RR LYRAE STARS; These stars belong to the Cepheids but have very short periods of less than 1 day.  W H van den Bos of the Union Observatory found one with a period of 1½ hours!

The prototype, RR Lyrae (RA 19h 25,3m; dec +42°47') has a period of 0,5668 day or 13h 36m 11,5s. It varies from maximum magnitude 7,06 to a minimum of 8,12. The periods of the RR Lyrae stars range from 0,2 to 1,2 days and the amplitudes of their variations in brightness are between 0,2 and 2 magnitudes. The maxima of their negative radial velocities during expansion coincide with their maxima of brightness in their light curves; and the minima of their positive radial velocities during contraction coincide with the minima of their light curves.

These stars are all of spectral type A (a few are late B types) and they are bright giants. At minimum their spectral types change to types F and G. They all have the same absolute magnitude of 0,8, i.e. 40 times the brightness of the Sun. The Sun's absolute magnitude is 4,85.
Therefore its brightness is .
The brightness of the RR Lyrae stars is .
The brightness of the RR Lyrae stars compared to the Sun is thus divided by .

This simplifies to 2,512^{4,85 - 0,8} = 2.512^4,05 and this works out to 41,687, so we can safely say they are 40 times brighter than the Sun.

Once a variable star has been identified by its short period and the shape of its curve as being a RR Lyrae star, it can be assumed that its absolute magnitude is 0,8. If we then measure it apparent visual magnitude m, we can use the formula M = 0,8 = m + 5 - 5 logD to determine its distance D in parsecs because D is the only unknown once m is found. m will be its average magnitude over its period of variation. If we multiply parsecs by 3,26, we obtain light years.

An amateur can start his study of RR Lyrae stars by monitoring the stars in the attached list. The maxima and minima have been rounded off so that the amateur can taste the pleasure of determining the exact values of the magnitudes. The periods are the exact values that have been determined. The spectral types are those at maximum and minimum respectively.

 Jan Eben van Zyl