What makes one star a cool red dot and another a fierce blue beacon? Astronomers answer that question with two basic labels: spectral class and evolutionary stage. The spectral classes—O, B, A, F, G, K, M—sort stars by color and surface temperature. Hotter stars shine blue-white; cooler ones glow red.Beyond color, a star’s life phase tells a different story. Many begin as protostars, then settle o...More