Curiosities of the Sky, Garrett Putman Serviss [free novel reading sites txt] 📗

There is a similar, but less perfect, ``coal-sack'' in the northern hemisphere, in the constellation of ``The Swan,'' which, strange to say, also contains a well-marked figure of a cross outlined by stars. This gap lies near the top of the cross-shaped figure. It is best seen by averted vision, which brings out the contrast with the Milky Way, which is quite brilliant around it. It does not, however, exercise the same weird attraction upon the eye as the southern ``Coal-sack,'' for instead of looking like an absolute void in the sky, it rather appears as if a canopy of dark gauze had been drawn over the stars. We shall see the possible significance of this appearance later.

Just above the southern horizon of our northern middle latitudes, in summer, where the Milky Way breaks up into vast sheets of nebulous luminosity, lying over and between the constellations Scorpio and Sagittarius, there is a remarkable assemblage of ``coal-sacks,'' though none is of great size. One of them, near a conspicuous star-cluster in Scorpio, M80, is interesting for having been the first of these strange objects noted by Herschel. Probably it was its nearness to M80 which suggested to his mind the apparent connection of such vacancies with star-clusters which we have already mentioned.

But the most marvelous of the ``coal-sacks'' are those that have been found by photography in Sagittarius. One of Barnard's earliest and most excellent photographs includes two of them, both in the star-cluster M8. The larger, which is roughly rectangular in outline, contains one little star, and its smaller neighbor is lune-shaped -- surely a most singular form for such an object. Both are associated with curious dark lanes running through the clustered stars like trails in the woods. Along the borders of these lanes the stars are ranked in parallel rows, and what may be called the bottoms of the lanes are not entirely dark, but pebbled with faint stellar points. One of them which skirts the two dark gaps and traverses the cluster along its greatest diameter is edged with lines of stars, recalling the alignment of the trees bordering a French highway. This road of stars cannot be less than many billions of miles in length!

All about the cluster the bed of the Galaxy is strangely disturbed, and in places nearly denuded, as if its contents had been raked away to form the immense stack and the smaller accumulations of stars around it. The well-known ``Trifid Nebula'' is also included in the field of the photograph, which covers a truly marvelous region, so intricate in its mingling of nebulæ, star-clusters, star-swarms, star-streams, and dark vacancies that no description can do it justice. Yet, chaotic as it appears, there is an unmistakable suggestion of unity about it, impressing the beholder with the idea that all the different parts are in some way connected, and have not been fortuitously thrown together. Miss Agnes M. Clerke made the striking remark that the dusky lanes in M8 are exemplified on the largest scale in the great rift dividing the Milky Way, from Cygnus in the northern hemisphere all the way to the ``Cross'' in the southern. Similar lanes are found in many other clusters, and they are generally associated with flanking rows of stars, resembling in their arrangement the thick-set houses and villas along the roadways that traverse the approaches to a great city.

But to return to the black gaps. Are they really windows in the star-walls of the universe? Some of them look rather as if they had been made by a shell fired through a luminous target, allowing the eye to range through the hole into the void space beyond. If science is discretely silent about these things, what can the more venturesome and less responsible imagination suggest? Would a huge ``runaway sun,'' like Arcturus, for instance, make such an opening if it should pass like a projectile through the Milky Way? It is at least a stimulating inquiry. Being probably many thousands of times more massive than the galactic stars, such a stellar missile would not be stopped by them, though its direction of flight might be altered. It would drag the small stars lying close to its course out of their spheres, but the ultimate tendency of its attraction would be to sweep them round in its wake, thus producing rather a star-swarm than a vacancy. Those that were very close to it might be swept away in its rush and become its satellites, careering away with it in its flight into outer space; but those that were farther off, and they would, of course, greatly outnumber the nearer ones, would tend inward from all sides toward the line of flight, as dust and leaves collect behind a speeding motor (though the forces operating would be different), and would fill up the hole, if hole it were. A swarm thus collected should be rounded in outline and bordered with a relatively barren ring from which the stars had been ``sucked'' away. In a general sense the M8 cluster answers to this description, but even if we undertook to account for its existence by a supposition like the above, the black gaps would remain unexplained, unless one could make a further draft on the imagination and suggest that the stars had been thrown into a vast eddy, or system of eddies, whose vortices appear as dark holes. Only a maelstrom-like motion could keep such a funnel open, for without regard to the impulse derived from the projectile, the proper motions of the stars themselves would tend to fill it. Perhaps some other cause of the whirling motion may be found. As we shall see when we come to the spiral nebulæ, gyratory movements are exceedingly prevalent throughout the universe, and the structure of the Milky Way is everywhere suggestive of them. But this is hazardous sport even for the imagination -- to play with suns as if they were but thistle-down in the wind or corks in a mill-race.

Another question arises: What is the thickness of the hedge of stars through which the holes penetrate? Is the depth of the openings proportionate to their width? In other words, is the Milky Way round in section like a rope, or flat and thin like a ribbon? The answer is not obvious, for we have little or no information concerning the relative distances of the faint galactic stars. It would be easier, certainly, to conceive of openings in a thin belt than in a massive ring, for in the first case they would resemble mere rifts and breaks, while in the second they would be like wells or bore-holes. Then, too, the fact that the Milky Way is not a continuous body but is made up of stars whose actual distances apart is great, offers another quandary; persistent and sharply bordered apertures in such an assemblage are a priori as improbable, if not impossible, as straight, narrow holes running through a swarm of bees.

The difficulty of these questions indicates one of the reasons why it has been suggested that the seeming gaps, or many of them, are not openings at all, but opaque screens cutting off the light from stars behind them. That this is quite possible in some cases is shown by Barnard's later photographs, particularly those of the singular region around the star Rho Ophiuchi. Here are to be seen somber lanes and patches, apparently forming a connected system which covers an immense space, and which their discoverer thinks may constitute a ``dark nebula.'' This seems at first a startling suggestion; but, after all, why should their not be dark nebulæ as well as visible ones? In truth, it has troubled some astronomers to explain the luminosity of the bright nebulæ, since it is not to be supposed that matter in so diffuse a state can be incandescent through heat, and phosphorescent light is in itself a mystery. The supposition is also in accord with what we know of the existence of dark solid bodies in space. Many bright stars are accompanied by obscure companions, sometimes as massive as themselves; the planets are non-luminous; the same is true of meteors before they plunge into the atmosphere and become heated by friction; and many plausible reasons have been found for believing that space contains as many obscure as shining bodies of great size. It is not so difficult, after all, then, to believe that there are immense collections of shadowy gases and meteoric dust whose presence is only manifested when they intercept the light coming from shining bodies behind them.

This would account for the apparent extinguishment of light in open space, which is indicated by the falling off in relative number of telescopic stars below the tenth magnitude. Even as things are, the amount of light coming to us from stars too faint to be seen with the naked eye is so great that the statement of it generally surprises persons who are unfamiliar with the inner facts of astronomy. It has been calculated that on a clear night the total starlight from the entire celestial sphere amounts to one-sixtieth of the light of the full moon; but of this less