Ahelium star is aclass O or Bstar (blue), which has extraordinarily strongheliumlines and weaker than normalhydrogen lines, indicating strong stellar winds and a mass loss of the outer envelope.Extreme helium stars (EHe) entirely lack hydrogen in their spectra. Pure helium stars lie on or near ahelium main sequence, analogous to themain sequence formed by the more common hydrogen stars.^[1]

Previously, ahelium star was a synonym for aB-type star, but this use of for the term is considered obsolete.^[2]

Ahelium star is also a term for ahypothetical star that could occur if two heliumwhite dwarfs with a combined mass of at least 0.5 solar masses merge and subsequently start nuclear fusion of helium, with a lifetime of a few hundred million years. This may only happen if these two binary masses share the same type of envelope phase. It is believed this is the origin of the extreme helium stars.

The helium main sequence is a line in theHR diagram where unevolved helium stars lie. It lies mostly parallel and to the left (i.e. higher temperatures) of the better-known hydrogenmain sequence, although at high masses and luminosities it bends to the right and even crosses the hydrogen main sequence. Therefore, pure helium stars have a maximum temperature, between about100,000 K and150,000 K depending onmetallicity, because high luminosity causes dramatic inflation of the stellar envelope.^[3]

Helium stars' great capability of transforming into other stellar objects has been observed over recent years since they were first identified. The blue progenitor system of thesupernova type IaxSN 2012Z in the spiral galaxyNGC 1309 is similar to the progenitor of theGalactic helium novaV445 Puppis, suggesting thatSN 2012Z was the explosion of a white dwarf accreting from a helium-star companion. It is observed to have caused a growing helium star that has the potential to transform into a red giant after losing its hydrogen envelope in the future.^[4]

• Carbon star
• R Coronae Borealis variable

• Star types
• Chemically peculiar stars
• Helium

• Articles with short description
• Short description is different from Wikidata