Variable star research is an important branch of modern astronomy – amateur observers make very valuable contributions. Variable stars are of many types; elaborate systems of classifying them have been proposed, and the data given here are not intended to be more than a general guide. Seven major categories are now recognised.

1. (1) Eclipsing stars (more properly eclipsing binaries, because they are not intrinsically variable).

2. (2) Pulsating variables: either radial or non-radial pulsations.

3. (3) Eruptive variables, where the changes are caused by flares or the ejection of shells of material.

4. (4) Cataclysmic variables, where the changes are due to explosions in the star or in an accretion disc round it. Novæ dwarf novæ and supernovæ come into this category.

5. (5) Rotating variables, where the changes are caused by star-spots, non-spherical shape or magnetic effects.

6. (6) X-ray variables, usually inherent in the neutron star or black hole companion of a binary.

7. (7) Unclassifiable stars, which do not fit into any accepted category.

We have already noted what are termed secular variables: stars which have permanently brightened or faded in historic times. Thus Ptolemy ranked β Leonis and θ Eridani as of the first magnitude, whereas today they are below magnitude 2 and 3 respectively: α Ophiuchi was ranked of magnitude 3, but is now 2.1. However, these changes must be regarded as highly suspect. It is unwise to trust the old observations too far.