The Energy of Starlight

I

THE development of science presents a dual aspect. When it is regarded, as it were, microscopically, the mind is staggered by the vision of ever-growing complexity, greater and greater diversity, whether it be of stars or stones, men or microbes, molecules or atoms — new types, new species, new affinities, new isotopes. Regarded macroscopically, however, very different is the prospect it affords — the vision of allembracing natural law, a great underlying unity gradually disclosing itself, the intrinsic harmony of all things. This is the vision all-compelling, the vision which sends the man of science on a lifelong quest as sacred as that of the Holy Grail — the quest of Truth. And in the reality of this vision his faith is unwavering and undaunted. All faith is dynamic, and to this vital faith of men of science that there exists ultimate order in the universe Professor A. N. Whitehead has ascribed the rescue of Western thought in the later Middle Ages from the futility of ‘unbridled rationalism.’

In no branch of science is this gradually emerging unity more evident than in the realm of physics. Five or six hundred years before the beginning of the Christian Era we find the Greek philosophers experimenting and speculating upon the phenomena of mechanics, sound, heat, light, electricity, and magnetism. Each of these was at first an entirely separate and distinct branch of knowledge, no connecting links being apparent. Archimedes investigated the laws of mechanical advantage and buoyancy; Pythagoras studied sound, discovering a relation between the length of a stretched string and the note which it would produce when caused to vibrate; Hero of Alexandria probed some of the mysteries of heat; Thales, in 600 B.C., knew something of the manifestations of electrostatics and of magnetism, but to him they were phenomena unrelated in any way; Aristotle, about 350 B.C., and five hundred years later Ptolemy of Alexandria, recorded their observations on the behavior of light, but of its true nature and its kinship to the phenomena studied by Thales and Hero even Ptolemy knew nothing.

Our picture is thus of six streamlets of knowledge having their beginnings in the far-distant past, each growing in breadth and depth as the centuries rolled by. But not the inspired vision of even the greatest thinkers of early times could foresee that these streams would prove to be tributaries of one mighty river — the vast, deep, broad river of energy.

The first confluence took place almost imperceptibly with the gradual realization that sound was essentially a mechanical phenomenon, produced by mechanical means and in itself a pulsation of material particles. Thus two of the streamlets had merged their waters before the time of Galileo and Newton. The momentous contributions to knowledge made by these two giants among the great thinkers of all time swelled this stream during the seventeenth century until it became a rushing torrent.

The other streamlets, meanwhile, grew each to the proportions of a river, but each maintained its individuality until the nineteenth century. With the discoveries of Coulomb, Volta, Gauss, Oersted, Ampère, Ohm, and the great Faraday, the rivers of knowledge regarding the phenomena of electricity and magnetism became merged into one, the river of electromagnetic energy. Contemporaneous with Faraday, Carnot and Joule were establishing the relation between mechanical energy and heat, and so another mingling of waters was accomplished and the comprehensive river of mechanical energy flowed on toward the present century, to be augmented from within by the work of Helmholtz, Clausius, Kelvin, Boltzmann, Gibbs, and Planck.

But what of the river of light? ‘Light, the prime work of God,’ as Milton has written; light, to this day the chief marvel and mystery to the natural philosopher. The stream of light grew into a river during the lifetime of Newton and Huygens. Its volume increased with the investigations of Fraunhofer, Fresnel, Foucault, and Kirchhoff, but it was the mathematical researches of James Clerk Maxwell about 1875 that brought about the union of the river of light with the great river of electromagnetic energy. The Maxwellian electromagnetic theory of light at once led men of science to search for a radiation akin to light but invisible and of very great wave length. The infectious enthusiasm and pioneer researches of Sir Oliver Lodge stimulated many to the search, but to Hertz in 1888 fell the lot of first detecting the ‘wireless’ waves. Gradually it was realized that to the same category of electromagnetic waves or vibrations belonged the heat rays, much shorter in wave length than the Hertzian waves, but longer than those of visible light. Thus, too, the mysterious X-rays of Röntgen found their true place among the electromagnetic waves shorter than light, shorter even than the invisible ultraviolet light whose chemical activity is so great.

