Planetary nebula

“ And the surmise was confirmed by his detection, in a planetary nebula, of a sharp nucleus, or “generating star,” possibly to be completed in time by the further accumulation of luminous matter.
His conjectures developed in 1811 into a formal theory. The cosmical fluid was met with in all stages of condensation. Nebulous tracts of almost evanescent72 lustre were connected in an unbroken series with slightly “burred” objects, wanting only a few last touches to make them finished stars. ”

“ It will readily be seen, from previous considerations, that large masses of the resulting nebula will be thrown out from the sphere of sensible attraction of the remaining mass, with velocities exceeding 300 miles a second, while other portions will emerge with less and less velocities, till finally some portions barely reach those limits.
The remaining nebula should then begin to contract, and might, possibly, form a small planetary system. Of the out-thrown masses, those which moved with the greatest velocity would retire to the greatest distances ”

“ The theory which at present seems more nearly than any other to offer a reasonable explanation of most planetary phenomena is based upon the supposition that the nebula from which the sun and planets were evolved was in the shape of a spiral, and not the gaseous mass that the original nebular hypothesis supposed. The fact that among the many thousands of nebulæ that have been discovered and observed a very large proportion of them are in this form, aside from any other consideration, suggests a great probability that the one from which the solar system was evolved was a spiral. ”

“ Gravity eventually brings the nebular particles into closer aggregations, and increased collisions finally vaporize the entire mass, forming planetary nebulae and gaseous stars. Continued condensation may make the stellar mass hotter and more luminous for a time, but eventually leads to its liquefaction, and ultimate consolidation—the aforetime nebulae becoming in the end a dark or planetary star. ”

“ During this conservational activity the layers of gas are rapidly rarefied, to be replaced by new masses from the inner parts of the nebula, until this centre is depleted, and the nebula has been converted into a star cluster or a planetary system which circulates about one or several suns. ”

“ Also, the closest affinity is shown between nebulae and stars. Some nebulae are found to contain stars, singly or in groups, in their actual midst; certain condensed "planetary" nebulae are scarcely to be distinguished from stars of the gaseous type; and recently the photographic film has shown the presence of nebulous matter about stars that to telescopic vision differ in no respect from the generality of their fellows in the galaxy. The familiar stars of the Pleiades cluster, for example, appear on the negative immersed in a hazy blur of light. ”

“ We might further mention the star-clusters, dense, ball-shaped agglomerations of stars, and the so-called planetary nebulæ, which—at least in their visible outer shell—also consist of gas accumulations with a spherical or ellipsoid conformation. The numerous spiral nebulæ on the other hand, those strange stellar bodies to which we45 later shall have occasion to return, are beyond comparison more frequent in the regions surrounding the poles of the Galaxy than in the rest of the heavens.
Many astronomers have considered the Milky Way itself a nebula. ”

“ The forms of many of the nebulæ are very curious. Some are quite irregular, as the Orion nebula, the Keyhole nebula in the southern hemisphere, and many others. Some show a decidedly spiral form, as those in Andromeda and Canes Venatici; others again are annular or ring-shaped, as those in Lyra and Cygnus, while a considerable number are termed planetary nebulæ, from their exhibiting a faint circular [ 71] disc like that of a planet. Many have stars or groups of stars evidently forming parts of them, and this is especially the case with those of the largest size. ”

“ When we compare the stars of the first magnitude, which, on an average, are certainly the nearest to us, with the non-nebulous telescopic stars, and further, when we compare the nebulous stars with unresolvable nebulae, for instance, with the nebula in Andromeda, or even with the so-called planetary nebulous vapor, a fact is made manifest to us by the consideration of the varying distances and the boundlessness of space, which shows the world of phenomena, and that which constitutes its causal reality, to be dependent upon the 'propagation of light'. ”

“ It has now attained the zenith of its evolution, and is breaking up into molecules, or solar systems, which again are composed of stellar bodies, or atoms within the molecule. In violent exuberance of youth these rush through space in fulfilment of their individual life. Many will in due time undoubtedly become dust again and then serve to nourish a new youthful nebula. Others succumb to a freezing-death but will be restored to life in collision with a nebulosity or some other stellar body and give form to “new stars” or planetary clouds. ”

“ New and ingenious considerations of the different influence exercised by distance* on the intensity of light of a disk of appreciable diameter, and of a single self-luminous point, render it not improbable that the planetary nebulae are very remote nebulous stars, in which the difference between the central body and the surrounding nebulous covering can no longer be detected by our telescopic instruments. ”

“ Unsuspected by any in the Galaxy the mighty nebula was hollow, its gigantic globe of flaming gas holding at its heart this mighty empty space, a space mighty in extent to our eyes, but small compared to the thickness of the great shell of fire that enclosed it. And down through that fire, that vast ocean of flame, the currents of the nebula had brought us, from its outer surface, down into this great space at its heart of which none had ever dreamed, and into which we had been the first in all the Galaxy to penetrate. ”

“ We have even dared to change the course of suns, to prevent collisions between them that would have annihilated their circling worlds. It might seem, indeed, that we, the massed peoples of the Galaxy, have risen to such power that all things in it are subject to our will, obedient to our commands. But we have not. One thing alone in the Galaxy remains beyond our power to change or alter, one thing beside which all our power and our science are as nothing. And that is the nebula.
"A nebula is the vastest thing in all our universe, and the most mysterious. ”

“ Look now again at the glowing spirals. We observe that hardly have they left the central mass before they begin to coagulate. In some places they have a “ropy” aspect; or they are like peascods filled with growing seeds, which eventually will become stars. The great focus itself shows a similar tendency, especially around its circumference. The sense that it imparts of a tremendous shattering force at work is overwhelming. There is probably more matter in that whirling and bursting nebula than would suffice to make a hundred solar systems! ”

