How are stars classified primarily?

Stars are primarily classified by their temperature and color because these characteristics intrinsically relate to their physical properties and stages of stellar evolution. The classification system known as the Harvard classification scheme categorizes stars into spectral types (O, B, A, F, G, K, M) which reflect their surface temperatures, ranging from the very hot, bright blue O-type stars to the cooler, dimmer red M-type stars.

The temperature of a star significantly affects its color; for instance, hotter stars emit more light in the blue and ultraviolet spectrum, while cooler stars emit light in the red and infrared spectrum. This relationship between temperature and color allows astronomers to determine not only a star's classification but also other vital details, such as its age, mass, and potential lifecycle.

Classifications based on brightness, distance, and size can provide additional context about a star's characteristics or position but do not serve as the primary basis for categorizing stars in an astrophysical sense. The intrinsic properties of temperature and color, however, offer direct insights into the nature of stars and are foundational in understanding their behavior and interactions with other astronomical bodies.