13-15]
The Galactic System
15
Table II. Star-densities ( Kapteyn ).
Ellipsoid
No.
Semi
major
axis
(parsecs)
Semi
minor
axis
(parsecs)
Relative
Star-
density
Stars
per cubic
parsec
Volume
of Shell
(cubic
parsecs)
Number
of Stars
Centre
1
602
118
1-000
0631
o-ioo
0063
1-79X10 8
0-1 X 10 8
II
1010
198
0-398
0-040
6-66
0-3 X 10«
III
1510
296
0-251
0-025
19-8
0 -6 X 10 8
IV
2106
413
0-158
0-016
48-4
1-OxlO 8
V
2820
553
0100
0 010
107
1-3 xlO 8
VI
3656
717
0-063
0-006
226
1-8 X 10 8
VII
4600
902
0040
0-004
388
2-0 X 10 8
Vili
5675
1114
0-025
0-002
702
2-3 x 10 8
IX
6960
1365
0-016
0-002
1270
2-5 xlO 8
X
8465
1660
o-oio
0001
2210
2-6X10 8
—
—
—
0-03
0003
4980
14-5X10 8
The first four columns are given by Kapteyn. The next column gives the
number of stars per cubic parsec over the spheroid in question, calculated
for a central density of one star per 10 cubic parsecs, our estimated density of
stars in the neighbourhood of the sun. Kapteyn’s own figures were rather less
than half of these, as he estimated the central density to be only 00451
stars per cubic parsec, or about one star per 22 cubic parsecs.
The general distribution implied in Kapteyn’s scheme is shewn dia-
grammatically in fig. 2. This represents a section through the central axis of
the system; if the section is supposed to be parsec in thickness, each
dot represents a single star.
0 1000 2000 3000 4000 5000
Scale, par secs
Fig. 2. Density of Distribution of Stars surrounding the Sun (Kapteyn).
(On right, the Andromeda Nebula M. 31 (Plate I) on the same scale.)