Ultraviolet spectroscopic measurements of globular clusters

Ultraviolet spectra of six globular clusters (M15, M92, NGC 1851, NGC 6624, 47 Tuc, and NGC 6752) have been taken with the International Ultraviolet Explorer satellite in order to investigate the surface brightness distributions in the central regions. The ultraviolet emission indicates composite stellar spectra. Short-wavelength emission (1200-1900 A) arises from blue horizontal-branch stars; long-wavelength emission (2100-3000 A) is characteristic of late-type horizontal-branch and giant stars. The surface brightness distribution is more compact at short wavelengths than at long wavelengths, suggesting a segregation of horizontal-branch stars. A dense core appears to be present in X-ray-emitting clusters and absent in those clusters without X-ray sources. The X-ray source in NGC 6624 may have been detected at short wavelengths (1300-1900 A).