Subdwarf stars are a type of star that have lower luminosity and temperature compared to main sequence stars of the same spectral type. They are found to lie below the main sequence on the Hertzsprung-Russell diagram. Subdwarf stars are classified into two main categories: subdwarf O and subdwarf B stars, with the latter being more common. Subdwarf stars are thought to be old stars that have exhausted their nuclear fuel, causing them to burn at a lower temperature and luminosity. They are often found in clusters or binary star systems, and are of interest to astronomers for studying the late stages of stellar evolution and the effects of metallicity on stellar properties. Research on subdwarf stars includes studies of their chemical composition, evolutionary history, and their potential use as standard candles for distance measurements in astronomy. By studying subdwarf stars, astronomers can gain insights into the formation and evolution of stars and galaxies in the universe.