Page:Popular Science Monthly Volume 41.djvu/410



O ordinary observation the light of the stars seems to be constant. Although of various degrees of brilliancy, the brightness of each individual star appears to most people to be invariable. This is, of course, true with reference to the great majority of the stars which deck our midnight sky. There are, however, many objects the light of which is subject to considerable fluctuations. These are known as variable stars, and form one of the most interesting classes of objects visible in the stellar heavens. Over two hundred are now known to be certainly variable in light, and many others have been suspected of inconstancy. In some of these stars the changes of brightness can only be detected by careful watching, but in many the light is variable to a considerable extent. In the variable star Chi Cygni, for example, the star's light at maximum is about sixteen hundred times the light at minimum. At its brightest it is sometimes fairly visible to the naked eye, whereas in its faintest phase a pretty good telescope is required to see it at all.

These most interesting and mysterious objects have been divided into different classes, according to the character of the light-variation and the length of the period which completes the cycle of their curious changes. The classification now generally adopted is that proposed by Prof. Pickering, of the Harvard Observatory, U. S. A. This includes five classes, which are as follows: 1. Temporary or "new stars"; 2. Stars with regular periods of considerable length; 3. Irregular variables, having no definite period; 4. Variables of short period—say, under thirty days; and, 5. Variables of the type of Algol, or those which, at regular intervals, undergo a sudden (or comparatively sudden) diminution of brightness lasting for a few hours only, the star remaining constant in light (or nearly so) during the remainder of its period. A short account of these different classes may prove of interest to the general reader.

1. "Temporary" or "new stars" are perhaps the rarest phenomena visible in the heavens. Comets—at least, those visible to the naked eye are rare celestial visitors. Telescopic comets are, however, tolerably numerous, and scarcely a year passes without the discovery of several of these faint objects. Very few "new stars" have, however, been recorded in the annals of astronomical history. I refer, of course, to those which can properly be termed "new"—that is, stars the existence of which was previously unknown to astronomers, and which, blazing out suddenly, remained visible for a short time, and then faded away without again