The Luminosities of the O and B Type Stars

Abstract

When the B type stars are classified, they are arranged into groups of differing luminosity at each spectral type by considering the strengths and shapes of lines which are sensitive to changes in pressure and to the extent of atmosphere. In the atmospheres of the stars of high luminosity, the pressure is lower than in the atmospheres of the stars of lower luminosity and the atmosphere is more extended. The most striking spectral features demonstrating this in B type spectra are the Balmer lines of hydrogen and the Hei lines. In main-sequence stars the Balmer lines have wide wings of a typical shape due to Stark effect. For a given spectral type, as the luminosity increases and the electron pressure in the atmosphere decreases, the Stark effect becomes less and the Balmer lines of hydrogen become narrower. In the most luminous stars, the supergiants, large-scale motions in the atmosphere dominate over Stark effect as a broadening agent, and the absorption lines have a bell shape rather than a sharp, narrow core with extended wings. These changes in line profile produce readily measurable changes in the equivalent width of each Balmer line.