The Spectral Classification System is used by astronomers to group stars according to the properties of their electromagnetic spectra. All stars emit electromagnetic radiation in a pretty continuous spectrum going from gamma rays to radio waves. However, each star emits different parts of the spectrum with different intensity, giving rise to colours for stars.
For example, a red giant star such as Betelgeuse emits red light most intensely, so it looks red, although it emits other colours too. In addition to this, each star has various absorption lines where it doesn't emit any light at all. One such set of lines are those caused in the visible spectrum by the gas hydrogen, also known as the Balmer Series. When the spectral classification system was invented, stars were assigned a letter of the alphabet depending on how pronounced the Balmer lines were in the star's spectrum. In this system, class A stars had the most pronounced Balmer lines.
However, after further study, it was decided that the temperature of a star was more important to classification than the absorption lines of hydrogen. The existing spectral classes were put into the following order:
O, B, A, F, G, K, M
With Class O stars being hottest, and M stars being coolest1. To remember the order of the spectral classes, astronomy students use mnemonics such as 'Oh Be A Fine Girl/Guy Kiss Me!'.
This classification scheme, known as the Harvard Classification Scheme system, and the mnemonic were devised by the American astronomer Annie Jump Cannon. She made major contributions to the Henry Draper Catalog of stellar spectra, adding over 230,000 stars over a 40 year career.
Here are the main features of each of the spectral classes:
| Spectral Class | Colour | Surface Temperature | Spectral Features of this Class | Examples |
|---|
| O | Blue | 30,000 | These stars have few absorption lines, and weak hydrogen lines. There are some lines of ionized helium and other ionised atoms. | Naos |
| B | Blue-white | 11,000 - 30, 000 | These stars have more pronounced hydrogen lines, as well as neutral helium lines. | Rigel, Spica |
| A | White | 7,500 - 11,000 | This class has the strongest hydrogen lines. Ionised metal lines are also visible, with a few weak neutral metal lines. | Sirius, Vega |
| F | Yellow-white | 6,000 - 7,500 | This class has less strong hydrogen lines than class A, but they are still quite strong. There are also lines from singly ionised metals and neutral metals. | Canopus, Procyon |
| G | Yellow | 5,000 - 6000 | The most conspicuous lines for G stars are ionised calcium, but there are other ionised and neutral metal lines present. Hydrogen lines are weak. | Sun, Alpha Centauri A, Capella |
| K | Orange | 3,500 - 5000 | Neutral Metal lines are most visible in this class. | Aldebaran |
| M | Reddish | 3,500 | This class has strong neutral metal lines, also lines from molecules. | Betelgeuse, Antares |
In addition, each spectral class is divided into numbers from 0 - 9, 0 being the hottest. The sun, for example, is a G2 star.
Classifying stars in this manner is useful, because it means that if you know the colour of a given star, you can work out its temperature as a rough approximation.
1 Class A stars still have the strongest Balmer/hydrogen absorption lines.
