Stellar classification is a system used in astrophysics to categorize stars based on their measurable physical properties, primarily surface temperature and luminosity. The most widely recognized method is the spectral class system, which uses letter designations (O, B, A, F, G, K, M) that correlate directly to a star's effective surface temperature, with 'O' denoting the hottest and 'M' denoting the coolest. Further classification is often accomplished using the Hertzsprung-Russell diagram, which plots a star's absolute luminosity against its spectral class, allowing astronomers to determine a star's evolutionary stage, size, and eventual fate. The classification is essential for understanding the composition and physical processes occurring within stellar populations.