What could hardly have been foreseen fifty years ago, is that these various classes of results are now made to combine and converge upon the greatest problem which the mind of man has ever attempted to grasp--that of the structure of the universe. The study of variable stars has suddenly fallen into line, so to speak, so that now, it is uniting itself to the study of all the other subjects to give us at least a faint conception of what the solution of this problem may be. One of the principal objects of the present chapter is to make a comparison of these various researches, and discuss the views respecting the constitution of the stars individually, as well as of the universe as a whole, to which they lead us. But there are a number of details to be considered singly before we can combine results in this way. Our early chapters will therefore be devoted to the special features and individual problems of stellar astronomy which have occupied the minds of astronomers from the beginning of their work to the present time. Keeping these details in mind, we can profitably proceed to the consideration of the general conclusions to be drawn from them. We may begin by refreshing our memories on some points, an understanding of which must be taken for granted. What are familiarly known as the heavenly bodies belong to two classes. Those nearest to us form a sort of colony far removed from all the others, called the solar system. The principal bodies of this system are the sun and eight great planets with their moons, revolving round it. On one of the planets, small when compared with the great bodies of the universe, but large to our every-day conceptions, we dwell. The other planets appear to us as stars. Four of them, Venus, Mars, Jupiter and Saturn, are distinguished from the fixed stars by their superior brightness and characteristic motions. Of the remaining three, Mercury will only rarely excite notice, while Uranus and Neptune are as good as invisible to the naked eye. The dimensions of the solar system are vast when compared with any terrestrial standard. A cannon shot going incessantly at its utmost speed would be a thousand years in crossing the orbit of Neptune from side to side. But vast as the dimensions are, they sink into insignificance when compared with the distance of the stars. Outside the solar system are spaces which, so far as we know, are absolutely void, save here and there a comet or a meteor, until we look far outside the region which a cannon shot would cross in a million of years.