“ Nearer and nearer we flashed toward it, with Sar Than and Jor Dahat beside me gazing forward, their interest as tense as my own. And now we saw that the pit was of gigantic size, its circular mouth all of five miles in diameter, and that from its center there drove up toward the zenith a flickering beam of pale and ghostlike white light, so pale as hardly to be visible, a livid white ray that stabbed straight up toward the fires of the nebula far above. ”

“ The nebular theory at once supplies the explanation of the unanimity in the rotation of the planets, just as it supplied the explanation of the unanimity in the directions of their revolutions. To explain the rotation of a planet on its axis, let us imagine that one portion of the contracting nebula has acquired exceptional density. In virtue of its superior attraction it absorbs more and more material from the adjacent parts of the nebula, and this will ultimately be consolidated into the planet, though in its initial stages this contracting matter will remain part of the nebula. ”

“ Ages passed over them while they lived thus in their world's depths, but still the nebula contracted, closed in upon them, in that vast, remorseless cycle that is nature's law throughout the universe. Closer and closer crept its fires toward the metal-sheathed world of the nebula people, until at last they saw that soon those fires would envelop their world and annihilate it, unless they were turned back in some way. ”

“ All that we can see is a long, shuttle-shaped nebulous object, having a brighter point near the center. Many stars are scattered over the field in its neighborhood, but the nebula itself, although its spectrum is peculiar in resembling that of a faint star, is evidently a gaseous or at any rate a meteoritic mass, since photographs show it to be composed of a series of imperfectly separated spirals surrounding a vast central condensation. ”

“ But we have learned too late, now, for in less than twenty hours the nebula's whirling will have reached its critical point, will have sent its vast flaming mass hurtling out across our universe, our Galaxy, in all directions, to carry destruction and death to all the peoples of our suns and worlds! ”

“ This brings us directly back to “Nova Persei,” for among the many suggestions offered to explain its outburst, as well as those of other temporary stars, one of the most fruitful is that of a collision between a star and a vast invisible nebula. Professor Seeliger, of Munich, first proposed this theory, but it afterward underwent some modifications from others. Stated in a general form, the idea is that a huge dark body, perhaps an extinguished sun, encountered in its progress through space a widespread flock of small meteors forming a dark nebula. ”

“ In subsequent chapters we are to present the evidence for the belief that this earth of ours, as well as the sun and all the other bodies which form the solar system, did once originate in a nebula. According to this view the materials which at present are found in 50the globes of the solar system were once distributed over a vast extent of space as a fire-mist, or nebula. It is surely very pertinent to be able to show that a nebula, such as we suppose to have been the origin of our system, is not a mere figment of the imagination. ”

“ In fact, we can hardly say that the nebula has any definite boundary, for with each increase of telescopic power faint new branches can be seen. There seems to be an empty space in the nebula immediately surrounding the multiple star, but this is merely an illusion, produced by the contrast of the brilliant light of the stars, as the spectroscopic examination of the nebula shows that the nebulous matter is continuous between the stars. ”

“ As the process advances still further the body may pass from a mere nebula into a star-like object. With increase of contraction the pressure also increases and materials which were originally gaseous will assume more and more a density resembling that of solid bodies.” He says further that should the sun contract into a globe less its present size by one ten-thousandth part of its diameter it would amount to a shrinkage in its diameter of 87 miles. ”

“ We know that the great nebula is growing slowly smaller, that through the eons it contracts to form new blazing stars, but what its constitution may be, what mysteries it may hide, has never been known, since it would be annihilation for any ship to approach too near to its fiery splendor, and all our interstellar traffic has detoured always far around its flaming mass. ”

“ The velocity of escape is 280 km/sec.
At the lower limit of average distance of spirals the typical nebulae would be fifteen light-years in diameter, forty-five million times as massive as our sun, and 3 × 1013 as dense as our atmosphere. On the other hand, if the typical spiral nebula is placed at a suitable distance, its dimensions assume the same order of magnitude as those of our own stellar system. ”

“ A few weeks ago, however, it was discovered by our astronomers that the great flaming sphere of the nebula had begun slowly to revolve, to spin, and that as the days went by it was spinning faster and faster. Through the weeks since then our astronomers have watched it closely, and ever faster it has spun, until now it is revolving at a terrific rate, a rate that is still steadily increasing. And that accelerating spin of the huge nebula must result, inevitably, in the doom of our universe. ”

“ Is not this a weighty piece of evidence in favour of the theory that earth, sun, and planets are all portions of the same primæval nebula in which these elements were blended?
THE SOLAR SPECTRUM.
We do not, of course, mean to affirm that the great primæval nebula was homogeneous throughout its vast extent. The waters of ocean are not strictly the same in all places; even the atmosphere is not 291absolutely uniform. Nature does not like homogeneity. The original nebula, we may well believe, was irregular in form, and denser in some places than in others. ”

“ It is accepted quite generally by astronomers at the present day, though in a greatly modified form; for there are many difficulties in the way of a full belief of the theory. Sir Robert Ball in a late work says of Herschel’s belief of the transmutation of nebulae into stars; “To establish [Pg 9] this theory it would be necessary to watch the actual condensation of one single nebula from the primitive gaseous condition down to the stellar points. ”

“ But once we grant that the system originated from the contraction of the primæval nebula, dynamics offers ready aid, and the difficulty vanishes. Not only do we see most excellent reasons why all the planets should revolve in the same direction; we are also provided with illustrations of similar evolutions in progress in other parts of the universe; we learn that the evolving nebula, however erratic may have been its primitive motion, whatever cross currents may have agitated it in the early phases of a possibly violent origin, will ultimately attain a rotation uniform in direction. ”