Stellar Structure

Abstract

ZANSTRAS recent determination of the temperatures of the O-stars in planetary nebulæ¹ makes it appear extremely likely that these stars are all generically in the ‘white dwarf’ class, with mean densities far above anything known on the earth. He has found temperatures between 30,000° and 100,000° for about twenty of these objects, and yet their luminosities are comparatively small. For a fairly typical nebular nucleus we may assume a photographic magnitude of 12.5 with a parallax of 0.002″, making the absolute magnitude + 4, or about eight magnitudes fainter than typical galactic O-stars. It is, of course, true that the luminous efficiency falls off with rising temperature in this range, but an increase in temperature at constant radius must always involve an increase in brightness, proportional, even in the farthest part of the Rayleigh-Jeans region, to at least the first power of the temperature. Thus these stars must be of very small radius.