A helium star or helium strong star is a class O or B star (blue), which has extraordinarily strong helium lines and weaker than normal hydrogen lines, indicating strong stellar winds and a mass loss of the outer envelope. Extreme helium stars (EHe) entirely lack hydrogen in their spectra.
Previously, a helium star was a synonym for a B class star, but this usage is considered obsolete.
A helium star is also a term for a hypothetical star that could occur if two helium white 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. It is believed this is the origin of the extreme helium stars.
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