Overview
Spectral type categorizes stars by analyzing their electromagnetic spectra, revealing key traits like temperature, composition, and evolutionary stage. This system, rooted in the Harvard Classification Scheme and later refined by the Morgan-Keenan (MK) system, organizes stars into classes denoted by letters (O, B, A, F, G, K, M) and numerical subdivisions (0–9). Each class corresponds to a temperature range, with O-type stars being the hottest (≥30,000 K) and M-type the coolest (≤3,500 K). The sequence is often memorized via the mnemonic "Oh Be A Fine Girl/Guy, Touch Me".Spectral classification relies on absorption lines in a star’s spectrum, which vary with temperature. For example, hydrogen lines peak in A-type stars, while ionized calcium dominates in G-type stars like the Sun. The MK system further divides classes by luminosity (e.g., I for supergiants, V for main-sequence stars), adding depth to stellar characterization.
History/Background
The foundation of spectral classification began in the 1860s with Angelo Secchi, who grouped stars into five empirical categories. However, the modern system emerged in the late 19th century at Harvard College Observatory, led by Annie Jump Cannon. By 1910, she had classified over 350,000 stars, streamlining the system into the O–M sequence based on temperature.The Morgan-Keenan system, introduced in 1943 by William Wilson Morgan, Philip C. Keenan, and Edith Kellman, added luminosity classes to distinguish between giants, dwarfs, and supergiants. This innovation allowed astronomers to correlate spectral types with stellar evolution models. Later, classes L, T, and Y were added in the 1990s–2000s to classify brown dwarfs, sub-stellar objects too cool to sustain hydrogen fusion.
