Most stars consist of hydrogen as their primary component. However, there are certain stars that are extremely low or almost devoid of hydrogen. This category of stars is termed “hydrogen-deficient stars”.
Hydrogen deficient stars
Cool carbon stars like R Coronae Borealis, helium-rich spectral type WC and transition stars like PG 1159 are all hydrogen deficient. An extreme helium star, often abbreviated as “EH”, is a low-mass supergiant that is almost devoid of hydrogen.
Discovery
Daniel M. Popper discovered the first known extreme helium star at the McDonald Observatory in Austin, USA, in 1942. By 1996, 25 possible helium stars were identified. This was further narrowed to 21 by 2006. Extreme helium stars are characterized as those that display no lines of hydrogen in their spectrum, but strong helium lines as well as the presence of carbon and oxygen.
Size and composition
The known extreme helium stars are “supergiants”. Hydrogen is less abundant by a factor of 10,000 or more, and surface temperatures range from 9,000 to 35,000 K. There are two popular theories as to how these stars are formed and why they have their unique composition.
The double-degenerate (DD) model :
This explains stars forming in a binary system. It has a small helium white dwarf and a more massive carbon-oxygen white dwarf. Gravity causes them to collide and form a dwarf that ignites into a supergiant.
The final-flash (FF) model :
It says that helium ignites in a shell around the core, causing the dwarf to rapidly expand.