Harvard spectral classification

A system for classifying stars by spectral features, especially absorption lines shaped by stellar temperature. Its main sequence runs from hot O-type stars to cool M-type stars.

Stellar spectrum: The distribution of light emitted by a star across wavelengths, including characteristic absorption and emission lines. The classification is built from recurring patterns in stellar spectra.
O-type star: A very hot, blue star with strong ionized-helium lines and surface temperatures typically above 30,000 kelvins. O stars occupy the hottest end of the Harvard sequence.
Williamina Fleming: A Scottish-American astronomer who developed early stellar spectral classifications at Harvard College Observatory. Fleming classified spectra and helped create the alphabetical scheme that preceded the modern sequence.
Morgan–Keenan classification: A stellar classification system combining spectral type with luminosity class to distinguish stars’ temperatures and sizes. It extends Harvard spectral types with luminosity classes such as dwarf and giant.
Absorption line: A dark feature in a spectrum produced when atoms or molecules absorb light at particular wavelengths. The strengths and patterns of absorption lines distinguish Harvard spectral classes.
B-type star: A hot blue-white star whose spectrum commonly shows prominent neutral-helium lines. Helium lines are strongest in this hotter class, just below O.
Antonia Maury: An American astronomer whose detailed stellar classifications distinguished spectral line widths and profiles. Maury’s finer distinctions revealed information that the basic Harvard classes did not capture.
Luminosity class: A category, usually expressed with a Roman numeral, indicating a star’s luminosity and evolutionary state. Harvard spectral type alone does not distinguish a giant from a dwarf of similar temperature.
Stellar classification: The broader practice of grouping stars by observable properties such as spectra, temperature, and luminosity. Harvard classification is one influential system within this broader practice.
A-type star: A white star whose spectrum has especially strong hydrogen Balmer absorption lines. Balmer lines reach their greatest strength near this class.

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