Interstellar Material and the OB Stars

Abstract

It was early noticed that although most of the apparently bright O and B stars looked very blue to the naked eye, many of the fainter O and B stars looked quite yellowish. According to the ideas then in vogue, the most probable explanation of this observation was that the latter group of stars was significantly cooler than the first group. However, all these stars had spectra suggesting temperatures in the neighbourhood of 20000°. The solution to this riddle was not found until Trumpler (1930) from his study of open clusters realised that interstellar space was not transparent in all directions and that particularly in the plane of the Milky Way the line of sight often passed through layers of material which attenuated the light from the stars lying behind it. It was soon established that the attenuation was due to scattering by particles of interstellar dust and that the scattering was not equally efficient at all wave lengths, but varied about as λ⁻¹.