Page:Astronomy for Everybody.djvu/324

the stars are not equally thick in all directions, there being more in a given space around the belt of the Milky Way, and the number growing smaller as we pass away from that belt. This is true even of the brightest stars, and yet more true of the fainter ones. The poles of the Milky Way are those two points in the heavens which are ninety degrees from every point of the Milky Way. If we imagine one to hold a rod in his hand, so that the Milky Way shall be at right angles to it, the two ends of the rod will point to the two poles in question. To give an idea of the thickness of the stars we may say that, near the poles of the Milky Way, a round circle of the sky one degree in diameter will commonly contain two or three stars visible in quite a small sized telescope. In the region of the Milky Way, such a circle may contain eight, ten, perhaps even fifteen or twenty such stars.

No one can look at the sky without seeing that the stars differ enormously in their brightness, or, in the language of astronomy, in their magnitude. They resemble men in that a very few far outshine all their fellows, a greater number are less bright, and, as we come down to smaller and smaller stars, we find the number to continually increase. Those visible to the naked eye were classified by the ancient astronomers as of six orders of magnitude. About twenty of the brightest in the sky were designated as of the first magnitude. The forty next in order of brightness were called of the second magnitude; a larger number were of the third, and so on to