Spectral Classification
Spectral classification is the categorisation of stars into ordered types based on the characteristic absorption lines present in their spectra, which primarily reflect surface temperature. The modern Harvard spectral sequence — O, B, A, F, G, K, M — runs from the hottest blue-white O-type stars (~30,000 K) to the coolest red M-type stars (~3,000 K). Each spectral class is subdivided into ten numerical subclasses (0–9) and luminosity classes (I–V) in the MKK system, enabling astronomers to infer temperature, luminosity, radius, and evolutionary stage from a star's spectrum.
Harvard Spectral Classification Summary
| Class | Temperature (K) | Colour | Key Spectral Lines | Example Star |
|---|---|---|---|---|
| O | ≥ 30,000 | Blue | Ionised He, N, O | Zeta Puppis |
| B | 10,000–30,000 | Blue-white | Neutral He, H Balmer | Rigel |
| A | 7,500–10,000 | White | Strong H Balmer, Ca II | Sirius |
| F | 6,000–7,500 | Yellow-white | Weak H, strong Ca II | Procyon |
| G | 5,200–6,000 | Yellow | Ca II, ionised metals | Sun |
| K | 3,700–5,200 | Orange | Ca II, TiO weak | Arcturus |
| M | 2,400–3,700 | Red | Strong TiO, VO bands | Betelgeuse |
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Related Terms
Absolute Magnitude
Absolute magnitude is the intrinsic brightness of a celestial object expressed as the apparent magnitude it would have if placed at a standard distance of 10 parsecs (32.6 light-years) from the observer. It provides a true measure of luminosity independent of the object's actual distance, allowing direct comparison between stars. Astronomers use absolute magnitude to classify stellar populations, construct the Hertzsprung–Russell diagram, and estimate distances via the distance modulus.
Stellar Nucleosynthesis
Stellar nucleosynthesis is the process by which nuclear fusion reactions inside stars create heavier atomic nuclei from lighter ones, releasing energy that sustains the star against gravitational collapse. Main-sequence stars primarily fuse hydrogen into helium via the proton–proton chain or CNO cycle, while more massive stars in later evolutionary stages fuse helium, carbon, oxygen, and silicon up to iron (Fe-56), the most tightly bound nucleus. Elements heavier than iron are synthesised through neutron-capture processes (s-process in AGB stars; r-process in neutron star mergers and supernovae), making stars the principal factories of the chemical elements in the universe.
Variable Star
A variable star is any star whose observed brightness (apparent magnitude) changes over time, whether due to intrinsic physical changes in the star itself or due to geometric effects such as eclipses or rotation. Intrinsic variables include pulsating stars (Cepheids, RR Lyrae, Mira), eruptive variables (novae, flare stars), and cataclysmic variables (dwarf novae, Type Ia supernovae). Of particular cosmological importance are Cepheid variable stars, whose pulsation period is directly related to their intrinsic luminosity (the period–luminosity relation), making them crucial standard candles for measuring distances to nearby galaxies.
From Latin "spectrum" (appearance, image) and Latin "classificatio" (arranging in classes). The Harvard Classification system was developed by Annie Jump Cannon and Edward Pickering between 1890 and 1924 at Harvard Observatory.