1. Delta Cepheid Types

The monitoring of variable stars should form a good part of the observing done by an amateur astronomer and it can be work of a professional standard as was shown by the late Danie Overbeek whose work received world-wide acclaim.

There are two main types of variable stars: those that revolve around each other as eclipsing binaries and those that undergo intrinsic changes in their interiors. A very important type of the latter is the Cepheid variables of which the star Delta Cephei is the prototype. It was discovered to be variable in brightness by a friend of John Goodricke, the deaf and dumb amateur who died at the age of 22. However, John is credited with the discovery in 1784. He correctly diagnosed the variability as being due to pulsations of the star, whereby it alternately expands and contracts, swelling up and shrinking. The curve of apparent magnitude against time could not be explained by any form of eclipse. The rise to maximum brightness is steeper than the decline to minimum. Actually the brightness reaches a maximum one-quarter period after the star reaches maximum size and minimum one-quarter period after it reaches minimum size, shown by the points a and c in the diagram. When the star expands, its surface area increases and its temperature drops. The subsequent contraction raises the temperature so that the brightness increases. Further contraction decreases the size of the radiating surface so that the total flux of energy decreases. In the diagram the sizes of the star are exaggerated.. According to C Hoffmeister, G Richter and W Wentzel in their book Veränderliche Sterne (Springer-Verlag, London), the pulsating of the Cepheids leads to the absorption of energy from the star's centre by doubly ionised Helium (He III) at a depth of a few hundred thousand kilometres below the surface. An undamped oscillation then causes the over- lying layers to expand so that the temperature drops and the apparent magnitude changes from 3,8 to 4,6. While so doing the spectral class changes from F to G, proving that intrinsic changes do take place in the star. The temperature changes from 6500° to 5500°. The absolute magnitudes of these stars range from -2 to -7 so they are giants. The visual magnitudes change by only 0,6 to 1,7 magnitudes.

The periods of Cepheid variables range from less than 2 days to hundreds of days. It is best to classify these variables according to the lengths of their periods:

(a) Less than 2 days, the RR Lyrae variables

(b) 2 to 20 days        Classical Cepheids.

(c) 20 to 40 days      Classical Cepheids

(d) Long period variables having periods of more than 40 days. They are of types K and M

Below is appended a list of Cepheids which an amateur can use as starting material. The values of apparent magnitudes and periods in days have been rounded off so that the amateur can taste the pleasure of determining them accurately. With an electronic photometer the values of apparent magnitude can be determined correct to two places of decimals. The stars have been selected to cover right ascensions around the clock and the declinations are within the range of southern observers.

Jan Eben van Zyl