Each spiral arm of the Milky Way consists of millions of stars. These stars are so far away from us that they are too faint to be pointed out individually without the help of powerful telescope. If one begins counting stars in the part of the space where the Milky Way does not pass, he finds certain numbers of them; as he counts in the regions nearer the Milky Way, the numbers are greater. The numbers are greatest in the middle line of the Milky Way. Another important point: The number of very faint stars in the direction of the Milky Way is very high. While it is true that we cannot judge by the apparent brightness of a star whether it is a nearby one or a distant one, when we deal with numerous stars we can say that, on an average, the faint stars are distant ones. So if we find many faint stars in the Milky Way we can say that there are many distant stars there.
Astronomers have been able to build up a picture of the system of stars, the Milky Way, in which we live. It is shaped somewhat like a convex lens. (Picture, above). To tell the number of stars by direct counting of all of them would take us many lifetimes, so what astronomers do is to take samples here and there all over the space, then make allowance for the parts of the sky where we have not counted. As a result, astronomers believe there are at least 1,00,00,00,00,000 stars in the Milky Way Galaxy. Our solar system is not situated in the middle of the Milky Way Galaxy; we are more than halfway from the center toward one edge. When we look toward the center, in the direction of the constellation Sagittarius and the tail of Scorpius, we see the greatest number of stars. That is why that part of the Milky Way appears so bright. When we look in the opposite direction, away from the center, we see many stars, but not so many as in the center. When we look up or down through the thin part of the ‘lens,’ we see the part of the space where there is no Milky Way. There is not such a thick array of stars there, so we do not see so many.
The Milky Way is very large. The distance from one side of rim to the opposite is 1,00,000 light years; and we are about 30,000 light years from the center. Outside, there is empty space; inside, there is much empty space, too, for there is much space between any two of the stars. If you can imagine one tiny particle of dust in the center of an average-sized room, it will compare with the space which the average star has for itself. One of these stars is the Sun, the particular star around which the Earth, the other planets and the comets revolve. From outside our Galaxy, the Sun would be quite indistinguishable from the cluster of other stars like it. Far off among the stars, we find not only pairs, but even clusters of dozens or hundreds of stars.
One kind of cluster is found principally in the Milky Way, so it is called the galactic cluster. We can call it a loose cluster, for the stars are loosely assembled with no apparent regularity. One of the best-known of these loose star clusters is the little group called the Pleiades, or Seven Sisters. Only six stars are visible to the naked eye, but we can see hundreds of stars using a powerful telescope.