Page:Popular Science Monthly Volume 58.djvu/17

Rh The system of α Centauri is interesting from the shortness of the period, the brightness of the stars and the fact that it is the nearest star to ns so far as known. We reproduce a diagram of the apparent orbit from Dr. See's work. The period of revolution found by Dr. See is eighty-one years. The major axis of the apparent orbit is 32"; of the minor axis 6".

The pairs of which, so far as known, the period of revolution is the shortest, are these:

Systems of three or more stars so close together that there must be a physical connection between them are quite numerous. There is every variety of such systems. Sometimes a small companion of a brighter star is found to be itself double. A curious case of this sort is that of γ Andromedæ. This object was observed and measured by Struve as an ordinary double star, of which the companion was much smaller than the principal star. Some years later Alvan Clark found that this companion was itself a close double star, of which the components, separated by about 1", were nearly equal. Moreover, it was soon found that these components revolved round each other in a period not yet accurately determined, but probably less than a century. Thus we have a binary system revolving round a central star, as the earth and moon revolve round the sun.

In most triple systems there is no such regularity as this. The magnitudes and relative positions of the components are so varied that no general description is possible. Stars of every degree of brightness are combined in every way. Observations on these systems extend over so short an interval that we have no data for determining the laws of motion that may prevail in any but one or two of the simplest cases. They are, in all probability, too complicated to admit of profitable mathematical investigation. There is, therefore, little more of interest to be said about them.

There is a very notable multiple system known as the Trapezium of Orion, from the fact that it is composed of four stars, one of which is plainly visible to the naked eye, while the others may he well seen in the smallest telescope. There are also two other very faint stars, each of which seems to be a companion of one of the bright ones. This system is situated in the great nebulæ of Orion, to be described in the next