sdB

Subdwarf B stars, or sdB stars, are hot, compact stars that occupy the blue end of the horizontal branch in the Hertzsprung–Russell diagram. They are low-luminosity, high-temperature objects with spectra dominated by hydrogen and helium lines. Typical sdBs have effective temperatures around 20,000 to 40,000 K and surface gravities (log g) in the range of about 5 to 6.5. Their masses are commonly near 0.47 solar masses, and they possess very thin hydrogen envelopes, which prevents prolonged hydrogen-shell burning and places them on the extreme horizontal branch as helium-core-burning stars with only a small outer layer of hydrogen.

Formation and evolution of sdB stars are strongly linked to binary interactions. Most sdBs are believed to form when a red-giant star loses most of its envelope, often through binary processes such as common-envelope ejection or stable Roche lobe overflow. A fraction may result from the merger of two helium white dwarfs. The high observed frequency of sdBs in close binary systems supports the importance of binary evolution in their origin. While single-star channels exist, they are considered less common for sdB production.

Observational properties and variability further define the class. sdBs show strong hydrogen lines, and in many cases helium lines as well. A subset exhibits stellar pulsations, with short-period p-modes (V361 Hya stars) and longer-period g-modes (V1093 Her stars), enabling asteroseismic studies that probe their interior structure and core mass. They occur in the Milky Way and other galaxies, contributing to the ultraviolet flux of old stellar populations and providing important tests of binary evolution and stellar pulsation theory.

Overall, sdB stars are key objects for understanding late stages of low-mass stellar evolution, binary interactions, and the UV contribution of old stellar systems.