Spectral Classification System
Created | Updated Feb 23, 2005
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 intensly, 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 furthur 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 this order:
O B A F G K M
With Class O stars being hottest, and M stars being
coolest.1
To remember the order of the spectral classes, astronomy students use mnemonics
such as Oh Be A Fine Girl/Guy Kiss Me!
Here are the main features of each of the spectral
classes:
| Spectral Class | Colour | Surface Temp. | Spectral Features of this Class | Examples |
| O | Blue-white | 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- | These stars have more pronounced hydrogen lines, as well as neutral helium lines. | Rigel, Spica |
| A | Blue-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 | Blue-white to 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 | White to yellow-white | 5,000- 6000 | The most conspicuos lines for G stars are ionized calcium, but there are other ionised and neutral metal lines present. Hydrogen lines are weak. | Sun, Alpha Centauri A, Capella |
| K | yellow-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 hottest. The sun, for example, is a G2 star.
Classifying stars in this manner is useful, bacause it means that if you know
the colour of a given star, you can work out its temperature.
