PRESIDEN'T’S ADDRESS. 21 
The Density of Stellar Distribution at Different Distances from 
our Sun. 
Consider, lastly, the distribution of stellar density, that is, the number 
of stars contained in the unit of volume. 
We cannot determine absolute star-density, because, for example, some 
of the stars which we know from their measured parallaxes to be com- 
paratively near to us are in themselves so little luminous that if removed 
to even a few light-years greater distance they would appear fainter than 
the ninth magnitude, and so fall below the magnitude at which our data 
at present stop. 
But if we assume that intrinsically faint and bright stars are dis- 
tributed in the same proportion in space, it will be evident that the 
comparative richness of stars in any part of the system will be the same 
as the comparative richness of the same part of the system in stars of a 
particular luminosity. Therefore, as we have already found the arrange- 
ment in space of the stars of different degrees of luminosity, and con- 
sequently their number at different distances from the Sun, we must also 
be able to determine their relative density for these different distances. 
Kapteyn finds in this way that, starting from the Sun, the star-density 
(i.e., the number of stars per unit volume of space) is pretty constant till we 
reach a distance of some 200 light-years. Thence the density gradually 
diminishes till, at about 2,500 light-years, it is only about one-fifth of the 
density in the neighbourhood of the Sun.!_ This conclusion must, however, 
be regarded as uncertain until we have by independent means been 
enabled to estimate the absorption of light in its course through inter- 
stellar space, and obtained proof that the ratio of intrinsically faint to 
bright stars is constant throughout the universe. 
Thus far Kapteyn’s researches deal with the stellar universe as a 
whole ; the results, therefore, represent only the mean conditions of the 
system. The further development of our knowledge demands a like study 
applied to the several portions of the universe separately. This will 
require much more extensive material than we at present possess. 
As a first further approximation the investigation will have to be 
applied separately to the Milky Way and the parts of the sky of higher 
galactic latitude. The velocity and direction of the Sun’s motion in space 
may certainly be treated as constants for many centuries to come, and 
these constants may be separately determined from groups of stars of 
various regions, various magnitudes, various proper motions, and various 
spectral types. If these constants as thus separately determined are 
different, the differences which are not attributable to errors of observa- 
tion must be due to a common velocity or direction of motion of the group 
or class of star to which the Sun’s velocity or direction is referred. Thus, 
for example, the Sun’s velocity as determined by spectroscopic observations 
Publications Astron. Lab. Groningen, No. 11. 
