Careful observation shows that the stars differ slightly in color. It is true that the colors are less marked than those of natural objects, there being no such red as The ruby and no such bright green as the emerald among the stars. But there is a regular gradation from a yellowish red through white, to a well-marked, bluish tint. These differences of color are supposed to indicate different degrees of tem perature of the stars, or different periods in their growth and development.
Number of The whole number of stars in the heavens, visible to the naked eye, is about 5,000. As one-half of these are neces sarily below the horizon at any one time, it fol lows that not more than 2,500 can be seen at once. But this is greater than the actual num ber visible under ordinary conditions, because small stars near the horizon are rendered dim or invisible by the thickness of the air through which we see them. Practically, it is not likely that more than 2,000 stars can ever be seen at once by the best eye, unless at elevated points where the air is very clear down to the horizon.
Up to the present time it has been impossible to make anything like an enumeration of the telescopic stars, because, as we have already said, every increase in the power of the tele scope brings out new stars that otherwise would be invisible. There is, however, no doubt that the stars visible in a large telescope are to be counted by tens of millions. It is now found that stars can be photographed with a large telescope which are so small that the eye would not perceive them in the telescope itself. It is probable that the whole number found on the best photographic plates amounts to more than 100,000,000. But we must not suppose that this number would actually comprise all the stars in the sky. Although in some direc tions we seem to have reached the limit of the visible stars, we do not know, when the entire heavens is considered, how many fainter stars there may be for every one that can be pho tographed. A very recent estimate fixes the total number of the stars at 1,600,000,000, but this is as yet necessarily very uncertain.
The Milky feature of the sky connected with the stars is the Milky Way or Galaxy. This really stupendous though not striking object can be seen at some hour of any night in the year, its position in the sky depending on the season. In our latitudes it is quite near, partly below the horizon during the evenings of May, and is then not readily seen, but even then it may be seen later in the evening. At the proper time, on any night, it
passes nearly through the zenith, spanning the heavens like a complete arch, resting on two opposite points of the horizon.
Galileo, pointing his first telescope to the sky, found that this object was composed of congeries of stars too faint to be separately visible. This fact has been abundantly proved by modern observations. A very little study of the sky without a telescope will show that the milky light of these stars is by no means uni form, but is frequently gathered into clouds. These show thal the Milky Way is in part made up of vast groups or collections of stars, rather irregular in form. The aspect of the Milky Way is also very different in different parts of its course. One of its most remarkable features is best seen in summer and autumn. It will then be noticed that, in the region of the zenith, the Milky Way seems to he divided into two separate streams with a comparatively dark space between them. This seeming cleft ex tends toward the south, nearly to the celestial equator, where one of the streams branches off toward the west and comes to an end, while the other continues on its course. It will also be noticed that at some points of its course the Galaxy is narrow and well defined, while in others, notably in the extreme south, it is wide and diffuse.
Many astronomers have occupied themselves in making photographs of various portions of the Milky Way. Barnard has used a large camera for this purpose, and, in several obser vatories, the largest telescopes have been so employed. One curious feature is brought out by these photographs. In that part of the Milky Way which in autumn is seen between the celestial equator and the southern horizon, numerous clefts and vacant spaces occur, as though there were actual openings through the mass of stars. This fact seems to show that the thickness of the Milky Way, or the distance, through which the line of sight extends when we look at it, is less here than elsewhere. Going farther south, into regions invisible in our northern latitudes, other vacant spaces are found, known as coal-sacks. But it is found that stars really exist in these spaces, though they are comparatively few and faint.