The phenomena of the red stars indicate that this final stage is close at hand, and curiously enough, in further testimony of the remarkable power of the spectroscope, the total extinction of a star's light does not always prove sufficient to place that star beyond the reach of this instrument. It is true that a spectroscope cannot reveal the chemical composition of a solid body which is devoid of intrinsic light, but such a body may form a system with another object which is still luminous,
Lunar Crater Theophilus and surrounding region. Photographed with the forty-inch Yerkes telescope (Ritchey).
and its gravitational power may cause the luminous body to move in an orbit. As we have already seen, the spectroscope is capable of revealing the motions of such a body. From a knowledge of these motions and the time in which the revolution is effected it is possible to determine the mass and dimensions of the system, and in some special cases like that of Algol, the diameter and density of the invisible component of the pair.