Stars are classified into different types based on their temperature, size, and brightness. The main categories include main sequence stars, giants, supergiants, and white dwarfs. Main sequence stars, like our Sun, are in a stable phase of hydrogen fusion. Giants and supergiants are larger and more luminous, while white dwarfs are the remnants of stars that have exhausted their nuclear fuel. Another way to classify stars is by their spectral types, which are determined by their surface temperatures and the absorption lines in their spectra. The main spectral types are O, B, A, F, G, K, and M, ranging from the hottest and most massive O-type stars to the cooler and smaller M-type stars. Each type has distinct characteristics that help astronomers understand the life cycles of stars.