It follows that in any given system there can be at most only one or two planets upon which life can find a home, and this only where the right conditions of size and mass, of rotation period, inclination of axis, and shape of orbit, all co-exist in a globe at the proper distance. But the type of system offered by our Sun and his planets is not the only one that exists. A very large proportion of stars are binaries--two suns revolve round their common centre of gravity. In many cases the two suns are separable in the telescope, and their relative movements can be measured; in other cases, termed "spectroscopic binaries," we only learn that a star which appears absolutely single has two components from the evidence of its spectrum; the spectroscope revealing two sets of lines that vibrate to and fro with respect to each other. Yet, again, a third class of double stars has made itself known in the "Algol variables." The optical double stars are cases where the two components are far distant from each other, and hence can be distinguished in our telescopes as separate points of light. The "spectroscopic binaries" are cases where the two components are too close to be separately perceived, but where the two are not greatly unequal in brightness, so that the spectrum of the one does not overpower that of the other. The "Algol variables" are cases where the two components are of very unequal brightness, and, being very close to each other, are so placed with respect to the Earth that the fainter partly eclipses the brighter in its revolution round it, and so causes a temporary diminution in its light at regular intervals. All these three classes of binary systems are now known to be very numerous. Prof. Campbell estimates that fully one star in six is a spectroscopic binary. But there must be many binary systems that do not reveal themselves--double stars where the companion is too faint or too close to be detected, Algol systems where the companion does not pass before its primary--and it seems almost certain that simple systems, like that of which our Sun is the unchallenged autocrat, must be comparatively rare.