Spectral classification is a fundamental system that categorizes stars based on their surface temperature by analyzing the characteristic absorption lines in their light spectra. Developed in the early 1900s by astronomers at Harvard Observatory, including Annie Jump Cannon, this system uses the sequence O-B-A-F-G-K-M, ranging from hottest to coolest stars.</p><p>Each spectral class reveals distinct temperature ranges and stellar properties: O-type stars burn at scorching 30,000-50,000K with prominent ionized helium lines, appearing blue-white like Alnitak in Orion's Belt. Our Sun exemplifies a G-type star at 5,800K, showing strong hydrogen and metal absorption lines. Cool M-type red dwarfs like Proxima Centauri operate around 3,000K with prominent molecular bands.</p><p>The famous mnemonic "Oh Be A Fine Girl/Guy, Kiss Me" helps students remember the sequence. Modern astronomers have expanded the system to include ultra-cool L, T, and Y classes for brown dwarfs below 2,500K.</p><p>Spectral classification proves invaluable for determining stellar masses, ages, and evolutionary stages. It enables astronomers to estimate distances through spectroscopic parallax and understand galactic structure. This system transformed astronomy from merely cataloging stars to understanding their physical nature, making it one of the most important tools in stellar astrophysics for over a century.