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
Planetary nebula exceeding about 30,000 K, there are enough emitted ultraviolet photons to ionize the ejected atmosphere, causing the gas to shine as a planetary nebula. After a star passes through the asymptotic giant branch (AGB) phase, the short planetary nebula phase of stellar evolution begins as gases blow away from the central star at speeds of a few kilometers per second. The central star is the remnant of its AGB progenitor, an electron-degenerate carbon-oxygen core that has lost most of its hydrogen envelope due to mass loss on the AGB. As the gases expand, the central star undergoes a two-stage evolution,