into momentary freedom. Ionised atoms, electrons, and radiation thus take part in a process of continual interchange, rapid beyond conception and without pause or dimi nution of intensity. But there is an important difference in the behaviour of the atoms and electrons on one hand and the radia tion on the other. The headlong careering of the former is kept within a limited range by the gravitational consolidation of the whole mass, while electrostatic forces preserve a constant pro portion between the number of nuclei and the number of electrons in each region. Radiation, however, is not so controlled. It works its way from the centre, where it is most intense, out to the surface and thence to space at the observed rate of 3.79 x 1o" ergs per second. The sun loses radiation at this enormous rate, and has been doing so for countless millions of years, while the electrons and atomic nuclei remain chained within its boundaries. How is it possible for the process of interchange to be maintained? There appears to be only one answer to this question. Electrons and nuclei must in some measure cancel one another, producing fresh radiation by their complete or partial annihilation. Such a process is theoretically possible. It has never been known to occur on the earth, and the only evidence that it occurs in the sun and stars is that otherwise we cannot explain the prodigal generation of radiant energy which those bodies manifest. What conditions give rise to a process which terrestrial circumstances do not countenance is at present an unanswered question.