-stars which happen to be embedded in a nebula of great extent which accompanies them in their motion. The Doppler effect due to the radial velocity would be the same for the star as the nebula, but the gravitational effect predicted by Einstein would not be the same, inasmuch as the gravitational field at the surface of the star will vary considerably from that at the outer edge of the nebula. Hence it would be reasonable to attribute any difference in the magnitude of the spectral shifts in the case of the star and the nebula to the difference in gravitational fields at each place.
The stars of the nebular group of Orion have hitherto offered the only possibility of applying this method. The results have fulfilled Einstein's expectations qualitatively, and it remains to be seen whether the agreement will hold quantitatively. A general shift of the star-spectrum as compared with the corresponding lines of the associated nebula was observed.
Some very bright
-stars in the constellation of Orion are considered to form an entity with their attendant nebula. This conclusion was reached as the result of independent research.
The radial velocity of the Orion-nebula has been measured by various observers. The values obtained are: 17.7 (Wright), 17.4 (Vogel and Eberhardt), 18.5 (Frost and Adams). The mean value is 17.4 kms. per sec. This velocity is derived from the brightest part of the nebula, the so-called trapezium. The values obtained in the case of the stars almost all exceed 20 kms. per sec., and hence it seems likely that part of this radial velocity, viz. the excess over that of the nebula, is due to the Einstein effect. When the difference between the radial velocities of the stars and the associated nebula are tabulated for each star, we find that in the case of all members except two the difference is positive, i.e. indicative of a shift towards the red end, in agreement with the statistical investigation applied to the
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