UntitledThe Scale of the UniverseHow many stars are there? A glance at a photograph of star-clouds willtell at once that it is quite impossible to count them. The finephotograph reproduced in Figure 2 represents a very small patch of thatpale-white belt, the Milky Way, which spans the sky at night. It is truethat this is a particularly rich area of the Milky Way, but the entirebelt of light has been resolved in this way into masses or clouds ofstars. Astronomers have counted the stars in typical districts here andthere, and from these partial counts we get some idea of the totalnumber of stars. There are estimated to be between two and threethousand million stars.Yet these stars are separated by inconceivable distances from eachother, and it is one of the greatest triumphs of modern astronomy tohave mastered, so far, the scale of the universe. For several centuriesastronomers have known the relative distances from each other of the sunand the planets. If they could discover the actual distance of any one
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