THE INTEGRATED SPECTRAL TYPES OF GLOBULAR CLUSTERS

Abstract

At the present time it is possible to assign the spectrum of a nonpeculiar star to one out of a total of approximately 170 spectrum-luminosity classes in the range B0-M2 on the MK system. On passing to spectra due to two or more stars, the precision of the classification decreases greatly. For unresolved binary systems, the most favorable case for classifying the two stars uniquely is when the system consists of a K giant and an early A main-sequence star. If the photographic magnitudes of the two components are about equal, a fairly accurate spectral type and luminosity class of each can be determined from the blended spectrum. In the case of more complex systems, such as globular clusters and extragalactic nebulae, the precision becomes even lower ; however, certain general conclusions can be reached even here ; the presence of a considerable proportion of G8-K3 normal giants can be detected even when the light passing through the slit of the spectrograph comes from thousands of stars of varying spectral type and luminosity. This is due to the fact that the absorption bands of CN, on spectrograms of low dispersion, are well marked only in giants within the above spectral range. Thus, the one criterion the presence of CN bands is sufficient to identify stars within a certain range, both of spectral type and luminosity class. A similar situation is encountered in the case of B9-A3 mainsequence stars ; in a complex, blended source such as a spiral galaxy the occurrence of a considerable proportion of such stars can be detected from the presence of strong hydrogen lines with broad wings. In this case again, a single criterion is sufficient to recognize stars within a fairly narrow range of spectral type and luminosity class.