Hydrogen-deficient star a faint blue star in 1994 to a yellow supergiant in 1996. One proposed mechanism for this migration is the final helium flash scenario. He-sdB are subdwarfs with class B spectra with broader than usual H, HeI, and HeII lines. JL 87 in 1991 was the first He-sdB star to be reported. Since then this class of stars has been shown to have a wide range of hydrogen-to-helium ratios. Compact He-sdO stars have class O spectra, are typically nitrogen-rich, and may or may not be carbon-rich. Low-gravity He-sdO stars overlap with their compact cousins, but have lower surface gravity. It
Gas torus no clear evidence of such a torus. While neutral nitrogen could not be measured, the ions near the orbit of Titan were primarily hydrogen or water group (O, OH, HO and HO) from the Enceladus torus. Trace amounts of nitrogen ions were detected but at levels consistent with an Enceladus source. A fictional gas torus is the setting for Larry Niven's novels "The Integral Trees" and "The Smoke Ring", in which a gas giant in orbit around a neutron star generates a gas torus of sufficient density and free oxygen to support life (including humans). Gas torus A gas torus