A helium 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. 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 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. This may only happen if these 2 binary masses share the same type of envelope phase. It is believed this is the origin of the extreme helium stars.
The helium star's great capability of transforming into other stellar objects has been observed over the years. In 2014, a helium nova named V445 Puppis exploded, along with a following explosion of the star SN2012Z, causing a high-mass transfer between the two. 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.
- Daviddarling.info: helium star
- Yoon, S. -C; Langer, N. (2004). "Helium accreting CO white dwarfs with rotation: Helium novae instead of double detonation". Astronomy and Astrophysics. 419 (2): 645–652. arXiv:astro-ph/0402288. Bibcode:2004A&A...419..645Y. doi:10.1051/0004-6361:20035823.
- McCully, Curtis (2014). "A luminous, blue progenitor system for the type Iax supernova 2012Z". Nature. 512 (7512): 54–56. arXiv:1408.1089. Bibcode:2014Natur.512...54M. doi:10.1038/nature13615. PMID 25100479.
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