1921.] 
Astronomical Notes. 
9 
use Planck’s formula for the energy from unit area radiated by a particular 
wave-length, A :— 
(7/V-j 
where C = *353 in ergs per square centimetre per second, 
a = 1-445 centimetre degrees, and 
0 is the absolute temperature, 
Plotting the values for E\ with respect to A for the three temperatures 
1,500°, 6,000°, and 6,000,000° absolute, we get the curves shown in fig. 2. 
The curves are necessarily drawn to different scales. The values of the 
ordinates read from the diagram must be multiplied by 10 14 , 10 18 , and 
10 23 respectively. 
By counting the squares it appears that between A 4,000 and A 7,000 
angstroms the area below the 6,000,000° curve is one-quarter of that 
below the 6,000° one. This implies that the actual energy in the visible 
part of the spectrum at 6,000,000° is 25,000 times that at 6,000°. If 
40 per cent, of the radiation from the Sun falls within the visible spectrum, 
the visible radiation from the nova is about 10,000 times the total radiation 
from the Sun. The total radiation from the nova being a billion (10 12 ) 
times the solar, it follows that only of the latter radiation is visible. 
It appears also that at this stage the visible light from the nova must 
be very blue, as the blue and violet end of the spectrum is very much 
stronger than the red end. If, therefore, during observation at any time 
an unfamiliar star should attract attention by the exceptionally blue colour 
of its light, it would be well to examine it carefully in case it should be a 
nova at an earlier stage than has yet been observed. During this stage 
also the photographic magnitude of the nova must be much greater than 
the visual, and this fact may play some part in enabling so many small 
novae to be detected by photography. 
The Great Velocities of the Planetary Nebulae. 
By A. C. Gtfford. 
In Publications of the Lick Observatory , vol. 13, 1918, the following table 
is given showing the velocities in space of stars and nebulae :— 
Diffuse nebulosities 
Class B stars 
5? A ,, 
F 
5 9 9 9 
n 
99 
99 
K 
M 
99 
99 
Planetary nebulae 
Spiral nebulae 
Velocities low—almost at rest relativelv to the 
*/ 
galaxy. 
Average velocities 12 km. (8 miles) per second. 
,, 21 km. (13 miles) ., 
,, 29 km. (18 miles) ,, 
,, 32 km. (20 miles) ,, 
,, 34 km. (21 miles) ,, 
,, 34 km. (21 miles) ,, 
,, 77 km. (45 miles) ,, 
,, 770 km. (480 miles) ,, 
The high average velocity of planetary nebulae makes it difficult, as 
Curtis points out, to adjust them to a position in the gamut of stellar 
