Figure 24 is a superb effort made by rhe Mount Wilson Observatory with the largest telescope on Earth, the 100- inch reflector. It shows a general view of the great Nebula M31 in Andromeda, or Region 2. It is among the most impressive of all astronomical photographs. M31 just barely is visible to the unaided eye. Its existence has been known since 1612, but only when the fine photographic equipment of recent years became available did we recognize what, this colossal aggregation of matter was like. It is estimated to be 900,000 to 1,000,000 light years distant. If this estimate is approximately correct, then its diameter must be something of the order of 30,000 light years. Its spectrum indicates that it is not a gas, but instead a colossal aggregation of separate suns, too far away for us Earthians yet to resolve into individual stars.
When we look upon this photograph, we may feel the thrill of contemplating another universe. It has countless thousands of suns like our Sun, for the haze we see is made up of individual suns, each operating in its own orbit and possibly some accompanied by a family of planets. An Earthian contemplating this picture acquires a feeling of reverence. It causes him to realize faintly the omnipotence of the Architect of this great Cosmic Machine and to be glad that we are a part of it, small and unimportant though this part nay be. It inspires a feeling of superlative satisfaction and hope to behold the spectacle that you and I present, sitting here on our insignificant little speck of dust, looking out upon the transcendent scene with at least partly understanding eyes, studying, measuring, and weighing with an attitude of confidence and detachment.
M31 is approaching the locality in space that we now occupy at a rate of 180 miles per second, or 648,000 miles per hour. At least it was doing so 900,000 years, ago, which is the latest news we have received. If we photograph the nebula to-night, we do it by light that left there 900,000 years ago. My ancestors and yours were swinging from trees and chattering at each other at that time. It is indeed impressive to think that light leaving M31 and traveling a velocity of 186,000 miles per second has required to reach Earth all the time that man has spent in developing from a wild animal to the creature that you and I represent to-day.
Figure 25 is another picture of M31. It is an enlarged view of the central portion of the nebula. The very center is seen to be a solid white light. We are not positive that this center is composed of individual stars, for it may be a mass of glowing gas still giving birth to millions of suns. The black portions are probably opaque matter in our own galaxy which shuts off the light from M31. This picture is another of those profoundly impressive productions of the 100-inch reflector at Mount Wilson.
Figure 26 is still a third picture of M31, and it is an attempt to resolve a small portion of its haze into individual stars. It is the last word that the astronomers are able to tell us about the “great Beyond.” Again does it lead us to ask the question, of all these suns, are there not some that have evolved planets? And if so, are not some of them hospitable to life?
In Figure 27 we have an enlargement of a selected portion of a photograph of M31. This is a masterpiece and represents the peak of our achievements in astronomical magnification. In the original photograph the haze can be seen to be made up of distinguishable specks. Each of these little specks is a sun that averages thousands of times the size of our Earth.
Figure 28 is a picture of Nebula M33 in Triangulum. It has the peculiarity of possessing a large number of “knots,” or concentrations of matter scattered through its spiral arms. It is fainter than M31, yet we estimate it to be some 850,000 light years distant. It is approaching in the line of present sight at the rate of about 44 miles per second. It is believed to represent a late stage in the development of a partially "finished" galaxy, such as our Milky Way galaxy.
Figure 29 is intended to show the types-of extra-galactic nebulae that exist and also the stages of evolution of a galaxy. EO NGCC 3379 represents an- early stage of development of a "Milky Way." It is thought to have a vast mass of gas. E2 NGC 221 represents a later stage of development. The rotary motion has begun to swell the center of the mass. E5 NGC 4621 shows a still later development with the "dish” shape well begun. E'7 NGC 3115 shows the "dish" in the stage of development of our own Milky Way galaxy. Are we looking at galaxies in the process of development when we regard these pictures? Our descendants will know the answer if they advance in their achievements as fast as we have advanced.
Figure 30 shows a late stage of development The Nebula has spread out in diameter and shrunk in central thickness, and we have something that certainly resembles our own Mi1ky Way. The thin outer edge has actually radiated enough hat to show black in a photograph. This photograph is regarded as another masterpiece It is one of those extraordinarily lucky combinations. The nebula is almost precisely edge on, and also its stage of development appears to be just right- to offer the best sort of evidence. The picture is tremendously impressive. It typifies our intellectual status, in that it evidences the solution of the host of highly technical problems we had to master in order that we could bring to the pages of this book this detailed picture of a nebula so deeply sunk in space that its existence would never be suspected had we to depend upon our unaided eyes.
It now becomes desirable to get an idea of what views other than edge-on views would disclose. Fortunately there are so many of these island universes that we are able to get views from any desired angle.
Figure 31 is a picture of an object to be found in Canes Venatici or Region 5, where we also find our old friend the "Great Dipper" or Ursa Major. It is known as M51 or the "Whirlpool Nebula." One look at this "plan view” is enough to show the spiral effect. We shall see as we proceed that it is common to nebulae that have passed out of the gaseous stage. The spectroscope shows dark lines in the spectrum of M51, and we know it is made up principally of individual stars. There appears to be a movement of matter outward along the spiral arms. One large lump at the end of one arm indicates a cataclysm of some sort. We do not know what to say about such evidence, but we are reminded of that episode which happened to our Sun and gave us birth. We have not had time to understand what is going on. Ali we dare say is that we are looking at another universe which probably has passed out of the gaseous stage and is on its way through its life cycle.
Figure 32 is a picture of a nebula tnat appears to have reached a still later stage of development. It is M101 in Region 5. The spiral effect is still evident. The matter in this nebula seems to have segregated into clots more than has been the case in those previously considered. The spectroscope shorn s these dots to be clusters of stars rather than chaotic, disordered masses of gas. We believe there is evidence of the movement of matter outward along the spiral arms. To prove ir photographically would mean that several thousands of years must elapse between photographs. Since we have only begun taking really good photographs within tire last 50 years, it is evident that we shall have to find some other way to detect motion.
This completes the pictorial evidence ir is necessary to have in order to form a concept of what exists in outer space. There is no evidence that anything markedly different in general character would be found were we able to penetrate deeper than we have penetrated thus far. In the seventeenth century the advanced astronomer had a fairly definite idea of our Solar System and suspected there was something out beyond it. In the eighteenth century the Milky Way was fairly well understood, and suspicions were aroused that there was something out beyond it. In the nineteenth century it was certain that there were other systems remote in space; and in the twentieth century we have coordinated these systems and acquired another suspicion, this time that there is nothing in the cosmos radically different from that which we already have seen.
Our knowledge is limited to what the 100-inch reflector at Mount Wilson has told us. We expect that the 200-inch, now in course of construction, will increase the number of island universes that we can photograph, but not that it will disclose any new and different sort of celestial organization from what we now know.
The number of island universes that the 100-inch reflector indicates to-day is of the order of 2 millions. The new 200-inch is expected to disclose the existence of something like 16 millions. The separation between these universes appears to be something like 2 million light years. The greatest depth that we have been able to penetrate into space so far is about 140 million light years.
Each nebula or island universe may be considered as being made up of not less than 2,000 million suns. We suspect our Milky Way island universe may be the largest we have seen. It has been estimated as being of the order of 250,000 light years in diameter, and we have reason to believe that it rotates at the rate of something like one revolution, in 300 million years. We suspect that Nebula M31 rotates once in 19 million years.
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