Thus the great river of mechanical energy mingles its waters with those of the great river of electromagnetic energy, and even as the waters swirl together new knowledge regarding the energy of matter itself springs suddenly forth in astounding abundance to increase the already swollen river. The momentous discovery by Sir J. J. Thomson of the electron, the ultimate charge of negative electricity and the smallest known particle of matter, closed the century. Following up the work of Becquerel and the Curies in radioactivity, the present century saw the birth of Rutherford’s nuclear theory of the atom and his discovery that the electron and the proton, or ultimate positive charge of electricity, are the two building bricks of all the elements of which matter is composed. On Rutherford’s experiments arose Bohr’s theory of the atom, which supplied for the first time a picture of the mechanism within the atom which permits of the emission of electromagnetic energy as X-rays, light, or some other form. The culminating step in this conception that there is one great entity, energy, which can manifest itself in many forms, was taken by Lorentz and by Einstein independently when they propounded the relation which links matter with energy. Matter itself, or, to think of its ultimate constituents, the proton and the electron— these are tremendous concentrations of the fundamental entity of the universe, energy. Not that this is available energy — the secret of the concentration of energy whereby matter is created and the secret of the annihilation of matter whereby its energy is dissipated into space are held tightly by Nature beyond the grasp of man. Sometime and somewhere in the space-time universe this transformation has taken place and matter has come into existence. Man can alter its form to a limited extent by bringing about chemical and physical change, but he cannot as yet make or unmake matter.

II

After contemplating this mysterious fundamental entity, energy, manifesting itself in so many ways, whether bound as in matter or freely transformable as from light to heat, from heat to mechanical energy, to electrical or chemical or any other of the wellrecognized forms, the question naturally presents itself, Whence comes all this terrestrial energy with which we are familiar? The earth beneath our feet, the air we breathe, and our bodies themselves are tremendous concentrations of energy. Whence comes it? And the answer is — star dust. Our earth was once a portion of the surface material of the sun, and our sun is just a star among the stars of a great galaxy of many millions, a quite typical star in most respects, neither one of the largest nor one of the smallest, neither one of the hottest nor yet the coolest. A chance fragment of a great star is our planet; and man, as far as his physical framework is concerned, being ‘of the earth, earthy,’ is therefore of the stars, starry, or, rewriting a famous line of the great poet, ‘ We are such stuff as stars are made on.’

When we seek the source of the unbound energy of the earth we find that some of it — its gravitational energy and a very small amount of its heat — is within it; but the energy which maintains the surface temperature of the earth at a reasonable degree of warmth, the energy which makes possible the existence of life upon the earth, life vegetable, life animal, the life of man — all this energy comes from without. It is brought to the earth in the sunlight and in the starlight; but, since the latter term in its fullest sense includes the former, we may simply and with absolute accuracy say, Of star dust are we made, and by starlight we live.¹

If astronomy be the study of the stars, astrophysics may be said to be the study of starlight. Whereas the former is the oldest of the sciences, the latter is one of the youngest, yet in the few-score years that have elapsed since the birth of astrophysics man’s knowledge of the universe has expanded many fold, so almost overwhelming have been the revelations resulting from the study of the energy of starlight.

One never ceases to marvel at the achievements of the early astronomers. They mapped the heavens; they recognized that the stars are the timekeepers; that their positions give us our sense of direction; that the seven celestial bodies—the sun, moon, and five naked-eye planets—which wander across the background of the ‘fixed’ stars move with an ordered precision which they could foresee though they could not explain. But, because they knew not the real significance of energy, they could read in the starlight only its most obvious message.

