The class of double star disclosed in this manner is known as the 'spectroscopic binary,' and has various other types differing from the Algol type. Thus the type of which Xi Ursæ Majoris was the first detected instance has two component bodies not differing greatly in brightness from one another. In such a case the fact of the star being binary is revealed through the consideration that in any binary system the two components must necessarily always be moving in opposite directions. Hence the shift of the lines of their spectrum will be in opposite directions also, and when one of the stars (A) is moving towards us, and the other (B) away from us, all the lines of the spectrum which are common to the two will appear double, those of A being displaced towards the violet and those of B towards the red. After a quarter of a revolution, when the stars are momentarily in a straight line with us, the lines will all appear single; but after half a revolution they will again be displaced, those of A this time towards the red and those of B towards the violet. There has thus been opened up an entirely new field of research, and the idea, long cherished, that the stars might prove to have dark, or, at all events, invisible, companions attendant on them, somewhat as our own sun has its planets, has been proved to be perfectly sound. So far, in the case of dark companions, only bodies of such vast size have been detected as to render any comparison with the planets of our system difficult; but the principle is established, and the probability of great numbers of the stars having real planetary systems attendant on them is so great as to become practically a certainty. 'We naturally infer,' says Professor Newcomb, 'that ... innumerable stars may have satellites, planets, or companion stars so close or so faint as to elude our powers of observation.'