CHAPTER 1. THE STARS AND NEBULÆ. LESSON I.-Magnitudes and Distances of the Stars. Shape of our Universe. 66 22. THE first thing which strikes us when we look at the stars is, that they vary very much in brightness. All of those visible to the naked eye are divided into six classes of brightness, called "magnitudes," so that we speak of a very brilliant one as a star of the first magnitude:" of the feeblest visible as a star of the sixth magnitude, and so on. The number of stars of all magnitudes visible to the naked eye is about 6,000; so that the greatest number visible at any one time-as we can only see one half of the sky at once-is 3,000. If we employ a small telescope this number is largely increased, as that instrument enables us to see stars too feeble to be perceived by the eye alone. For this reason such stars are called telescopic stars. The stars thus revealed to us still vary in brightness, and the classification into magnitudes is continued down to the 12th, 14th, 16th, or even lower magnitudes, according to the power of the telescope; in powerful telescopes at least 20,000,000 stars down to the 14th magnitude are visible. 23. A star of the sixth magnitude is, as we have seen, the faintest visible to the naked eye. It has been estimated that the other stars are brighter than one of the sixth magnitude, by the number of times shown in the following table :- 24. Now it is evident that these stars, as they all shine out with such different lights, one star differing from another star in glory, are either of the same size at very different distances, the furthest away being of course the faintest; or are of different sizes at the same distance, the biggest shining the brightest; or are of different sizes at different distances. Where the actual distances of the stars are known we can be certain; but from other considerations it is most probable that the difference in brilliancy is due to difference of distance, and not to size. 25. The distances of the stars from us are so great that it scarcely conveys any impression on the mind to state them in miles; some other method, therefore, must be used, and the velocity of light affords us a convenient one. Light travels at the rate of 186,300 miles in a second of time—that is to say, between the beats of the pendulum of an ordinary clock, light travels a distance equal to eight times round the earth. 26. In spite, however, of this great remoteness, the distances of some of them are known with considerable accuracy. Thus, leaving the sun out of the question, we find that the next nearest is situated at a distance which light requires four years and four months to traverse. 27. From the measurements already made, we may say that, on the average, light requires fifteen and a half years to reach us from a star of the first magnitude, twenty-eight years from a star of the second, forty-three years from a star of the third, and so on, until, for stars of the 12th magnitude, the time required is 3,500 years. 28. Winding among the stars, a beautiful belt of pale light spans the sky, and sometimes it is so situated, that we see that it divides the heavens into two nearly equal portions. This belt is the Milky Way; and the smallest telescope shows that it is composed of stars so faint, and apparently so near together, that the eye can only perceive a dim continuous glimmer. 29. We find the largest stars scattered very irregularly, but if we look at the smaller ones, we find that they gradually increase in number as their position approaches the portion of the sky occupied by the Milky Way. In fact, of the 20,000,000 stars visible, as we have stated, in powerful telescopes, at least 18,000,000 lie in and near the Milky Way. This fact must be well borne in mind. 30. Adding this fact to what has been said about the distances of the stars, we can now determine the shape of our stellar system. It is clear that it is most extended where the faintest stars are visible, and where they appear nearest together; because they appear faint in consequence of their distance, and because their close packing does not arise from their actual nearness to each other, but results from their lying in that direction at constantly increasing distances. Indeed, the stars which give rise to the appearance of the Milky Way, because in that part of the heavens they lie behind each other to an almost infinite distance, are probably as far from each other as our sun is from the nearest star. 31. The Milky Way, then, indicates to us, and traces for us, the direction in which the system has its largest dimensions; the absence of faint stars in the parts of the sky furthest from the Milky Way shows us that the limits of our system in that direction are much sooner reached than in the direction of the Milky Way itself. We gather, therefore, that its thickness is small compared with its length and breadth. This flat stratum of stars is split, as we might split a round piece of thick cardboard, in those regions where we see the Milky Way divided into two branches, and here its edge is double. Our sun is situated near the point at which the mass of stars begins to divide itself into two portions; and, as there are more stars on the south side of the Milky Way than there are on the north, we gather that our earth occupies a position somewhat to the north of the middle of its thickness. 32. But although the Milky Way thus enables us to get a rough idea of the shape of our system as we might get a rough idea of the shape of a wood from some point within it by seeing in which direction the trees appeared densest and thickest together, and in which direction it was most easy to pierce its limits, still what the telescope teaches us shows that its boundaries are most probably very irregular. 33. The Magellanic Clouds, called the Nubecula Major and Nubecula Minor, visible in the southern hemisphere, are two cloudy oval masses of light, and are very like portions of the Milky Way, but they are apparently unconnected with its general structure. LESSON II.-The Constellations. Movements of the Stars. Movements of our Sun. 34. We have in the last lesson considered our starsystem as a whole; we have discussed its dimensions, and given an idea of its shape. Before we proceed with a |