stars is placed, at the present stage of investigation, at the top of the stellar sequence. These spectra indicate higher excitation than those of any other class, and ionization theory distributes their temperatures between 25,000° and 40,000°. Their spectra are among the most puzzling encountered in the whole stellar sequence, and theory has hitherto been unsuccessful in suggesting the conditions that produce them.
A hundred and forty non-Magellanic
stars[439] are enumerated in the Draper catalogue, and in addition a small number of apparently faint
stars should probably be transferred to Class
, as Victoria has already done[440] for a group of stars in Monoceros. The
stars have a very definite distribution; they lie either very near the galactic plane, or in one of the Magellanic clouds, or they constitute the nuclei of planetary nebulae.