94 CELESTIAL SPECTROSCOPY. 
tenuity, is pobably more nearly at rest relatively to the stars of our 
system than any other celestial object we know; still it would seem 
more likely that even here we have some motion, small though it may 
he, than that the motions of the matter of which it is formed were so 
absolutely balanced as to leave this nebula in the unique position of 
absolute immobility in the midst of whirling and drifting suns and sys- 
tems of suns. 
The spectroscopic method of determining celestial motions in the line 
of sight has recently become fruitful in a new but not altogether unfore- 
seen direction, for it has, so to speak, given us a separating power far 
beyond that of any telescope the glassmaker and the optician could 
construct, and so enabled us to penetrate into mysteries hidden in stars 
apparently single, and altogether unsuspected of being binary systems. 
The spectroscope has not simply added to the list of the known binary 
stars, but has given to us for the first time a knowledge of a new class 
of stellar systems, in which the components are in some cases of nearly 
equal magnitude, and in close proximity, and are revolving with veloci- 
ties greatly exceeding the planetary velocities of our system. 
The K line in the photographs of Mizar, taken at the Harvard Col- 
lege Observatory, was found to be double at intervals of fifty-two days. 
The spectrum was therefore not due to a single source of light, but to 
the combined effect of two stars moving periodically in opposite diree- 
tions in the lineof sight. It is obvious that if two stars revolve round 
their common centre of gravity in a plane not perpendicular to the line 
of sight, all the lines in a spectrum common to the two stars will appear 
alternately single or double. 
In the case of Mizar and the other stars to be mentioned, the spec- 
troscopic observations are not as yet extended enough to furnish more 
than an approximate determination of the elements of their orbits. 
Mizar especially, on account of its relatively long period—about 
ove hundred and five days—needs further observations. The two stars 
are moving each with a velocity of about 50 miles a second, probably in 
elliptical orbits, and are about 143,000,000 miles apart. The stars, of 
about equal brightness, have together a mass about forty times as great 
as that of our sun. 
A similar doubling of the lines showed itself in the Harvard photo- 
graphs of 6 Aurige at the remarkably close interval of almost exactly 
two days, indicating a period of revolution of about four days. Accord- 
ing to Vogel’s later observations, each star has a velocity of nearly 70 
miles a second, the distance between the stars being little more than 
7,900,000 miles, and the mass of the system 4.7 times that of the sun. 
The system is approaching us at the speed of about 16 miles a second. 
The telescope could never have revealed to us double stars of this 
order. In the case of 6 Aurige, combining Vogel’s distance with 
Pritchard’s recent determination of the star’s parallax, the greatest 
angular separation of the stars as seen from the earth would be one 
