Spectral type

A classification of stars based chiefly on absorption features in their spectra, which reveal surface temperature and related atmospheric properties.

Stellar spectrum: The distribution of a star’s emitted light by wavelength, including absorption and emission features. Spectral types are assigned by comparing characteristic features across stellar spectra.
Harvard spectral classification: The stellar classification scheme that orders spectral classes mainly by temperature and absorption-line patterns. It supplies the familiar letter sequence used to describe spectral type.
Annie Jump Cannon: An American astronomer who classified hundreds of thousands of stellar spectra at Harvard College Observatory. Cannon organized the letter classes into the sequence that became standard.
Stellar temperature: The effective temperature of a star, defined as the temperature of a black body emitting the same total energy per area. Spectral type provides a practical temperature estimate from a star’s spectrum.
Stellar metallicity: The abundance of elements heavier than helium in a star, often expressed relative to the Sun. Metal content changes line strengths but is not the primary axis of spectral type.
Black-body radiation: Electromagnetic radiation whose spectrum depends only on the temperature of an ideal absorber and emitter. A star’s continuum spectrum roughly follows a thermal curve that helps estimate its temperature.
O-type star: A very hot, blue star whose spectrum contains prominent ionized helium lines. O stars anchor the hottest end of the main spectral sequence.
Williamina Fleming: A Scottish-American astronomer who developed early methods for classifying stellar spectra at Harvard College Observatory. Fleming’s spectral cataloging helped establish the observational basis for stellar classes.
Stellar luminosity class: A Roman-numeral classification of stars by luminosity and surface gravity, used alongside spectral type. Combining luminosity class with spectral type distinguishes giants from dwarfs of similar temperature.
MK classification: The Morgan–Keenan system that classifies stars by both spectral type and luminosity class. It places temperature-based spectral type within a broader two-part stellar classification.

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