MOUNT WILSON OBSERVATORY. 201 



which is the group of Halm and with which the B-type stars probably are 

 associated. 



The results of a similar investigation of stars of types F to M show that 

 the giant stars form a single group with a small systematic motion relative 

 to the commonly adopted origin. Their velocity distribution is ellipsoidal 

 and shows a regular diminution of ellipticity with spectral type, being nearly 

 spherical for stars of the later K and M types. The dwarf stars also form a 

 distinct group as regards their motions. Their distribution is ellipsoidal, 

 but the directions of the axes and the systematic motion of the group 

 differ from those of the giant stars. Rapidly moving stars, regardless of 

 spectral type or absolute magnitude, show an extraordinary avoidance of 

 motion in the direction of the first quadrant of longitude relative to our 

 Galaxy. 



Mr. Hubble has completed an investigation of the relationship between 

 stars and diffuse nebulae. Particular stars, obviously involved in luminous 

 nebulosity, are found to have spectra similar to those of the nebulae. If the 

 stellar spectra are earlier than Bl, the nebulae give emission spectra; if later 

 than Bl, the spectra of the nebulae are continuous. At the critical type Bl 

 the nebular spectra are mixed. The relationship thus suggested is strikingly 

 confirmed by the observed dependence of the extent and brightness of the 

 nebulae upon the apparent magnitude of the stars involved. The photo- 

 graphic magnitudes of the stars, over a range of more than 14 magnitudes, 

 are in linear relationship with the logarithm of the greatest angular extent of 

 associated nebulosity. The results indicate that the amount of light radiated 

 from the nebulosity, whether it have an emission or a continuous spectrum, 

 is equal to the amount of light received from the dominating star or stars. 

 This indicates, as Russell had previously suggested on theoretical grounds, 

 an absorption and re-emission of starlight by the nebulosity rather than simple 

 reflection, and points to important possibilities of further researches (p. 222). 



The great division in the Milky Way between Ophiuchus and Aquila con- 

 tains within its borders a number of objects catalogued as nebulae. Hubble 

 has photographed many of these and finds them all to be clusters of faint 

 stars, in some cases extremely condensed. The rather high color-index of 

 these stars may be due to their type, or perhaps to scattering by the extensive 

 dark nebulosity which Hubble believes to account for the forked appearance 

 of the Milky Way. 



From a theoretical investigation of the dark nebulae, Professor Russell 

 concludes that these objects, catalogued in great numbers by Barnard, prob- 

 ably consist mainly of dust particles a few millionths of an inch in diameter, 

 driven away from stars by radiation pressure. Particles of the most effective 

 size (with a circumference 1.12 times the wave-length) and of the density of 

 water are repelled from the sun with a force 10 times, and from white (B) 

 stars 100 times, greater than gravitational attraction. Dwarf stars, on the 

 contrary, hardly repel dust at all. The final effect of the repulsion of fine 

 dust would be to scatter it to an indefinitely great distance, were it not for 

 the gravitational forces of vast masses of dust, which may account for the 

 sharp outlines of dark nebulae. In Russell's opinion, such an object as the 

 Great Nebula in Orion consists of wisps and clouds of dust, slowly drifting 

 about and carrying gas with them. When in the radiation field of the Trape- 



