Tidal friction has also been operant in producing sun-raised tides upon the early plastic substances which composed the planets: more powerfully in the case of planets nearer the sun; less rapidly if the planet"s ma.s.s is large; also less completely if the planet has solidified earlier on account of its small dimensions. So Darwin would account for the present rotation periods of all the planets: both Mercury and Venus powerfully acted on by the sun on account of their nearness to him, and their rotational energy completely exhausted, so that they now and for all time turn a constant face toward him, as the moon does to the earth; earth and possibly Mars even yet undergoing a very slight lengthening of their day; Jupiter and Saturn, also Ura.n.u.s and probably Neptune, still exhibiting relatively swift axial rotation, because of their great ma.s.s and great original moment of momentum, and also by reason of their vast distances from the central tide-raising body, the sun.
By applying to stellar systems the principles developed by Darwin, See accounted for the fact, to which he was the first to direct attention, that the great eccentricity of the binary orbits is a necessary result of the secular action of tidal friction. The double stars, then, were double nebulae, originally single, but separated by a process allied to that known as "fission" in protozoans. Indeed, Poincare proved mathematically that a swiftly revolving nebula, in consequence of contraction, first undergoes distortion into a pear-shaped or hour-gla.s.s figure, the two ma.s.ses ultimately separating entirely; and the observations of the Herschels, Lord Rosse and others, with the recent photographic plates at the Lick and Mount Wilson observatories, afford immediate confirmation in a mult.i.tude of double nebulae, widely scattered throughout the nebular regions of the heavens.
Jeans of Cambridge, England, among the most recent of mathematical investigators of the cosmogony, balances the advantages and disadvantages of the differing cosmogonic systems as follows, in his "Problems of Cosmogony and Stellar Dynamics": "Some hundreds of millions of years ago all the stars within our Galactic universe formed a single ma.s.s of excessively tenuous gas in slow rotation. As imagined by Laplace, this ma.s.s contracted owing to loss of energy by radiation, and so increased its angular velocity until it a.s.sumed a lenticular shape.... After this, further contraction was a sheer mathematical impossibility and the system had to expand. The mechanism of expansion was provided by matter being thrown off from the sharp edge of the lenticular figure, the lenticular center now forming the nucleus, and the thrown-off matter forming the arms, of a spiral nebula of the normal type. The long filaments of matter which const.i.tuted the arms, being gravitationally unstable, first formed into chains of condensation about nuclei, and ultimately formed detached ma.s.ses of gas. With continued shrinkage, the temperature of these ma.s.ses increased until they attained to incandescence, and shone as luminous stars. At the same time their velocity of rotation increased until a large proportion of them broke up by fission into binary systems. The majority of the stars broke away from their neighbors and so formed a cl.u.s.ter of irregularly moving stars--our present Galactic universe, in which the flattened shape of the original nebula may still be traced in the concentration about the Galactic plane, while the original motion along the nebular arms still persists in the form of "star-streaming." In some cases a pair or small group of stars failed to get clear of one another"s gravitational attractions and remain describing orbits about one another as wide binaries or multiple stars. The stars which were formed last, the present B-type stars, have been unusually immune from disturbance by their neighbors, partly because they were born when adjacent stars had almost ceased to interfere with one another, partly because their exceptionally large ma.s.s minimized the effect of such interference as may have occurred; consequently they remain moving in the plane in which they were formed, many of them still const.i.tuting closely a.s.sociated groups of stars--the moving star cl.u.s.ters.
"At intervals it must have happened that two stars pa.s.sed relatively near to one another in their motion through the universe. We conjecture that something like 300 million years ago our sun experienced an encounter of this kind, a large star pa.s.sing within a distance of about the sun"s diameter from its surface. The effect of this, as we have seen, would be the ejection of a stream of gas toward the pa.s.sing star.
At this epoch the sun is supposed to have been dark and cold, its density being so low that its radius was perhaps comparable with the present radius of Neptune"s...o...b..t. The ejected stream of matter, becoming still colder by radiation, may have condensed into liquid near its ends and perhaps partially also near its middle. Such a jet of matter would be longitudinally unstable and would condense into detached nuclei which would ultimately form planets."
CHAPTER LXI
COSMOGONY IN TRANSITION
We have seen how Wright in 1750 initiated a theory of evolution, not only of the solar system, but of all the stars and nebulae as well; how Kant in 1752 by elaborating this theory sought to develop the details of evolution of the solar system on the basis of the Newtonian law, though weakened, as we know, by serious errors in applying physical laws; how Laplace in 1796 put forward his nebular hypothesis of origin and development of the solar system, by contraction from an original gaseous nebula in accord with the Newtonian law; how Sir William Herschel in 1810 saw in all nebulae merely the stuff that stars are made of; how Lord Rosse in 1845 discovered spiral nebulae; how Helmholtz in 1854 put forward his contraction theory of maintenance of the solar heat, seemingly reinforcing the Laplacian theory; how Lane in 1870 proved that a contracting gaseous star might rise in temperature; how Roche in 1873 in attempting to modify the Laplacian hypothesis, pointed out the conditions under which a satellite would be broken up by tidal strains; how Darwin in 1879 showed that the theory of tidal evolution of non-rigid bodies might account for the formation of the moon, and binary stars might originate by fission; how Keeler in 1900 discovered the vast numbers of spiral nebulae; how Chamberlin and Moulton in 1903 put forward the planetesimal hypothesis of formation of the spiral nebulae, showing also how that hypothesis might account for the evolution of the solar system; and how Jeans in 1916 advocated the median ground in evolution of the arms of the spiral nebulae, showing that they will break up into nuclei, if sufficiently ma.s.sive.
