The absorption features present in stellar spectra allow us to divide stars into several spectral types depending on the temperature of the star. The scheme in use today is the Harvard spectral classification scheme which was developed at Harvard college observatory in the late 1800s, and refined to its present incarnation by Annie Jump Cannon for publication in 1924.
Originally, stars were assigned a type A to Q based on the strength of the hydrogen lines present in their spectra. However, it was later realised that there was significant overlap between the types, and some of the letters were dropped. Continuity of other spectral features was also improved if B came before A and O came before B, with the end result, the spectral sequence: OBAFGKM. This sequence is ordered from the hottest to the coolest stars, and is often remembered by the mnemonic ‘Oh Be A Fine Girl/Guy, Kiss Me’.
The following table summarises the main spectral types in the Harvard spectral classification scheme:
|more than 28,000K||10,000-28,000K||7,500-10,000K||6,000-7,500K||5,000-6,000K||3,500-5,000K||less than 3,500K|
|few visible absorption lines, weak Balmer lines, ionised helium lines||neutral hydrogen lines, more prominent Balmer lines||strongest Balmer lines, other strong lines||weaker Balmer lines, many lines including neutral metals||Balmer lines weaker still, dominant ionised calcium lines||neutral metal lines most prominent||strong neutral metal lines and molecular bands|
Unfortunately, proper classification of a stellar spectrum is not quite this simple. Within each spectral type there are significant variations in the strengths of the absorption lines, and each type has been subdivided into 10 sub-classes numbered 0 to 9. In addition, stars of a particular spectral type can differ widely in luminosity and must also be assigned a luminosity class. This distinguishes main sequence stars (dwarf stars) from giant and supergiant stars.
As an example of the full classification of a star, let us consider the Sun. It is a main sequence star (luminosity class ‘V’) with a temperature of about 5,700 Kelvin. In the modern Harvard classification scheme, our Sun is a G2V star.
See also: Hertzsprung-Russell Diagram.