to which our attention is directed in the present work. If we are asked what they are, we may reply that the stars are suns. But we might equally well say that the sun is one of the stars; a small star, indeed, surrounded by countless others, many of which are much larger and brighter than itself. We shall treat our theme as far as possible by what we may call the natural method, beginning with what, being most obvious to the eye, was first noticed by man, or will be first noticed by an observer, and tracing knowledge up step by step to its present state.
Several features of the universe of stars will be evident at a glance. One of these is the diversity of the apparent brightness, or, in technical language, of the magnitudes of the stars. A few far outshine the great mass of their companions. A greater number are of what we may call medium brightness; there is a yet larger number of fainter ones, and about one half of all those seen by a keen eye under favorable conditions are so near the limit of visibility as to escape ordinary notice. Moreover, those which we see are but an insignificant fraction of the number revealed by the telescope. The more we increase our optical power, the greater the number that come into view. How many millions may exist in the heavens it is scarcely possible even to guess. The photographic maps of the heavens now being made probably show fifty millions, perhaps one hundred millions or more.
Another evident feature is the tendency of the brighter stars to cluster into groups, known as constellations. The latter are extremely irregular, so that it is impossible to decide where one constellation should end and another begin, or to which constellation a certain star may belong. Hence, we can neither define the constellations nor say what is their number, and the division of the stars among them is a somewhat arbitrary proceeding.
A third feature is the Milky Way or Galaxy, which, to ordinary vision, appears as an irregular succession of cloud-like forms spanning the heavens. We now know that these seeming clouds are really congeries of stars too small to be individually visible to the naked eye. We shall hereafter see that the stars of the Galaxy form, so to speak, the base on which the universe appears to be constructed. Each of these three features will be considered in its proper place.
II. Magnitudes of the Stars.
The apparent brightness of a star, as we see it from the earth, depends upon two causes—its intrinsic brilliancy or the quantity of light which it actually emits, and its distance from us. It follows that if all the stars were of equal intrinsic brightness we could determine their relative distances by measuring the respective amounts of light which we receive from them. The quantity of light in such a case varies inversely as the square of the distance. This will be made
