130 
ASTRONOMY: CAMPBELL AND MOORE 
of the lines do not maintain the indination of the bright central section 
of the lines but turn backward slightly. We interpret this to mean that 
the corresponding faint outer strata of nebulosity do not rotate with 
speeds proportional to the radii of the strata — relatively to the speeds 
of the bright inner strata — but that they 'lag behind/ 
The maximum displacements of the nebular lines correspond to points 
in the nebula 9 or 10 seconds of arc west and east of the central nucleus, 
and the observed component of rotational speed is of the order of 6 km. 
per second. 
On a fourth spectrogram, with the slit upon the minor axis of the ellip- 
tical image, the nebular lines are not inclined to the 'zero' direction, and 
they therefore fail to indicate any components of rotational motion in 
the plane passing through the minor axis of the nebula and the observer 
— a result in accord with expectations. 
The observations which we have described, considered in connection 
with the geometrical form of the nebular image as photographed, leave 
essentially no room for doubt that the nebula is rotating about an axis 
through the central nucleus approximately at right angles to the plane 
passing through the observer and the major axis of the image, the nebu- 
lar materials lying nearest to us being carried from west to east by the 
rotation. 
Measures of the rotational velocity of the nebula enable us to draw 
some interesting conclusions concerning its mass. If we assimie that the 
axis of rotation is located as described above, then the orbital speed of 
the nebular materials lying at a distance of 9 seconds of arc from the 
center is about 6 km. per second. If we provisionally assume the mass 
of the central nucleus to equal that of the Sun, Kepler's law connecting 
the periodic time with the distance from the nucleus tells us definitely 
that the nebula is distant from us only 8.9 light years. This must be 
regarded as an improbably small value, in view of other evidence bear- 
ing on the question. For assumed distances of 100 and 1000 light years, 
which we have reason to believe are more probable orders of nebular 
distance, the masses of the nebula would be respectively 11.3 and 113 
times that of the Sun, and the corresponding periods of rotation 1371 
and 13,710 years. From these considerations it seems certain that the 
mass of the planetary nebula N. G. C. 7009 is several times that of the 
Sun. The nebula is therefore competent, from the point of view of its 
mass, to develop into a system more pretentious than is our solar system. 
A few speculations concerning this nebula may not be without inter- 
est and value. 
The faint extensions to the east and to the west of the elliptical fig- 
