118 
ASTRONOMY: H. N. RUSSELL 
Proc. N. a. S. 
boring stars of very high temperature. In the Orion nebula the stars 
of the Trapezium {6 Orionis) appear to be the source of excitation. 
There is no reason to beUeve that the luminous gas forms the whole, 
or even any large part, of the matter present within the region — only 
that it is selectively sensitive to the incident excitation, and therefore 
gives out most of the light, just as the gases (carbon compounds and nitro- 
gen) do in the coma and tail of a comet. 
If the turbulent motions of the various parts of this nebula are of the 
same order of magnitude in the other two coordinates as in the radial 
direction, they must correspond to an average proper motion of 1.5 as- 
tronomical units per year, or about 0".8 per century (with Kapteyn's 
parallax of 0".0055). In a million years this would carry a nebulous wisp 
through 2°, which is more than the whole extent of the nebula. 
It appears probable, therefore, that the aspect of the Orion nebula 
was entirely different a million years ago from what it is now, as regards 
its details. There is no reason, however, to suppose that the nebula 
was not there. We may rather imagine that wisps and clouds of dust, 
carrying gas with them, are slowly drifting about. Some of them pass 
through the field of excitation due to the radiations from the Trapezium 
stars, and, when in this field, the gas is set shining — faintly near its out- 
skirts, and without excitation of the nebular lines; more strongly, and 
with the nebular lines, near the middle. 
According to unpublished investigations by Hubble, it appears probable 
that the absorbing clouds in Orion, not far from the nebula, weaken the 
light of stars behind them by at least ten magnitudes. The exciting 
radiations probably penetrate to a relatively small depth into the mass 
and, even if they went deeper, little of the excited light could get out again. 
The Orion nebula, on this hypothesis, may be regarded almost as a super- 
ficial fluorescence of the gaseous portion of this vast dark cloud, in the 
limited region where it is stimulated by the influence of the exciting stars. 
1 Barnard, E. E., Aslrophys. J., Chicago, 49, 1919 (1-23). 
2 Scares, E. H., and E. P. Hubble, Ibid., 52, 1920 (8-22); Mt. Wilson Contr., No. 187. 
^ Schwarzschiid, K., Sitzungsherichte der K. B. Akad. der Wiss., Math. Phys. Kl., 
Munchen, 31, 1901 (293-338); Proudman, Monthly Not., R. A. S., London, 73, 1913 
(535-539). 
* Barnard, E. E., Astrophys. J., Chicago, 38, 1913 (496-501). 
^ Publications oj the Lick Observatory, Berkeley, Cat., 13, 1918 (98). 
6 Russell, H. N., The Observatory, London, 44, 1921 (72). 
^Hubble, E. P., "Annual Report of the Mount Wilson Observatory," 1921; 
Year Book of the Carnegie Institution of Washington, 1921. 
