The special problems of stellar spectroscopy are very numerous. We may mention the novae, the Class stars, stars that show emission lines, the problem of -star classification and the peculiar stars, stars that display both giant and dwarf spectral characteristics, the classification of the stars, the apparent splitting into three groups of the spectral sequence for temperatures below 4000°, the problem of the c-stars, the Cepheid variables with their variable spectra, and the variables of long period. It is not possible, in a work like the present, to touch upon many of these subjects, and the writer has selected for brief discussion three upon which she can contribute new material: the problem of the stars, the classification of the stars, and the interpretation of the spectra of the stars that display the c-characteristic. THE STARS OF CLASS The statistically negligible class containing the stars is placed, at the present stage of investigation, at the top of the stellar sequence. These spectra indicate higher excitation than [Pg 162] 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. The stars other than the nuclei of planetaries have high intrinsic luminosities, but the material is insufficient for a satisfactory estimate of the absolute magnitudes of the non-Magellanic stars; various indications point to a value at least as high as -4. For the Magellanic stars, absolute magnitudes as great as -6.7 have been derived.[441] The measured parallaxes of the planetary nebulae, however, give for the nuclei absolute magnitudes[442] in the neighborhood of +8. The wide difference in absolute magnitude can merely be pointed out; it has never received adequate explanation. The masses are presumably very high for the stars, though but few have been accurately measured. The star B.D. 6°1309, a spectroscopic binary reported by J. S. Plaskett,[443] has a minimum mass eighty times that of the sun, and the stars 29 Canis Majoris and Orionis also appear to be very massive.[444]