In all these theories, truth and error, or lack of complete knowledge, appear to be intermingled in varying proportions. Is it not early yet to say, either that any one of them must be abandoned as totally wrong, or on the other hand that any one of them, or indeed any single hypothesis, can explain all the evolutionary processes of the universe?
Clearly the great problems cannot all be solved by the kinetic theory of gases and the law of gravitation alone. Recent physical researches into sub-atomic energy and the structure and properties of matter, appear to point in the direction where we must next look for more light on such questions as the origin and maintenance of the sun"s heat, the complex phenomena of variable stars and the progressive evolution of the myriad bodies of the stellar universe. Because we have actually seen one star turn into a nebula we should not jump to the conclusion that all nebulae are formed from stars, even if this might seem a direct inference from the high radial velocities of planetary nebulae.
Quite as obviously many of the spiral nebulae are in a stage of transition into local universes of stars--even more obvious from the marvelous photographs in our day than the evolution of stars from nebulae of all types was to Herschel in his day.
The physicist must further investigate such questions as the building up of heavy atomic elements by gravitative condensation of such lighter ones as compose the nebulae; and laboratory investigation must elucidate further the process of development of energy from atomic disintegration under very high pressures. This leads to a recla.s.sification of the stars on a temperature basis.
Equally important is the inquiry into the mechanism of radiative equilibrium in sun and stars. Not impossibly the process of the earth"s upper atmosphere in maintaining a terrestrial equilibrium may afford some clue. What this physical mechanism may be is very incompletely known, but it is now open to further research through recent progress of aeronautics, which will afford the investigator a "ceiling" of 50,000 feet and probably more. Beneath this level, perhaps even below 40,000 feet, lie all the strata, including the inversion layer, where the sun"s heat is conserved and an equilibrium maintained.
Even ten years ago, had an astronomer been asked about the physical condition of the interior of the stars, he would have replied that information of this character could only be had on visiting the stars themselves--and perhaps not even then. But at the Cardiff meeting of the British a.s.sociation in 1920, Eddington, the president of Section A, delivered an address on the internal const.i.tution of the stars. He cites the recent investigations of Russell and others on truly gaseous stars, like Aldebaran, Arcturus, Antares and Canopus, which are in a diffuse state and are the most powerful light-givers, and thus are to be distinguished from the denser stars like our Sun. The term _giants_ is applied to the former, and _dwarfs_ to the latter, in accord with Russell"s theory.
As density increases through contraction, these terms represent the progressive stages, from earlier to later, in a star"s history. A red or M-type star begins its history as a giant of comparatively low temperature. Contracting, according to Lane"s law, its temperature must rise until its density becomes such that it no longer behaves as a perfect gas. Much depends on the star"s ma.s.s; but after its maximum temperature is attained, the star, which has shrunk to the proportions of a dwarf, goes on cooling and contracts still further.
Each temperature-level is reached and pa.s.sed twice, once during the ascending stage and once again in descending--once as a giant, and once as a dwarf. Thus there are vast differences in luminosity: the huge giant, having a far larger surface than the shrunken dwarf, radiates an amount of light correspondingly greater.
The physicist recognizes heat in two forms--the energy of motion of material atoms, and the energy of ether waves. In hot bodies with which we are familiar, the second form is quite insignificant; but in the giant stars, the two forms are present in about equal proportions. The super-heated conditions of the interior of the stars can only be estimated in millions of degrees; and the problem is not one of convection currents, as formerly thought, bringing hot ma.s.ses to the surface from the highly heated interior, but how can the heat of the interior be barred against leakage and reduced to the relatively small radiation emitted by the stars. "Smaller stars have to manufacture the radiant heat which they emit, living from hand to mouth; the giant stars merely leak radiant heat from their store."
So a radioactive type of equilibrium must be established, rather than a convective one. Laboratory investigations of the very short waves are now in progress, bearing on the transparency of stellar material to the radiation traversing it; and the penetrating power of the star"s radiation is much like that of X-rays. The opacity is remarkably high, explaining why the star is so nearly "heat-tight."
Opacity being constant, the total radiation of a giant star depends on its ma.s.s only, and is quite independent of its temperature or state of diffuseness. So that the total radiation of a star which is measured roughly by its luminosity, may readily remain constant during the entire "giant" stage of its history. As Russell originally pointed out, giant stars of every spectral type have nearly the same luminosity. From the range of luminosity of the giant stars, then, we may infer their range of ma.s.ses: they come out much alike, agreeing well with results obtained by double-star investigation.
These studies of radiation and internal condition of the stars again bring up the question of the original source of that supply of radiant energy continually squandered by all self-luminous bodies. The giant stars are especially prodigal, and radiate at least a hundredfold faster than the sun.
"A star is drawing on some vast reservoir of energy," says Eddington, "by means unknown to us. This reservoir can scarcely be other than the sub-atomic energy which, it is known, exists abundantly in all matter; we sometimes dream that man will one day learn how to release it and use it for his service. The store is well-nigh inexhaustible, if only it could be tapped. There is sufficient in the sun to maintain its output of heat for fifteen billion years."