They are pulsatinghorizontal branch stars ofspectral class A or F, with a mass of around half theSun's. They are thought to have shed mass during thered-giant branch phase, and were once stars at around 0.8 solar masses.
In contemporary astronomy, aperiod-luminosity relation makes them goodstandard candles for relatively nearby targets, especially within the Milky Way andLocal Group. They are also frequent subjects in the studies ofglobular clusters and the chemistry (and quantum mechanics) of older stars.
In surveys of globular clusters, these "cluster-type" variables were being rapidly identified in the mid-1890s, especially byE. C. Pickering. Probably the first star definitely of RR Lyrae type found outside a cluster wasU Leporis, discovered byJ. Kapteyn in 1890. The prototype starRR Lyrae was discovered prior to 1899 byWilliamina Fleming, and reported by Pickering in 1900 as "indistinguishable from cluster-type variables".[1]
From 1915 to the 1930s, RR Lyrae variables became increasingly accepted as a class of star distinct from theclassical Cepheids, due to their shorter periods, differing locations within the galaxy, and chemical differences. RR Lyrae variables are metal-poor, Population II stars.[1]
RR Lyraes have proven difficult to observe in externalgalaxies because of their intrinsic faintness. (In fact,Walter Baade's failure to find them in theAndromeda Galaxy led him to suspect that the galaxy was much farther away than predicted, to reconsider the calibration ofCepheid variables, and to propose the concept ofstellar populations.[1]) Using theCanada-France-Hawaii Telescope in the 1980s, Pritchet and Van Den Bergh found RR Lyrae variables in the Andromeda Galaxy's galactic halo.[2] More recently, observations with theHubble Space Telescope found them in its globular clusters.[3]
The RR Lyrae stars are conventionally divided into three main types,[1] following classification byS.I. Bailey based on the shape of the stars' brightness curves:
RRab variables are the most common, making up 91% of all observed RR Lyrae, and display the steep rises in brightness typical of RR Lyrae
RRc are less common, making up 9% of observed RR Lyrae, and have shorter periods and more sinusoidal variation
RRd are rare, making up between <1% and 30%[4] of RR Lyrae in a system, and are double-mode pulsators, unlike RRab and RRc
RR Lyrae-type variable stars close to the galactic center from theVVVESO public survey
RR Lyrae stars were formerly called "cluster variables" because of their strong (but not exclusive) association withglobular clusters; conversely, over 80% of all variables known in globular clusters are RR Lyraes.[5] RR Lyrae stars are found at all galactic latitudes, as opposed toclassical Cepheids, which are strongly associated with the galactic plane.
Because of their old age, RR Lyraes are commonly used to trace certain populations in the Milky Way, including the halo and thick disk.[6]
Several times as many RR Lyraes are known as all Cepheids combined; in the 1980s, about 1900 were known in globular clusters. Some estimates have about 85,000 in the Milky Way.[1]
Thoughbinary star systems are common for typical stars, RR Lyraes are very rarely observed in binaries.[7]
RR Lyrae stars pulse in a manner similar toCepheid variables, but the nature and histories of these stars is thought to be rather different. Like all variables on theCepheid instability strip, pulsations are caused by theκ-mechanism, when the opacity of ionised helium varies with its temperature.
