17 
is] The Galactic System 
in the number of stars. By extrapolation from known observational results, 
Seares and van Rhijn* have estimated the total number of stars at 30,000 
million. As their estimate relies on extrapolations to enumerate totally 
invisible stars, they do not of course claim great accuracy for it. For instance, 
they compute that even in the direction of the galactic poles, where the stars 
thin out most rapidly, there are three stars too faint to be seen in any 
existing telescope for each one that is telescopically visible; in directions 
in the galactic plane, they estimate the ratio of invisible to telescopically 
visible stars to be not less than 70 to 1 . 
The circular appearance of the Milky Way led Kapteyn to anticipate a 
general symmetry of arrangement about the galactic pole, and his scheme of 
star distribution presupposes this symmetry to exist. Recent investigations 
have shewn that the distribution of stars is by no means the same in the 
different planes through the galactic pole, being especially disturbed by the 
local condensations of stars known under the general name of “ star clouds.” 
A conspicuous example occurs in Cygnus, at a distance which Pannekoekf 
estimates as 600 parsecs, and there are others in Sagittarius, Scutum, Mono- 
ceros (200 parsecs) and Carina (300 parsecs). Charlier and Shapley have 
found that the brightest stars of the earliest spectral types form a star cloud 
which surrounds the sun and appears to have the same general biscuit shape 
as the main galactic system, but to be inclined to it at an angle of about 12 °. 
The sun appears to be only a small distance, 90 parsecs according to Charlier J, 
from its centre. Shapley calls this “the local system,” and a large proportion 
of the stars in the neighbourhood of the sun appear to belong to this system. 
Seares finds that it is only the nucleus of a much larger “local system,” some 
6000 parsecs or more in diameter, which is responsible for three-fourths of 
the stars in the neighbourhood of the sun, and for half of the stars in the 
galactic plane up to a distance of about 700 parsecs. 
As the Milky Way forms almost exactly a great circle in the sky, the sun 
cannot be far removed from the central plane of the galactic system. Seares § 
finds that it lies exactly in this plane to within about eight parsecs, but that 
it lies some 40 or 50 parsecs north of the central plane of the local cluster. 
Kapteyn, treating the sun as lying precisely in this central plane, discussed 
its distance from the centre of the system, and concluded that the approxi 
mate equality of star-density in different galactic latitudes was inconsistent 
with this distance being more than about 700 parsecs. Seares now finds, 
however, that the star-density in the main galactic system increases to a 
maximum at a distance of 1000 parsecs, or even more, from the sun. This 
main system is far from symmetrical, and its geometrical centre is probably 
* Astrophys. Journal, lxii. (1925), p. 320 or Mount Wilson Contribution, No. 301. 
t Publications of the Astronomical Institute of the University of Amsterdam, No. 1 (1924). 
t “ The Distances and the Distribution of Stars of Spectral Type B,” Lund Meddelanden, ii. 
(1916), No. 14. § Contributions from Mount Wilson Observatory, No. 347 (1928).