subdwarfs

Subdwarfs are a class of stars whose luminosities place them below the main sequence for a given color, reflecting their lower metal content and distinct evolutionary histories. They are typically metal-poor, Population II objects and many belong to the Galactic halo or thick disk. The term covers both hot and cool stars with subdwarf luminosities and spectra that differ from ordinary dwarfs of similar temperatures.

Hot subdwarfs, designated sdO and sdB, are compact, high–temperature stars that are believed to be core helium-burning with very thin hydrogen envelopes. They occupy the hot, blue end of the horizontal branch and are often found in binary systems. Their ultraviolet brightness makes them useful for studies of old stellar populations and binary evolution.

Cool subdwarfs include spectral types such as sdK and sdM. These are metal-poor counterparts to dwarf stars of the same spectral classes; metal lines are weaker and molecular band strengths reflect the low metallicity, typically accompanied by higher space velocities relative to thin-disk stars. Cool subdwarfs help trace the chemical and dynamical structure of the Galaxy’s halo and thick disk.

Classification schemes have evolved to include intermediate designations such as extreme subdwarfs (esd) and ultra subdwarfs (usd) to denote increasing metal deficiency. Subdwarfs have been identified in large numbers by surveys like SDSS, Gaia, and various proper-motion programs, providing important constraints on Galactic formation and chemical evolution while illustrating the diversity of low-metallicity stellar populations.