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
Red-giant branch terminology is still frequently used today. Modern stellar physics has modelled the internal processes that produce the different phases of the post-main-sequence life of moderate-mass stars, with ever-increasingly complexity and precision. The results of RGB research are themselves being used as the basis for research in other areas. When a star with a mass from about (solar mass) to ( for low-metallicity stars) exhausts its core hydrogen, it enters a phase of hydrogen shell burning during which it becomes a red giant, larger and cooler than on the main sequence. During hydrogen shell burning, the interior of the star goes