sdO

sdO, or subdwarf O stars, are a class of hot, compact, evolved stars that lie below the main sequence in the Hertzsprung–Russell diagram. They typically have high effective temperatures, around 40,000 to 80,000 K, and surface gravities of log g about 5.0 to 6.5, indicating small radii for their luminosities. Spectroscopically, sdO stars show strong helium features, especially He II lines, while hydrogen lines may be weak or absent in helium-rich objects. Some sdO stars also display enhanced lines of carbon, nitrogen, or oxygen, reflecting surface chemical peculiarities.

Origin and evolution: sdO stars are believed to be remnants of red giants that lost most of their hydrogen envelopes, exposing hotter, helium-burning cores. The majority are thought to form through binary evolution, including common-envelope ejection or stable Roche-lobe overflow, and some may be the result of mergers between helium white dwarfs. Depending on the object, an sdO can be in a phase of helium-core burning or on a post-asymptotic-giant-branch track heading toward a white dwarf.

Population and observations: sdO stars occur throughout the Milky Way, in both the disk and halo, and are identified in ultraviolet and optical surveys. Distances and motions are increasingly constrained by astrometric data from missions such as Gaia. A subset shows photometric or spectroscopic variability, including pulsations in some helium-rich examples, which provide a probe of their internal structure.

Relation to other hot subdwarfs: sdO stars are the hotter counterparts to sdB stars; together they constitute the hot subdwarf class, representing late-stage stellar evolution influenced by significant mass loss and binary interactions.