8 
ASTRONOMY: W. W. CAMPBELL 
ON THE RADIAL VELOCITIES OF NEBULAE 
By W. W. Campbell 
UCK OBSERVATORY. UNIVERSITY OF CAUFORNIA 
Presented to the Academy. October 31, 1914 
The discovery, early in 1910, that the velocities of the stars are func- 
tions of their spectral types — the stellar velocities increase rapidly, 
on the average, as we pass from the blue stars through the yellow stars 
and on to the red stars — and the prevailing opinion that the stars have 
been evolved from nebulae, gave special interest to the search for possible 
relationships existing between nebular and stellar velocities. The 
nebular velocities then available were the velocities of approach and 
recession (radial velocities) of 13 planetary nebulae and of the Orion 
nebula, as observed by Keeler in 1890-1891 with a visual spectroscope 
attached to the 36-inch refracting telescope. The average radial 
velocity of the 14 nebulae, as observed by Keeler, was 25 km. per second; 
or omitting the Orion nebula, whose velocity is close to zero, 26| km. 
per second for the 13 planetaries on his list. The average radial velocity 
of the helium stars, very generally supposed to be the stars most recently 
formed from nebulae, was found to be only 6 km. per second. The 
average nebular velocity was four times as great. Were Keeler 's results, 
only 14 in number, too few to furnish a safe statistical basis of com- 
parison? The basis was at least too small to support a superstructure 
of any pretensions to reliability. A knowledge of a greatly increased 
number of nebular velocities was strongly demanded. 
During the past three years, at the Lick Observatory and at the associ- 
ated D. O. Mills Observatory, Santiago, Chile, more than 200 measures 
of nebular radial velocities have been secured by spectrographic methods. 
These relate to 61 nebulae, but as 7 of these objects have been observed 
only once each, this note is hmited to the velocities of 54 nebulae. 
The distribution of the velocities, as to magnitude, tabulated below 
is remarkable. No distinction between velocities of approach and 
velocities of recession is made in this table. 
Velocities, km./sec 0-10 10-20 20-40 40-60 60-80 80-up 
Number of nebulae 11 6 17 11 2 7 
Average velocity 5 16 28 50 77 132 
Total number of nebulae 54. Grand average velocity 42 km./ sec. 
It is now well known that the radial velocities of four-fifths of the 
so-called helium stars are under 10 km. per second, and that the average 
