ArXiv Stellar

Different Types of Stars

 * Early-type stars with absorption spectra dominated by neutral or ionized helium are extremely rare and fall into one of (roughly) two classes:
 * Extreme helium stars: low-gravity stars of spectral types B and A (Jeﬀery 2008).
 * Extremely helium-rich hot subdwarfs are found (naturally) at higher gravities with early-B or late-O type spectra (Naslim et al. 2010).
 * A few low-gravity helium stars of spectraltype O are also known. These are almost certainly related to the cooler R Coronae Borealis variables, and are currently considered to be the product of the merger of a helium white dwarf with a carbon-oxygen white dwarf (Jeﬀery et al. 2011)

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