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
S-type star difficult for a red giant to have other s-process elements in its atmosphere without technetium. S-type stars without technetium form by the transfer of technetium-rich matter, as well as other dredged-up elements, from an intrinsic S star in a binary system onto a smaller less-evolved companion. After a few hundred thousand years, the Tc will have decayed and a technetium-free star enriched with carbon and other s-process elements will remain. When this star is, or becomes, a G or K type red giant, it will be classified as a Barium star. When it evolves to temperatures cool enough for ZrO