Astronomy for Everybody/Part 6/Chapter 2

Not only to the ordinary beholder, but to the learned student of the heavens, the most wonderful feature of the sky is the Milky Way. This is a girdle apparently spanning the sky and perhaps, in reality, spanning the entire universe of stars, uniting them, as it were, into a single system—one "stupendous whole." It may be seen at some time of the night every day of the year, and at some convenient hour in the evening of every month except May. During this month it extends round the horizon in the early evening, and is invisible through the denser strata of the air. Of course it will even then become visible in the east and northeast later at night.

The smallest telescope will show the Milky Way to be formed of immense congeries of stars, too faint in their light to be separately visible at their great distance from us. Careful observation, even with the naked eye, will show that these stars are not equally scattered along the whole extent of their course, but are frequently collected in great masses or clusters, with comparatively empty spaces around or between them. These are especially marked in the portions of the belt visible in the south in the evenings of summer and autumn.

A remarkable fact connected with the universe is that ​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 ​the sixth magnitude, which included the faintest stars that the best eye could see under a clear sky.

Modern astronomers carry this system down to the telescopic stars. Those which are one degree fainter than the smallest visible to the naked eye are called of the seventh magnitude; the next in brightness are of the eighth, and so on. The faintest that can be seen or photographed with the largest telescopes are probably of the fifteenth, sixteenth, or seventeenth magnitude.

The reader will of course understand that the magnitude of a star does not express its real brightness, because a shining body looks brighter the nearer it is to us. No matter how bright a star may be, if it were removed far enough away it would grow so faint as to be invisible. The smallest star in the heavens if brought near enough to us would shine as of the first magnitude.

It was formerly believed that the actual brightness of the different stars was nearly the same, and that some looked brighter than others only because they were nearer to us. But the case is now known to be different. Estimates of the distance of the stars show that among the nearest to us are many quite invisible to the naked eye, while some of the first magnitude are so far away that their distance is immeasurable. The brightest ones probably emit hundreds of thousands of times as much light as the smallest ones.

The whole number of stars in the heavens which can be seen by the ordinary eye is between five and six thou​sand. Possibly a very keen eye might see more than six thousand, but most eyes will see even less than five thousand. Of these only one half can be above the horizon at the same time, and of this half a great number will be so near the horizon as to be obscured by the great thickness of the atmosphere in that direction. The number which can readily be seen on a clear evening by an ordinarily good eye will probably range between fifteen hundred and two thousand. Stars visible to the naked eye are called lucid stars, to distinguish them from telescopic stars, which can be seen only by the aid of a telescope.

It is impossible to make even an estimate of the total number of telescopic stars. It is commonly supposed that between fifty and one hundred million can be seen with large telescopes, and it is now possible, with specially arranged telescopes, to photograph stars which are fainter than the smallest the eye can see in any telescope. There is no sign of any limit to the number. As we pass to fainter and fainter degrees of brightness the stars are found to be more and more numerous. All that we can say of the total number is that it must be counted by hundreds of millions.

We have, in fact, some reason for inferring that the great majority of the stars are invisible in the most powerful telescope we can make, owing to their distance. The distance of the great majority is such that only the brightest of them can become known to us.

Minute stars are here and there collected into clusters in various parts of the sky. Some of these clusters are ​visible to the naked eye. Those in and near the Milky Way frequently contain hundreds or even thousands of stars too small to be seen separately without a telescope. The stars differ from each other in colour, although not in so marked a degree as terrestrial objects. The most casual observer cannot fail to note the difference between the bluish white of Alpha Lyrae and the reddish light of Arcturus. There seems to be a regular gradation in the colour of the stars from blue, through yellow, to red. These differences of colour are connected with differences in the spectra of the stars. As a general rule, the redder a star is, the greater the number and intensity of the dark lines that can be seen in the green and blue parts of its spectrum.

A slight examination of the heavens shows that the stars are not scattered equally over the sky, but that there is more or less of a tendency to collect into constellations. This is especially the case with the brighter stars. But no well-marked dividing line between the constellations is possible; that is, we cannot draw a line showing exactly where one constellation ends and another begins. Nevertheless a division into constellations was made in ancient times and has been followed by astronomers down to the present time.

How and by whom the constellations were first mapped out and named no one knows. The Chinese had their asterisms—collections of stars smaller than what we call constellations—in the earliest years of their history. ​What we know of the constellations dates from Ptolemy, who lived in the second century after Christ. His names are still in use. As many of them are those of the gods, goddesses, and heroes of Grecian mythology—Perseus, Andromeda, Cepheus, Hercules, etc.—it seems likely that they were assigned during or after the heroic age.

In modern times quite a number of new constellations have been carved out of or drawn between the older ones. This is especially the case in the southern hemisphere, which was imperfectly known to the ancient Greeks.