Preliminary Results On The Color Of Nebulae (1916)(en)(5s)

PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES Volume 2 OCTOBER 15, 1916 Number 10 PRELIMINARY RESULTS ON THE COLOR OF NEBULAE By Frederick H. Seares MOUNT WILSON SOLAR OBSERVATORY, CARNEGIE INSTITUTION OF WASHINGTON Received by the Academy, August 4, 1916 Important differences in the spectra of gaseous nebulae, even those of the same general class, have been known for many years. Although the characteristic nebular lines X 5007 and X 4959 seem to maintain a constant ratio of brightness,* their behavior varies greatly with respect to H#, X 4686, X 3727, and other lines. Thus H, is usually fainter than the chief nebular line X 5007, but there is a wide range in the relative intensities, and intensity reversals may even occur. Again, lines usually present are as yet unobserved in certain objects, and in some we have apparently only the monochromatic radiation X 3727.1 This diversity is reminiscent of the differences in the spectra of stars. These are universally accepted as evidence of evolutionary change, and if nebulae have any part in the scheme of development, it is likely that their spectral peculiarities will find a similar interpretation. In this event spectral correlations must exist, more or less clearly defined according to the development of the nebulae compared. The important investigations of Wright have already revealed relationships presumably of this kind. Further, experience in other fields suggests the probability of spectrum differences within the same nebula. No mixture of luminous vapors is homogeneous in its radiation, for the pressure, temperature, electrical conditions, etc., are never constant throughout. Thus the spectrum changes from point to point in the flame and the electric arc; the radiation from the attenuated gases of a vacuum tube varies with the region examined; and, on a larger scale, the spectrum of