The Absorption-Line Spectra of the Central Stars of the Planetary Nebulae.

Under the terms of a co-operative arrangement with the University of Chicago and the University of Texas, a systematic spectroscopic study of the brighter central stars of the planetary nebulae has been undertaken at the McDonald Observatory. The intensities of all available absorption lines have been measured. From the Balmer lines it is possible to estimate the number of hydrogen atoms above the photosphere and the electron pressure, which is equal to haff the gas pressure if hydrogen is the most abundant element. If the atmosphere is in.hydrostatic equilibrium, the gas plus radiation pressure must just balance the weight of the overlying layers. Hence we may estimate the effective surface gravity of the star; and, if we can allow for the radiation pressure and find the stellar radius, we may obtain a' lower limit to the mass of the star. To within the limits imposed by observational uncertainties the spectra of these stars appear con- sistent with the suggestion that they are submain-sequence dwarfs with masses appropriate to their luminosities in accordance with the mass-luminosity law. There is no evidence from their spectra that they are white dwarfs or dense, extremely massive stars. The hotter planetary nuclei may resemble the old novae, although the absorption-line objects seem cooler and larger. Attention is called to the high-excitation character of the nucleus of NGC 246