 Pure helium stars lie on or near a helium main sequence, analogous to the main sequence formed by the more common hydrogen stars.Previously, a "helium star" was a synonym for a B-type 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.