Page:Popular Science Monthly Volume 58.djvu/25

Rh period is 11h. 5m. 7s. Apparently about 65 per cent, of the whole period is occupied by the increase of the light. This very slow rate of increase is especially striking from the fact that in many cases in this cluster the increase is extremely rapid, probably not more than ten per cent, of the whole period. In one case, No. 45, having a period of 14h. 8m., the rise from minimum to maximum, a change of two magnitudes takes place in about one hour, and in certain cases, chiefly owing to the necessary duration of a photographic exposure, there is no proof at present that the rise is not much more rapid.

"The marked regularity in the period of these stars is worthy of attention. Several have been studied during more than a thousand, and one during more than five thousand, periods without irregularities manifesting themselves."

It may be added that this regularity of the period, taken in connection with the case of η Aquilæ, already mentioned, affords a strong presumption that the variations in the light of these stars are in some way connected with the revolution of bodies around them, or of one star round another. Yet it is certain that the types are not of the Algol class and that the changes are not due merely to one star eclipsing another. That such condensed clusters should have a great number of close binary systems is natural, almost unavoidable, we might suppose. It will hereafter be shown to be probable that among the stars in general single stars are the exception rather than the rule. If such be the case, the rule should hold yet more strongly among the stars of a condensed cluster.

Perhaps the most important problem connected with clusters is the mutual gravitation of their component stars. Where thousands of stars are condensed into a space so small, what prevents them from all falling together into one confused mass? Are they really doing so, and will they ultimately form a single body? These are questions which can be satisfactorily answered only by centuries of observation; they must, therefore, be left to the astronomers of the future.

The first nebula, properly so-called, to be detected by an astronomical observer was that of Orion. Huyghens, in his 'Systema Saturnium,' gives a rude drawing of this object, with the following description:

"There is one phenomenon among the fixed stars worthy of mention which, so far as I know, has hitherto been noticed by no one, and, indeed, cannot be well observed except with large telescopes. In the sword of Orion are three stars quite close together. In 1656, as I chanced to be viewing the middle one of these with the telescope, instead of a single star, twelve showed themselves (a not uncommon circumstance). Three of these almost touched each other, and, with