SIMBAD references

Data in Solon Bailey's 1902 paper on variable stars in the globular cluster Omega Centauri showed, in crude form, the existence of a Cepheid period-luminosity relation several years before Henrietta Leavitt's discovery of period-luminosity relation among the Cepheid's of the Small Magnellanic Cloud. Bailey's pioneering observations remain useful in studies of changes in the pulsation periods of variable stars within globular clusters. The study of period changes amoung globular cluster variable stars is a field in which amateur astronomers may be able to make a significant contribution.