"The Stars," using that expression in its most general sense, do not lie at one fixed distance from us, set here and there upon a background of sky. There is in fact no background at all. The brilliant orbs are all around us in space, at different distances from us and from each other; and we can gaze between them out into the blackness of the void which, perhaps, continues to extend unceasingly long after the very outposts of the stellar universe has been left behind. Shall we then start our imaginary express train once more, and send it out towards the nearest of the stars? This would, however, be a useless experiment. Our express-train method of gauging space would fail miserably in the attempt to bring home to us the mighty gulf by which we are now faced. Let us therefore halt for a moment and look back upon the orders of distance with which we have been dealing. First of all we dealt with thousands of miles. Next we saw how they shrank into insignificance when we embarked upon millions. We found, indeed, that our sixty-mile-an-hour train, rushing along without ceasing, would consume nearly the whole of historical time in a journey from the sun to Neptune. In the spaces beyond the solar system we are faced, however, by a new order of distance. From sun to planets is measured in millions of miles, but from sun to sun is measured in billions. But does the mere stating of this fact convey anything? I fear not. For the word "billion" runs as glibly off the tongue as "million," and both are so wholly unrealisable by us that the actual difference between them might easily pass unnoticed. Let us, however, make a careful comparison. What is a million? It is a thousand thousands. But what is a billion? It is a million millions. Consider for a moment! A million of millions. That means a million, each unit of which is again a million. In fact every separate "1" in this million is itself a million. Here is a way of trying to realise this gigantic number. A million seconds make only eleven and a half days and nights. But a billion seconds will make actually more than thirty thousand years! Having accepted this, let us try and probe with our express train even a little of the new gulf which now lies before us. At our old rate of going it took almost two years to cover a million miles. To cover a billion miles--that is to say, a million times this distance--would thus take of course nearly two million years. Alpha Centauri, the nearest star to our earth, is some twenty-five billions of miles away. Our express train would thus take about fifty millions of years to reach it!