RR Lyrae stars are variable stars that pulsate -- they are photometrically variable because their photospheres move in and out in a regular way. This class of stars is named for the prototype RR Lyrae, a variable star in the constellation Lyra.

RR Lyrae stars were first discovered in the late nineteenth century, where they were found in great numbers in globular clusters. They were first called ``cluster variables,'' a name which they retained for many decades. In 1899, the field star RR Lyrae was found to be variable by Willamina Fleming of the Harvard College Observatory, and the connection between its pulsation behavior and that of the cluster variables was established. In 1902, Solon I. Bailey, also of Harvard, grouped the RR Lyrae stars into several classes according to the shapes of their light curves and their periods. In 1913, C. C. Kiess of the Lick Observatory published a short paper in Publications of the Astronomical Society of the Pacific (PASP 25, 121), relating the behavior of the cluster variables to that of the Cepheid variables. The RR Lyrae were simply considered to be short-period Cepheids, which while incorrect, is not far wrong. Harlow Shapley, also studied the RR Lyrae stars in great detail, publishing both a comprehensive analysis of RR Lyrae in 1916 (AstrophysicalJournal 43, 217), and a calibration of RR Lyrae absolute magnitudes and a determination of globular cluster distances in 1918 (Astrophysical Journal 48, 89). It was the latter work which Shapley used as the basis for his determination of the structure of the Milky Way.

Work continued on the cluster variables and field RR Lyrae for several decades. In 1940, Martin Schwarzschild first showed that the RR Lyrae stars in clusters are not true Cepheids, but are much more evolved stars which lie on the horizontal branch. (The Cepheids are in the brief stage of their lives between the main sequence and the red giant branch.) Now, the ``cluster variables'' and field RR Lyrae stars are all known to be the same kind of star. They are very evolved stars, with masses less than that of our Sun. They are all metal-poor compared to the Sun (meaning their chemical compositions contain fewer metallic elements than the Sun), and are all very old. Although they are not true Cepheids, they lie on the Cepheid instability strip, a region of the Hertzsprung-Russell Diagram where stars are capable of sustaining pulsations with no external driving mechanism. The RR Lyrae stars all have pulsation periods between 0.2 and 1.2 days, and thus have much shorter periods than most of the Cepheids.

The RR Lyrae stars are divided into several Bailey types, mentioned above, which are: RRa, RRb, RRab, RRc, and RRd. The RRa, b, and ab stars are all fundamental mode pulsators, meaning that the entire star ``breathes'' in and out at the same time. In one dimension, the fundamental mode is analogous to the lowest tone of a plucked guitar string, where there are no nodal points along the string. The RRc stars are first overtone pulsators, which means that at a certain radius from the center, there is a shell where the stellar material does not move (called a ``node''), and waves propagate in and out above and below that point. The RRc stars have shorter periods than the RRa,b stars. The RRd pulsate in both the fundamental and first overtone mode at the same time.

The RRa stars have very asymmetric light curves, almost like a sawtooth wave (just like the ascii art there, except mirror it left-to-right). The RRa stars have very large amplitudes, sometimes greater than one magnitude. This large amplitude corresponds to very large changes in radial velocity, and the high radial velocity amplitudes in turn cause a shock wave. The shock is strong enough in these stars to actually cause ionization in the stellar photosphere, generating emission lines of hydrogen during maximum light. The RRab and RRb stars have similar light curves, but with lower amplitude, and they do not generate emission lines during the pulsation cycle. The RRc stars have very rounded light curves, almost like a pure sine wave. The reason the RR Lyrae stars are divided into these classes is due to their location within the Cepheid instability strip. Stars on the hot (blue) side of the instability strip will be RRc stars, while stars on the cool (red) side of the instability strip will be RRab stars.

In his 1916 paper, Shapley was also the first to note the Blazhko effect, a long-period modulation in the light curves of some RR Lyrae stars. RR Lyrae shows a small modulation with a period of 41 days superimposed upon the dominant period of 0.56 days. The reason for the Blazhko effect is not known. One possibility is that the dominant period may excite non-radial or sectoral mode pulsations with similar oscillation periods. These modes interfere with the dominant mode, creating a Blazhko modulation at the beat period of the two modes. A paper on this theory was recently published by R. Nowakowski and W. Dziembowski (Acta Astronomica 51, 5), though there is currently no consensus as to the correct answer.

The RR Lyrae stars are very important in astrophysics, because they are used for determining the distances to globular clusters and nearby galaxies. There are also many field RR Lyrae stars in the halo of the Milky Way -- since we have a way of measuring their distances, we can use them to determine the shape and size of our Galaxy's halo. They are still important targets of study, because their exact luminosity calibration has yet to be determined -- their properties seem to be influenced heavily by their chemical composition. Their high amplitudes make them good targets for amateur astronomers interested in studying variable stars. In fact, the American Association of Variable Star Observers or AAVSO has ongoing observing campaigns on many RR Lyrae stars which backyard astronomers can contribute to.

I made use of the Encyclopedia of Astronomy and Astrophysics recently published by the Institute of Physics, as well as the NASA ADS Abstract Service at http://adsabs.harvard.edu. The AAVSO maintains a webpage at http://www.aavso.org.

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