Astronomy of To-day: A Popular Introduction in Non-Technical Language

by Cecil Goodrich Julius Dolmage

Certain remarkable groups of stars, of a nature similar to clusters, though not containing such faint or densely packed stars as those we have just alluded to, call for a mention in this connection. The best example of such star groups are the Pleiades and the Hyades ([see Plate XX.], p. 296), Coma Berenices, and Præsepe (or the Beehive), the last-named being in the constellation of Cancer.

Stars which alter in their brightness are called Variable Stars, or "variables." The first star whose variability attracted attention is that known as Omicron Ceti, namely, the star marked with the Greek letter ο (Omicron) in the constellation of Cetus, or the Whale, a constellation situated not far from Taurus. This star, the variability of which was discovered by Fabricius in 1596, is also known as Mira, or the "Wonderful," on account of the extraordinary manner in which its light varies from time to time. The star known by the name of Algol,[32] popularly called the "Demon Star"—whose astronomical designation is β (Beta) Persei, or the star second in brightness in the constellation of Perseus—was discovered by Goodricke, in the year 1783, to be a variable star. In the following year β Lyræ, the star in Lyra next in order of brightness after Vega, was also found by the same observer to be a variable. It may be of interest to the reader to know that Goodricke was deaf and dumb, and that he died in 1786 at the early age of twenty-one years!

It was not, however, until the close of the nineteenth century that much attention was paid to variable stars. Now several hundreds of these are known, thanks chiefly to the observations of, amongst others, Professor S.C. Chandler of Boston, U.S.A., Mr. John Ellard Gore of Dublin, and Dr. A.W. Roberts of South Africa. This branch of astronomy has not, indeed, attracted as much popular attention as it deserves, no doubt because the nature of the work required does not call for the glamour of an observatory or a large telescope.

The chief discoveries with regard to variable stars have been made by the naked eye, or with a small binocular. The amount of variation is estimated by a comparison with other stars. As in many other branches of astronomy, photography is now employed in this quest with marked success; and lately many variable stars have been found to exist in clusters and nebulæ.

It was at one time considered that a variable star was in all probability a body, a portion of whose surface had been relatively darkened in some manner akin to that in which sun spots mar the face of the sun; and that when its axial rotation brought the less illuminated portions in turn towards us, we witnessed a consequent diminution in the star's general brightness. Herschel, indeed, inclined to this explanation, for his belief was that all the stars bore spots like those of the sun. It appears preferably thought nowadays that disturbances take place periodically in the atmosphere or surroundings of certain stars, perhaps through the escape of imprisoned gases, and that this may be a fruitful cause of changes of brilliancy. The theory in question will, however, apparently account for only one class of variable star, namely, that of which Mira Ceti is the best-known example. The scale on which it varies in brightness is very great, for it changes from the second to the ninth magnitude. For the other leading type of variable star, Algol, of which mention has already been made, is the best instance. The shortness of the period in which the changes of brightness in such stars go their round, is the chief characteristic of this latter class. The period of Algol is a little under three days. This star when at its brightest is of about the second magnitude, and when least bright is reduced to below the third magnitude; from which it follows that its light, when at the minimum, is only about one-third of what it is when at the maximum. It seems definitely proved by means of the spectroscope that variables of this kind are merely binary stars, too close to be separated by the telescope, which, as a consequence of their orbits chancing to be edgewise towards us, eclipse each other in turn time after time. If, for instance, both components of such a pair are bright, then when one of them is right behind the other, we will not, of course, get the same amount of light as when they are side by side. If, on the other hand, one of the components happens to be dark or less luminous and the other bright, the manner in which the light of the bright star will be diminished when the darker star crosses its face should easily be understood. It is to the second of these types that Algol is supposed to belong. The Algol system appears to be composed of a body about as broad as our sun, which regularly eclipses a brighter body which has a diameter about half as great again.

Since the companion of Algol is often spoken of as a dark body, it were well here to point out that we have no evidence at all that it is entirely devoid of light. We have already found, in dealing with spectroscopic binaries, that when one of the component stars is below a certain magnitude[33] its spectrum will not be seen; so one is left in the glorious uncertainty as to whether the body in question is absolutely dark, or darkish, or faint, or indeed only just out of range of the spectroscope.

It is thought probable by good authorities that the companion of Algol is not quite dark, but has some inherent light of its own. It is, of course, much too near Algol to be seen with the largest telescope. There is in fact a distance of only from two to three millions of miles between the bodies, from which Mr. Gore infers that they would probably remain unseparated even in the largest telescope which could ever be constructed by man.

The number of known variables of the Algol type is, so far, small; not much indeed over thirty. In some of them the components are believed to revolve touching each other, or nearly so. An extreme example of this is found in the remarkable star V. Puppis, an Algol variable of the southern hemisphere. Both its components are bright, and the period of light variation is about one and a half days. Dr. A. W. Roberts finds that the bodies are revolving around each other in actual contact.

Temporary stars are stars which have suddenly blazed out in regions of the sky where no star was previously seen, and have faded away more or less gradually.