378 
DIFFUSE MATTER IN SPACE 
space to the observer. The material at a considerable distance from either 
component must be of very low density, and the exceptionally sharp and 
narrow appearance of the “fixed calcium lines” is in keeping with this. 
The detection of fixed calcium lines in binary stars may be compared 
with the detection of telluric lines in the solar spectrum; certain lines are 
found not to partake of the sun’s rotation, and these have been imprinted 
on the spectrum during the journey of the light to us, viz. in our own 
atmosphere. 
The phenomenon is shown only by the very hottest “early type” 
stars, viz. those of types 0 to B 3. It might, however, exist undetected 
in cooler stars, the fixed lines being masked by stronger and wider ab 
sorption lines in the atmospheres of the components. 
The question whether the calcium cloud producing this absorption is 
attached to the star or is a free cloud in space can be settled by measuring 
the radial velocity. The test is not an easy one because the early type stars 
have usually low individual velocities; but an investigation by J. S. 
Plaskett appears to be conclusive*. He measured the calcium velocity 
and the ordinary stellar velocity (determined from the lines of normal 
behaviour) of 40 stars earlier than B 3, the investigation not being re 
stricted to spectroscopic binaries. In a number of cases considerable 
differences ranging up to 50 km. per sec. were found, indicating that the 
cloud could not be attached to the star. Moreover, after correcting the 
calcium velocities for solar motion it was found usually that the cloud had 
little, if any, velocity referred to the standard of reference of stellar 
velocities—the so-called “mean of the stars.” This uniformity of motion 
(or rest) of the calcium cloud indicates that it is a continuous cloud 
extending through all parts of the stellar universe explored in the in 
vestigation. Presumably it is of the nature of the diffuse matter in space 
imagined in the last two sections; inequalities of motion have been 
smoothed out in the course of time by diffusion and collision of the atoms. 
The D lines of sodium also appear fixed in these early type stars and 
must therefore be produced in the cloud. No doubt other elements may 
be present but are not in a condition to produce prominent lines in the 
visible region of the spectrum. It is evident that only lines of a principal 
series are likely to be seen, since in the weak field of radiation the pro 
portion of atoms excited at any one moment must be exceedingly small. 
It is noteworthy that the H and K lines are produced by singly ionised 
calcium and the spectrum of the un-ionised element does not appear 
although it has strong principal lines that might have been observed. We 
deduce therefore that practically all the calcium is ionised. On the other 
hand, the D lines are absorbed by sodium atoms which are un-ionised and 
unexcited. Some difficulty has been felt with regard to the absence of 
* Pub. Dom. Obs. Viet. 2, p. 287.