Aristotle gave his support to the doctrine that all things terrestrial were made up of four constituents, — earth, air, fire, and water, — whereas the celestial bodies were composed of a fifth substance, the perfect immutable substance; and hence, unlike the earth, the heavenly bodies remained forever unchanged. No room is here for a study of the energy of starlight, for to recognize an outflow of energy presupposes change in the radiat ing body. Thus the minds of men had to be freed from the shackles of this Aristotelian fallacy before the science of astrophysics could come into being. This liberation was not achieved for seventeen centuries, and then it was Galileo who severed the chains once and for all by turning his pioneer telescope upon the sun and finding there every evidence of change — change in its surface brightness, dark areas whose shapes and positions altered from day to day. By this and other evidence he proved the universality of the law of change.

There remained now no obstacle to the study of starlight as energy save only the very essential fact that the means of analyzing light was still unknown. The discovery of the prismatic separation of light into its constituent rays was one of the many achievements of Sir Isaac Newton. If a ray of starlight be made to traverse a glass prism, it emerges not as one ray but as many; the composite starlight is analyzed, and each ray corresponding to a different energy value is set out in order, so that it can be studied apart from the others; and, what is quite as important, if the incoming starlight is lacking in some of the full range of expected energy values, their absence becomes at once apparent.

The road was now paved for the birth of astrophysics, and the investigations of Fraunhofer and Kirchhoff into the spectra of starlight mark the beginning of this new branch of science.

III

When the light of a star is viewed through a spectroscope attached to a telescope, a band of colored light is seen, the sequence of rainbow colors, but crossed at intervals by dark lines. Instead of merely looking at this spectrum, a photograph of it may be taken, and thus a permanent record is obtained which may be studied under the microscope. In this way precise measurements may be made of the wave lengths of those missing radiations in the stellar spectra. Now these dark lines in the spectrum are the hieroglyphics which hold the message of starlight, and the corresponding measurements of wave lengths are the code by means of which these hieroglyphics may be interpreted.

It is well known to the physicist that the atoms of every element can be identified by the distinctive radiations which they can emit or absorb according to the conditions of temperature and pressure under which the experiments are carried out. When the distinctive wave lengths of, let us say, the hydrogen atoms are found to agree precisely with a set of the dark lines in a stellar spectrum, the conclusion is obvious and unavoidable — hydrogen is not exclusively a terrestrial element, it is a constituent of every star, it is one of the essential building bricks of the material universe. So too carbon, nitrogen, calcium, iron, silicon, and, indeed, most of the elements of terrestrial occurrence, have added their hieroglyphics to the stellar spectra. Not only do the elements impress upon the starlight the record of their identity, but also of the conditions of temperature and pressure under which they are radiating or absorbing energy. The temperatures of stellar atmospheres are found to vary from about two thousand degrees Centigrade for the cool reddish stars to over twenty thousand degrees for the hot giant stars of bluish hue. This is read directly in the message of the starlight, but to form a picture of the conditions at the centre of a star is another matter. All the courage and all the insight of the mathematician are here required to reason from known surface conditions to the unknown central conditions. ‘Our object in diving into the interior,’ writes Professor Eddington, ‘is not merely to admire a fantastic world with conditions transcending ordinary experience; it is to get at the inner mechanism which makes stars behave as t hey do. ’

The starlight contains some valuable information regarding the motion of the star. Just as the whistle of an approaching locomotive is shrill, suddenly becoming deeper as the engine speeds away, so the hieroglyphics are displaced slightly toward the violet end of the spectrum if the star is approaching and toward the red or longer wave lengths if the star is receding. Much of what is known regarding the motions of the stars in space is learned in this way. These motions are sometimes very large, a velocity of one hundred kilometres per second being not uncommon, yet so vast are the distances between stars that ‘speeding’ is not a public danger! Even a ‘runaway star’ traveling one thousand kilometres per second would speed on t hrough empty space for many millions of years before approaching so closely to any other star as to cause apprehension or consternation in the star world. The stars are no more crowded together than would be four or five little minnows were they the sole inhabitants of the Atlantic Ocean.

