RR Lyrae stars o-

-10 12 Log period (days)

Figure 9.6. (a) Typical variation of flux from a cepheid variable star as a function of time. The ordinate is "apparent photographic magnitude" (mpg ^ mB) which is proportional to the logarithm of the detected flux F(see Section 8.3). In this case, the flux changes by a factor of about two, and the period is about 5.5 d. (b) Average luminosity (right hand labels) of cepheid and RR Lyrae stars as a function of the period of oscillation. The left hand axis gives the absolute photographic magnitude, a measure of luminosity. [Adapted from Abell, Exploration of the Universe, 3rd Ed., Holt Rinehart Winston, 1975, p. 407, p. 411]

There are two types of cepheid variables recognizable by the shape of their light curves, the classical cepheids (Type I) and the less luminous Type II cepheids (Fig. 9.6b). The former are young stars with relatively short lifetimes, ~108 yr, and the latter are older stars of ages ~1.5 x 1010 yr. The former are found in galaxies with star-forming regions such as the Magellanic clouds and spiral galaxies, while the latter are found in places with older star populations such as the halo of the (MW) Galaxy and globular clusters.

There is also a class of RR Lyrae variables that are less luminous and have periods ranging from 0.2 to 0.9 d. They are found in, and yield distances to, globular clusters as distant as our neighboring sister galaxy, the Andromeda nebula (M31) at a distance of 2.5 MLY.

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