Spectral classification

Spectral classification groups astronomical objects by patterns in their spectra. For stars, it organizes types chiefly by characteristic absorption lines and surface temperature.

Harvard spectral classification: A scheme that orders stars by spectral type, principally according to surface temperature. Its OBAFGKM sequence is the best-known implementation of stellar spectral classification.
Astronomical spectroscopy: The study of astronomical objects through the wavelengths and intensities of their emitted, absorbed, or scattered light. Spectral classification depends on spectroscopy to reveal each object's diagnostic pattern.
Annie Jump Cannon: An American astronomer who classified hundreds of thousands of stellar spectra and refined the Harvard sequence. Her cataloging established the practical OBAFGKM ordering used in modern stellar classification.
Yerkes spectral classification: A system classifying stars by spectral features associated with surface gravity and luminosity. It emphasizes luminosity-sensitive features rather than the Harvard sequence's temperature ordering.
Hertzsprung–Russell diagram: A graph relating stellar luminosity to surface temperature or spectral type. Spectral types provide one axis and reveal the main sequence and giant branches.
Stellar spectral type: A designation, such as G2, describing a star’s spectrum and approximate photospheric temperature. The type is the primary label assigned in the Harvard scheme.
Absorption spectrum: A spectrum with dark wavelength bands where intervening material absorbs light. Stellar classification often relies on the strengths and shapes of absorption lines.
Antonia Maury: An American astronomer who developed an early stellar classification scheme incorporating spectral-line sharpness. Her work highlighted line widths, an avenue later used to distinguish luminosity classes.
Morgan–Keenan spectral classification: A stellar scheme that assigns both spectral type and luminosity class using standardized spectra. It shows how a temperature sequence can be supplemented to distinguish stars of different sizes.
Stellar evolution: The changes in a star's structure and properties over its lifetime. Spectral types help place stars in evolutionary contexts when combined with luminosity.

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