ONE of the curious things in the clear night sky is an irregular, narrow, cloudike haze which crosses directly overhead and extends from horizon to horizon. It is difficult to believe it is not an ordinary cloud. But it persists night after night. No night is clear enough to eliminate it. On tire contrary, the clearer the night, the more dlistinctly it stands out.
It has caused wonder since remote antiquity and probably was among the early things noticed in the sky when men began to pick out details and to think about them. When men began to move about generally all over the face of the Earth, they noticed that this queer haze was visible in the Southern Hemisphere also, which, of course, looks out upon a different part of the cosmos from the Northern Hemisphere. This indicated that the cloud extended completely around the Earth. What possibly couid the thing be? Men asked themselves this question for centuries. They named it, but they could not account for it. It was referred to as the Miiky Way, a lowly sort of name, but pleasantly suggestive of the order of men's thoughts when the first crumbs of knowledge were being gathered up.
Galileo in 1610, only 323 years ago, solved the mystery when he pointed the first telescope skyward and impressed the first magnified image of the cosmic machine upon an intelligent retina. Under the magnification of his crude telescope the haze was resolved into an enormous number of individual stars. It probably was one of the great dramatic moments in the history of humanity, when, after all the millions of centuries, Galileo, the first intelligent creature on Earth to peer at the sky through a telescope, saw a part of the cosmos as it really is. A tremendous impetus was given to astronomical investigation, probably the greatest ever actuated before or since. Other men built better telescopes, now the way had been shown, and in three centuries it was established that the Milky Way was an aggregation of stars passing all comprehension as to numbers, was biscuit- or dish- shaped, and that our Sun was located nearly in its equatorial plane but somewhat off its center. The only way to acquire a definite conception of all this is to look at a picture. Figure 17 is a picture of our Milky Way galaxy as we imagine it looks when seen on edge. Our Sun is thought to be located where the black dot appears. It is doubtful if we shall ever photograph it, for we forever shall be inside it. All we can do to-day is to make drawings of it as we deduce from observation what it must be like.
Were we able to withdraw some 500,00 light years and view our galaxy from that distance, it would look about as shown in Figure 18. Quite evidently our Sun is part of a dish-shaped aggregation of suns. This "dish" is thicker in the center than it is at the edge. It is enormous in size, and it is spinning much as the fragments in Saturn's rings are spinning. Instead, however, of the units being small, cold fragments, they are hot suns. We shall see later that some immensely suggestive similarities exist between this aggregation of which we are a part and other aggregations that we shall see in various stages of development in other parts of space.
In order to gain a conception of what sections of the actual aggregation are like in "close up," turn to Figures 1, 19, and 20. These are reproductions of actual photographs of sections of the Milky Way galaxy. In looking at these photographs it should be realized that every individual white speck is a sun, more or less like our Sun; also that there is about as much space around each sun as there is around our Sun. The nearest body to our Sun, except its own planets, is Proxima Centauri, about 25 million million miles away or 4 1/4 light years. We might assume that the individual suns shown in these photographs are about this distance from each other. We must grasp this separation because it gives us an idea of the chances of collisions and near-collisions.
To bring the matter down to a comparison that we can come somewhere near comprehending, we may say that if we had a ship that could make the journey from Earth to Moon in a week, or across the Atlantic from New York to Southampton in two hours, this ship would take about two million years to travel from sun to sun in our Milky Way. It becomes quite evident that they are not as crowded as we might judge from the photographs. Thus, although we may speak of the suns as "dodging" each other as the 30,000 millions of them weave in and out amid the galaxy, yet we see that there is opportunity for considerable wandering about without their having actually to do any expert "dodging." Nevertheless, we shall find in a moment that the chances of the suns in the Milky Way having a near-collision are susceptible of fairly exact computation. Another feature in the photographs that calls for our consideration is the occurrence of dark areas. These dark areas appear in every picture of every star cloud. We do not believe they are "holes" through which we look into the depths of empty space. It is not likely that there would be so many holes all pointing exactly at our insignificant little Earth. It is generally agreed that these dark spaces are caused by some kind of opaque matter which shuts off the light from stars that are be, hind it.
When we study star-cloud photographs, we see that the dark areas are a considerable proportion of the entire area. One of the most striking examples of these mysterious areas is the "Horse's Head" in the great nebula in Orion, shown in Figure 21. It is particularly unfortunate that we are limited in our photography to luminous objects. Only an object that radiates or reflects light will impress itself upon a photographic plate. Our eyes are even more limited than the sensitive emulsion on a plate, for our eyes record only the light that exists between the violet and the red rays of the spectrum. All else is dark to us. The ordinary photographic plate will record the band beyond violet, known as the "ultra. violet," and also the X-rays. Very recently emulsions have been perfected that record bands beyond the red which we call the "infra-red." Whether we shall be able to photograph objects that radiate only to the extent, for example, that our Earth radiates remains for the future to determine. Our astronomers are so busy studying bright objects that almost no attention has been paid to the dark ones. Until we are able to measure temperature or obtain the spectra of dark bodies, or whatever will correspond to them, we shall be compelled to limit ourselves to speculation, as did our forefathers before the days of the telescope, the spectroscope and the camera. We are forced to pass on and leave a promising field unexplored. We shall return most certainly at a later day when we have the proper tools.
The satellites of the planets of our Solar System and the planets themselves are dark. They radiate very slightly so far as our knowledge goes. The radioactive materials in them radiate something, but it is not enough for us to utilize in our cameras as the latter exist to-day. Possibly this radioactive emanation will be of service to us as time goes on and we improve our equipment.
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