What does the spectral sequence OBAFGKM mean?

It is an ordering of stars from hottest (O) to coolest (M) by surface temperature, defined by which spectral lines are strong; the letters survive from an earlier alphabetical scheme that was later reordered by temperature.

Why do hydrogen lines peak in A-type stars rather than the hottest stars?

Hydrogen lines require atoms in a specific excited state; in the hottest stars hydrogen is mostly ionized and in the coolest it is mostly in the ground state, so the lines are strongest at the intermediate temperatures of A-type stars.

Spectral Classification

Spectral classification assigns stars to a sequence of spectral types and luminosity classes based on the pattern and strength of lines in their spectra, encoding temperature and luminosity.

Najít téma v PaperMindJiž brzyFind papers & topics

Tools & resources

Stáhnout prezentaci

Learn & explore

VideoJiž brzy

Definition

Spectral classification is the assignment of a star to a spectral type and luminosity class by comparing its spectrum to standard reference spectra, primarily reflecting surface temperature and luminosity.

Scope

This topic covers the empirical classification of stellar spectra, principally the Harvard temperature sequence OBAFGKM and the Morgan-Keenan two-dimensional system that adds luminosity classes. It addresses the spectral features that define each type, the physical basis of the temperature ordering, and extensions to cool dwarfs and unusual stars.

Core questions

What spectral features define the OBAFGKM temperature sequence?
How does the Morgan-Keenan luminosity class distinguish dwarfs, giants, and supergiants?
Why does the strength of a given line peak at a particular temperature rather than tracking abundance directly?
How have classification schemes been extended to brown dwarfs and peculiar stars?

Key theories

MK classification system: The Morgan-Keenan system classifies stars in two dimensions, a temperature-ordered spectral type and a luminosity class, by matching against a grid of standard stars with defining spectral criteria.
Temperature sequence of line strengths: The strength of each spectral line depends on the fraction of atoms in the right ionization and excitation state, which is governed by temperature, so the line pattern orders stars by surface temperature.

Clinical relevance

Spectral types provide a quick estimate of a star's temperature and luminosity, anchor the calibration of color and bolometric corrections, and identify peculiar and rare objects for follow-up.

History

The Harvard system, developed by Annie Jump Cannon and colleagues for the Henry Draper Catalogue, ordered stars by temperature; Morgan, Keenan, and Kellman added the luminosity dimension in the 1940s to create the enduring MK system.

Seminal works

morganKeenan1973
grayCorbally2009
gray2005

Frequently asked questions

What does the spectral sequence OBAFGKM mean?: It is an ordering of stars from hottest (O) to coolest (M) by surface temperature, defined by which spectral lines are strong; the letters survive from an earlier alphabetical scheme that was later reordered by temperature.
Why do hydrogen lines peak in A-type stars rather than the hottest stars?: Hydrogen lines require atoms in a specific excited state; in the hottest stars hydrogen is mostly ionized and in the coolest it is mostly in the ground state, so the lines are strongest at the intermediate temperatures of A-type stars.