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When gravity makes them contract again, the cycle repeats. These stars, all confined to a narrow region of the Hertzsprung-Russell diagram, pulsate for the same reason: pulsations are driven by radiation being partially blocked from escaping the star, and the resulting increase in pressure and temperature makes them expand. The RR Lyrae are members of an elite class of pulsating variables known as instability strip pulsators. The name RR Lyrae variable subsequently became a fitting title for this important class of stars. This effect, later known as the Blazhko Effect, has continued to provide a puzzle for astrophysicists to the present day.Īlthough RR Lyrae was not the first "RR Lyrae star" discovered - both the cluster variables and the two field stars U Lep and S Ara came first - RR Lyrae is by far the brightest, and its brightness made it an easy target for both photometrists and spectroscopists. Shapley also noted an important fact about RR Lyrae using the observations of Harvard's Oliver Wendell as well as his own: the times of maximum and the shape of RR Lyrae's light curve varies in a cyclical way with a period of around 40 days. In his comprehensive 1916 review paper on RR Lyrae, Harlow Shapley made it clear that the binary hypothesis for variations in the "Cepheid variables" (with which he included the cluster variables) was inconsistent with both the spectroscopic and photometric variations spectra suggested that the "orbits" of these binaries would have to be unphysically small, which photometry showing variations in the rise time to maximum required unphysical variations in the hypothetical orbital parameters. This enabled astronomers to measure changes in spectral type, as well as to detect the presence of emission lines. RR Lyrae's brightness (between 7th and 8th magnitude) made it bright enough to observe spectroscopically in such a way that the changes in its spectrum could be traced throughout its cycle of variability. Regular observations of this brightest "cluster variable" of the field commenced at Harvard as well as at other major observatories including Lick and Mt. The star, with a range of more than 3/4 of a magnitude and period of just over half a day, clearly resembled those of the cluster variables (also discovered by Fleming in her analysis of plates from Solon Bailey's cluster survey in 1893). Examination of this plate by one of Pickering's staff, Wilhelmina Fleming, revealed a short-period, high amplitude star. The result is that multiple exposures of a given star were obtained during an evening's observing, and that the periods for short stars might be more efficiently obtained.Ī 1901 Astrophysical Journal paper by Pickering provides a list of sixty four new variables, one of which - a star in the constellation Lyra - was found using the method above on a plate from July 13, 1899. A photographic plate was alternately exposed and covered over preestablished intervals in a telescope whose alignment and tracking rate were not precisely aligned with the sky. In it, Pickering describes a technique for obtaining multiple photographic exposures of a star in a short amount of time - a primitive but effective form of time-series photometry. One of these was a short paper in Harvard Circular Number 29 (1898) describing a simple technique for the study of short period variables. The director, Edward Charles Pickering, and his extensive staff of "computers" - women who carefully conducted many of the tedious calculations or searches of photographic plates at the observatory - released dozens of papers and catalogues detailing their efforts in stellar cartography and photometry, asteroid searches and photometry, and variable stars. Harvard College was a hive of variable star activity in the late 19th century. Both its visual prominence and its historical stature make it a fitting target for the September 2010 Variable Star of the Season. RR Lyrae itself is a variable easily within view of most northern observers with modest telescopes or binoculars, and yet it remains a target for major observatories and research programs. RR Lyrae and the class of pulsating variable stars that bears its name had a profound influence on astrophysics of the 20th Century, and it's likely that our understanding of both the size and nature of our Universe would be far more incomplete without these important stars.
HIGH RESOLUTION SMITH CHART SERIES
Our Variable Star of the Season series returns from hiatus with a long-neglected astronomical gem: RR Lyrae, the prototype of one of the most important classes of variable star in astronomy. RR Lyrae, 1 degree field, DSS I survey plate