84 THE UNIVERSE A VAST SYSTEM OF PARTS class of these variable stars known as the Cepheid Variables, named after one of the stars in the group, Delta Cephei, discovered that the period of variation is related to the intrinsic brightness. Once the intrinsic brightness of a star is known, its distance can be calculated, of course, by methods already outlined. W^k^. Fig. 18. The Spiral Nebula in Triangulum, Messier 33. (Photograph from the Yerkes Observatory, reprinted by permission of the Chicago University Press.) It was found that many of the exterior galaxies contain Cepheid Variables which enable us to form our present conclusions concerning the distance of these galaxies from ours. Note that it would no longer add meaning to refer such distances to the earth or the sun. Extragalactic Nebulae Move at Various Speeds. There is a definite rotational motion in spiral nebulae. Photographs taken at ten-year intervals show that the spiral nebula, Andromeda, makes one rotation about every 17,000,000 years. Some astronomers have suggested that our galaxy, too, is spiral in shape; but little evidence is available. We are too much a part of it to see it in the proper perspective. There is evidence, however, that our galaxy rotates once in about every 100,000,- 000,000 years. The exterior galaxies are the "speed demons" of the universe; at least the shift in the Fraunhofer lines indicates that Andromeda is approaching our galaxy at a speed of nearly 200 miles per second. of these galaxies seem to be moving in the same general direction, perhaps as a single organization of supergalaxies, which Shapley would call a metagalaxy. The fact that all of the galaxies seem to be receding from one another has led to the theory of an expanding universe. All
OUR GALAXY IS BUT ONE OF MANY GALAXIES 85 It is well at this point to emphasize the fact that our present ideas concerning the nature of the universe are founded on scientific observations but that they are subject to a considerable change due to the meagerness of our information and judgment. In the next Section we shall return to our own little Solar System. Do not think for a moment, however, that nothing more remains to challenge the imagination. For as we turn to the study of smaller things — the sun, the earth, ourselves, down to microscopic objects — and still others out beyond the range of the microscopic objects — we again become confronted with the unknown. But now let us hasten on to the study of the vast known that lies between these realms, for our penetration of the unknown is all based on this firm foundation. How Important Is Man in This Vast Scheme of Things? Man now wonders what his position in the universe is. It is true that the modern revelations of the universe have administered a rude shock to man's conception of his relative importance; but on the other hand, he gains respect for himself.as he comes to realize what knowledge his intellect has made possible. It is well to keep in mind that although, astronomically speaking, man is very small, still astronomically speaking, he is the astronomer. STUDY QUESTIONS 1. What is the nature of the extragalactic nebulae according to modern theories? 2. How many extragalactic nebulae are there? 3. What forms do extragalactic nebulae take? 4. What is the distance of the Andromeda Spiral from the earth? 5. Discuss the motion of extragalactic nebulae. 6. How has the picture of the universe, as revealed by modern astronomy, changed your thoughts concerning yourself and your importance in the vast cosmic scheme?