Page:Popular Science Monthly Volume 87.djvu/220

216 hemisphere, counted the stars visible in the eyepiece, 15 minutes of arc in diameter, in 7,300 regions distributed rather uniformly over the entire sky. They found that the number of stars decreased rapidly as they passed from the central plane of the Milky Way toward the north and south poles of the galaxy. Here is a table deduced by Struve from the Herschels' counts.

The average number of stars in the Milky Way zone 30° wide, that is, in galactic latitude +15 to -15, visible in the eyepiece of the telescope, was 56, whereas in the region surrounding the north and south galactic poles the average number visible in the same eyepiece was but 5. The great condensation in the Milky Way is not fully evident from the table. The stars are much more numerous near the central line of the Milky Way than they are near its borders. The average number along the central line, found by Sir William Herschel, was 122. There is no reason to doubt that the preponderance of stars visible in the direction of the Milky Way is due to the greater extension of the stellar system in that direction than in the direction of the galactic poles.

It has been noted by several observers that the faintest stars visible in telescopes of moderate size, that is, stars of the 14th, 15th and 16th magnitudes, are plentiful in the Milky Way and very scarce at a distance from the Milky Way. The contrast between Milky Way and non-Milky Way regions is scarcely noticeable in the naked eye stars, but it becomes stronger and stronger as we pass to the fainter stars.

If there is an absorption of light in its passage through space, such that the very distant stars are appreciably reduced in brightness, then the stars of average size and physical condition must be invisible to us when they are farther away than a certain limiting distance, and in that case the extent of the universe in the direction of the Milky Way