8 - Stellar Pulsation and Oscillation

That some stars vary in brightness has been known from time immemorial. That this variation is the result of intrinsic changes in the star itself has been known for less than 100 years and the causes of those variations have been understood for less than 30 years. It is not a simple matter to distinguish the light variations resulting from eclipses by a companion from those caused by physical changes in the star itself. However, for the cepheid variables (named for the prototype example δ Cepheii), the treatment of the light variations as if they resulted from eclipses by a companion leads to some absurd results. If this were the case, the orbit of the companion would have to be highly elliptical with the semimajor axes pointed toward the earth. This somewhat Ptolemaic view suggests that the binary hypothesis is incorrect but is far from conclusive. Further analysis shows that the sum of the radii for the two hypothetical stars would exceed their separation. Such a situation is even less likely. However, the coup degrace is administered to the binary hypothesis when one considers the radial velocity curve produced by the star throughout a period of light variation. Here one finds that maximum radial velocity occurs near minimum light, which is nearly the reverse of that required by the binary hypothesis. Thus, one is left with almost no other option than to conclude that this class of stars is intrinsically varying.