• Space Telescope Imaging Spectrograph images of the well-studied globular cluster NGC 2808 reveal a population of hot helium-burning stars that are much fainter than theoretical expectations.

• Our models demonstrate that some of the hottest helium-burning stars should evolve via a new theoretical path that produces a hydrogen-deficient stellar atmosphere.

• Compared to normal helium-burning stars with a hydrogen-rich atmosphere, these hydrogen-deficient stars would be much hotter and have far less extreme-UV opacity. Thus, more of their flux is emitted at shorter (unobserved) wavelengths, explaining why they appear fainter in the observed bandpasses.

• This mechanism can account for other unexplained observational anomalies in globular clusters, and provides further insight into the formation of H-deficient stars.

• This work was published in The Astrophysical Journal: Brown, T.M., Sweigart, A.V., Lanz, T., Landsman, W.B., & Hubeny I. 2001, ApJ, 562, 368.

                    STIS Image of globular cluster NGC 6681 (35 arcsec x 30 arcsec)

                            Blue = far-UV, green = near-UV, red = optical
                                    Blue stars are hot helium-burning stars

This cluster is used for calibration purposes.  None of the helium-burning stars in this cluster are as hot as the unusual stars in NGC 2808, so we do not see evidence for subluminous stars in this cluster.