IV

Whence comes the energy of starlight? This has been one of the puzzling questions to which one answer after another has been given, only to be rejected as insufficient. Kelvin suggested a gradual contraction of the star; then radioactivity seemed to offer a solution; more recently the slow synthesis of the elements was regarded as the source of energy — hydrogen atoms in the stellar crucibles being so locked together in mutual embrace as to be transmuted into the successively more complex elements of increasing atomic weight, a process accompanied by a liberation of electromagnetic energy. But now a new and yet more startling theory holds the field, championed by some, doubted by some, unproven but not disproven — the theory of the spontaneous annihilation of matter within a star. Reference has already been made to the LorentzEinstein hypothesis of the intrinsic oneness of matter and energy. It is not inconceivable, then, that under the extraordinary conditions existing deep down within a star the ultimate particles of matter, proton and electron, might so collide that each unkinked the other’s energy, thus producing their mutual annihilation as matter, or, expressed otherwise, thus bringing about the physical degradation of the energy of matter to the energy of radiation.

Perhaps this is the true explanation of the seemingly inexhaustible store of energy poured out continuously into space from every star that shines in the heavens. To consider just one typical star, our own sun is radiating energy equivalent to the annihilation of four million tons of its mass every second, and has been radiating thus for a million million years if we are justified in assuming that it was once as massive a star as Sirius. But, vast as is the store of stellar energy, it is not limitless. Professor Whitehead paints a majestic but a solemn picture of the universe ‘passing with a slowness, inconceivable in our measures of time, to new creative conditions, amid which the physical world, as we at present know it, will be represented by a ripple barely to be distinguished from non-entity. ’ ‘Degradation of energy, yet conservation,’ is the summing up of the whole matter by the physicist; ‘physically wasting, yet spiritually ascending,’ is the dictum of the philosopher.

That the processes of nature are irrevocably proceeding toward a lowest ebb is a thought intolerable to some types of mind. In spite of the fact that there is no supporting evidence, there are those who would believe that somehow and somewhere radiant energy — the energy of starlight — is being re-formed into protons and electrons, these aggregating into atoms, the atoms forming nebulæ which condense into stars whose matter gradually becomes transformed again into radiant energy. Thus the whole order of nature becomes one vast cycle indefinitely repeated.

V

It would be difficult to overestimate the influence which astronomy has had upon the human mind. From the geocentric standpoint of pre-Copernican days to the heliocentric point of view was a tremendous advance and involved a revolution of thought of farreaching consequences. But the study of starlight has not left us there; it has forced upon mankind the realization that, though the sun is the centre of the solar system, it is not the centre of the universe. Our glorious sun is but one of the lesser stars in a galaxy of a thousand million; and far out in space beyond our galaxy are the spiral nebulæ to the certain number of many million, and the probable number of a thousand billion² — and each spiral nebula is a galaxy of myriad stars!

Where is man in such a picture? Certainly not where Gray infers him to be when he sings of gems and blossoms unseen by mortal eye: —

Nature has strewn her stars with even greater prodigality, but can we say they ‘waste’ their energy of radiant light and heat upon surrounding space because it may be devoid of little ‘earths,’ the habitats of human life? Is it ‘waste’? That word is unknown in the economics of nature. The stars in their courses fight against our petty, anthropocentric sentimentalisms; they fight and they prevail. We must change our philosophy; we must widen our vision and sing with another poet, Robert Service,—

All down the ages we find men of science and poets alike drawing inspiration from the starlight. Ptolemy of Alexandria, in terms characteristic of his period, expressed it thus: ‘Mortal though I be, yea, ephemeral, if but a moment I scan the multitudinous circling of the stars, no longer on earth I stand, but sit with Zeus himself and take my fill of the ambrosial food of gods. ’

Human thought has passed through many phases since Ptolemy’s day. Materialism is inadequate and unsatisfying. When man endeavors to grasp the significance of the workings of nature, tries to realize the vastness of the universe and to grapple with the great mysteries still unsolved, perhaps it is a something within him akin to mysticism which sets some chord vibrating in harmony with those thoughts expressed by Pope in his pæan of praise to an Immanent Divinity, who