RR Lyraes are old, relatively low mass,Population II stars, in common withW Virginis andBL Herculis variables, thetype II Cepheids.Classical Cepheid variables are higher masspopulation I stars. RR Lyrae variables are much more common than Cepheids, but also much less luminous. The averageabsolute magnitude of an RR Lyrae star is about +0.75, only 40 or 50 times brighter than theSun.[8] Their period is shorter, typically less than one day, sometimes ranging down to seven hours. Some RRab stars, including RR Lyrae itself, exhibit theBlazhko effect in which there is a conspicuous phase and amplitude modulation.[9]
Unlike Cepheid variables, RR Lyrae variables do not follow a strict period-luminosity relationship at visual wavelengths, although they do in the infraredK band.[10] They are normally analysed using a period-colour-relationship, for example using a Wesenheit function. In this way, they can be used asstandard candles for distance measurements although there are difficulties with the effects of metallicity, faintness, and blending. The effect of blending can impact RR Lyrae variables sampled near the cores of globular clusters, which are so dense that in low-resolution observations multiple (unresolved) stars may appear as a single target. Thus the brightness measured for that seemingly single star (e.g., an RR Lyrae variable) is erroneously too bright, given those unresolved stars contributed to the brightness determined. Consequently, the computed distance is wrong, and certain researchers have argued that the blending effect can introduce a systematic uncertainty into thecosmic distance ladder, and may bias the estimated age of the Universe and theHubble constant.[11][12][13]
TheHubble Space Telescope has identified several RR Lyrae candidates in globular clusters of theAndromeda Galaxy[3] and has measured the distance to the prototype star RR Lyrae.[14]
TheKepler space telescope provided accuratephotometric coverage of a single field at regular intervals over an extended period. 37 known RR Lyrae variables lie within the Kepler field, including RR Lyrae itself, and new phenomena such as period-doubling have been detected.[15]
TheGaia mission mapped 140,784 RR Lyrae stars, of which 50,220 were not previously known to be variable, and for which 54,272interstellar absorption estimates are available.[16]
ThePanSTARRS1 3π survey identified ~45,000 RR Lyrae stars, representing the widest (covering 3/4 of the sky) and deepest (reaching up to 120 kpc) sample of RR Lyrae stars to date. In 2017, Sesar et al. used these stars to develop a novel template-fitting technique, achieving highly accurate period estimates with precision better than 2 seconds in over 80% of cases.[17]
TheDark Energy Survey (DES) was used to identify ~6000 RR Lyrae candidates in the southern sky , ~31% of which are previously undiscovered. The survey also improved period-luminosity relations, advancing distance measurements and studies of galactic structure.[18]
Feng et al. (2024) used deep imaging from the Next Generation Virgo Cluster Survey (NGVS) to identify 180 faint RR Lyrae candidates located at galactocentric distances of roughly 20–300 kpc, extending well into the outer Milky Way halo. About 100 of these sources were previously uncataloged in Pan-STARRS 1 (PS1).[19]
^Pritchet, Christopher J.; Van Den Bergh, Sidney (1987). "Observations of RR Lyrae stars in the halo of M31".Astrophysical Journal.316: 517.Bibcode:1987ApJ...316..517P.doi:10.1086/165223.
^Lee, Jae-Woo; López-Morales, Mercedes; Hong, Kyeongsoo; Kang, Young-Woon; Pohl, Brian L.; Walker, Alistair (2014). "Toward a Better Understanding of the Distance Scale from RR Lyrae Variable Stars: A Case Study for the Inner Halo Globular Cluster NGC 6723".The Astrophysical Journal Supplement.210 (1): 6.arXiv:1311.2054.Bibcode:2014ApJS..210....6L.doi:10.1088/0067-0049/210/1/6.S2CID119280050.
^Neeley, J. R.; Marengo, M.; Bono, G.; Braga, V. F.; Dall'Ora, M.; Stetson, P. B.; Buonanno, R.; Ferraro, I.; Freedman, W. L.; Iannicola, G.; Madore, B. F.; Matsunaga, N.; Monson, A.; Persson, S. E.; Scowcroft, V.; Seibert, M. (2015). "On the Distance of the Globular Cluster M4 (NGC 6121) Using RR Lyrae Stars. II. Mid-infrared Period-luminosity Relations".The Astrophysical Journal.808 (1): 11.arXiv:1505.07858.Bibcode:2015ApJ...808...11N.doi:10.1088/0004-637X/808/1/11.S2CID117031686.
^Sridhara, Shravan Calvin; Thomas, Ethan; Dong, Hannibal; Chenrayan, Meiyappa Sugam; Guhathakurta, Puragra; Feng, Yuting; Peng, Eric (2025). "Identification of Faint Quasars via Spectroscopy of Distant Milky Way Halo RR Lyrae Candidates".American Astronomical Society Meeting Abstracts.245.Bibcode:2025AAS...24547004S.
^Kappagantula, Chandrasekhar; Gilligan, Finn; Chen, Sihan; Feng, Yuting; Guhathakurta, Puragra; Peng, Eric; Komiyama, Yutaka (2025). "Testing the Classification and Verifying the Pulsational Parameters of RR Lyrae candidates using a pilot sample of Hyper Suprime-Cam survey data".American Astronomical Society Meeting Abstracts.245.Bibcode:2025AAS...24547003K.
^Ingkasampan, Rasmiwan (Pear); Anand, Anya; Pramanik, Chetana; Agarwal, Tannuvi; Feng, Yuting; Guhathakurta, Puragra; Peng, Eric (2025). "Testing the Performance of a RR Lyrae Light Curve Template Fitting Algorithm".American Astronomical Society Meeting Abstracts.245.Bibcode:2025AAS...24547006I.
