RR RR Lyrae stars

Radially pulsating A-F stars having amplitudes from 0".'2 to 2"' in V. Cases of variable light-curve shapes as well as variable periods are known. If these changes are periodic, they are called the "Blazhko effect." Traditionally, RR Lyr stars are sometimes called short-peripd Cepheids or cluster-type variables. The majority of these stars belong to the spherical component of the Galaxy; they are present, sometimes in large numbers, in some globular clusters, where they are known as pulsating horizontal-branch stars. Like Cepheids, maximum expansion velocities of surface layers for these stars practically coincide with maximum light. RR(B) (subtype) - RR Lyrae variables showing two simultaneously operating pulsation modes, the fundamental tone with the period P0 and the first overtone P,. The ratio PJP0 «i 0.745. RRAB (subtype) - RR Lyrae variables with asymmetric light curves (steep ascending branches), periods from Oi'3 to 1*2, and amplitudes from 0".'5 to 2'" in V. RRC (subtype) RR Lyrae variables with nearly asymmetric, sometimes sinusoidal, light curves, periods from 0'!2 to fffs, and amplitudes not greater than 0".'8 in V. GCVS

RR Lyrae stars have been studied for over 100 years. Their study initiated from the end of the nineteenth century when astronomers began an increasingly close scrutiny of globular clusters. It was during these observations that the first short-period variable stars were discovered. Although the first globular cluster variable to be discovered was a nova which erupted in M80 in the year 1860, it took another three decades before E.C. Pickering reported the discovery of a second globular cluster variable near the center of M3.

During the following few years a small number of bright globular cluster variables began to be discovered. In response to these discoveries, Solon I. Bailey began a program of globular cluster photography at the Harvard College Observatory station in Arequipa, Peru in 1893. Upon close examination of these photographic plates, Williamina Fleming discovered a variable star in the globular cluster w Centauri in August of that year. Following Fleming, Pickering detected six more variable stars in the same globular cluster in 1895 and this began the flood of ensuing discoveries. Between 1895 and 1898 more than 500 variable stars were discovered in globular clusters. These first short-period variables became known as the RR Lyrae variable stars and during the past century the number of RR Lyrae stars has grown to outnumber known members of any other well-defined class of variable star.

Of historical note, E.C. Pickering's 1889 discovery of a variable in M3 was probably a Cepheid rather than an RR Lyrae star but it does serve to demonstrate the initial confusion in distinguishing these two types of variable stars. As another example, Sir Arthur Edding-ton included RR Lyrae in a table of important Cepheid variables in his influential book The Internal Constitution of the Stars. Other astronomers, such as Henry Norris Russell, drew the distinction between Cepheids and RR Lyrae stars more precisely. Despite similarities with pulsating Cepheid variables the RR Lyrae variable stars from the first decades of this century were usually looked upon as a distinct class of variables, though having considerable kinship to the Cepheids.

Today, it is known that all RR Lyrae star are low-mass horizontal branch stars in the core helium burning stage of evolution and this has provided additional argument for distinguishing them from the higher-mass classical Cepheids. However, there have been adjustments from time to time in the types of variable star included in the class of RR Lyrae stars. A particular confusion arose with the short-period pulsating variable stars which are now usually called S Scuti when they are classed as Population I stars, or SX Phoenicis stars when determined as Population II stars.

Referred to as cluster-type variables in the past for obvious reasons since they can occur in considerable numbers within globular clusters, you may occasionally find this classification still used today. RR Lyrae variable stars are giant radial pulsating stars with periods of ~0d2 to IrO. Based on their light curves, the RR Lyrae stars can be subdivided into two main groups. The RRab stars have relatively large light amplitudes with visual amplitudes of about one magnitude being common. Their light curve is asymmetrical with a steep rising branch. The RRab Lyrae stars are believed to be pulsating in their fundamental mode and generally have periods of -0^4 to li*0. The RRc variables have smaller light amplitudes and their light curves are more nearly sinusoidal in shape. These variable stars are believed to be pulsating in their first overtone (sometimes called the first harmonic mode) and generally have periods of ~0?2 to 0^5.

Many of the RR Lyrae variables display long-term modulations of their light curves. This phenomenon is known as the Blazhko effect. Its cause is still not well understood. The modulation periods are generally in the range of 20-200 days and the effect on the light curve can be quite marked. For instance, in RR Lyrae itself the visual amplitude of the star varies by about 01'3 over the Blazhko cycle and the shape of the light curve changes. For some variables the Blazhko effect itself is modulated on an even longer time-scale. In the case of RR Lyrae the tertiary period is -3.8-4.8 years.

The study of RR Lyrae stars has contributed to almost every branch of modern astronomy including: being used as tracers of the chemical and the dynamic properties of old stellar populations within our own and nearby galaxies; serving as standard candles, indicating the distances to globular clusters, to the center of the Galaxy, and to neighboring Local Group systems; and serving as test objects for theories of the evolution of low-mass stars and for theories of stellar pulsation.

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