RR Lyrae variable

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The RR Lyrae variable stars fall in a particular area on a Hertzsprung–Russell diagram of color versus brightness.

RR Lyrae variables are periodic variable stars, commonly found in globular clusters, and often used as standard candles to measure galactic distances.

This type of variable is named after the prototype, the variable star RR Lyrae in the constellation Lyra.

RR Lyraes are pulsating horizontal branch stars of spectral class A (and sometimes F), with a mass of around half the Sun's. They are thought to have previously shed mass and consequently, they were once stars with similar or slightly less mass than the Sun, around 0.8 solar masses.

RR Lyrae stars pulse in a manner similar to Cepheid variables, so the mechanism for the pulsation is thought to be similar, but the nature and histories of these stars is thought to be rather different. In contrast to Cepheids, RR Lyraes are old, relatively low mass, metal-poor "Population II" stars. They are much more common than Cepheids, but also much less luminous. The average absolute magnitude of an RR Lyrae is about 0.75, only 40 or 50 times brighter than our Sun.[1] Their period is shorter, typically less than one day, sometimes ranging down to seven hours. The RR Lyrae subtype RRab (see Subtypes section below) sometimes exhibit the Blazhko effect in which there is a conspicuous phase and amplitude modulation.

The relationship between pulsation period and absolute magnitude of RR Lyraes makes them good standard candles for relatively near objects, especially within the Milky Way. They are extensively used in globular cluster studies, and also used to study chemical properties of older stars.

Abundances and distribution[edit]

RR Lyrae stars were formerly called "cluster variables" because of their strong (but not exclusive) association with globular clusters; conversely, about 90% of all variables known in globular clusters are RR Lyraes. RR Lyrae stars are found at all galactic latitudes, as opposed to classical Cepheid variables, which are strongly associated with the galactic plane.

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 85000 in the Milky Way.[2]

Though binary star systems are common for typical stars, RR Lyrae are very rarely observed in pairs.[3]

Discovery and recognition[edit]

In surveys of globular clusters, these "cluster-type" variables were being rapidly identified in the mid-1890s, especially by E. C. Pickering.

Probably the first star of definitely RR Lyrae type found outside a cluster was U Leporis, discovered by J. Kapteyn in 1890.

The prototype star RR Lyrae was discovered prior to 1899 by Williamina Fleming, and reported by Pickering in 1900 as "indistinguishable from cluster-type variables".

From 1915 to the 1930s, the RR Lyraes became increasingly accepted as a class of star distinct from the Cepheids, due to their shorter periods, differing locations within the galaxy, and chemical differences from classical Cepheids, being mostly metal-poor, Population II stars.[2]

RR Lyraes have proven difficult to observe in external galaxies because of their intrinsic faintness. (In fact, Walter Baade's failure to find them in the Andromeda galaxy led him to suspect that the galaxy was much farther away than predicted, to reconsider the calibration of Cepheid variables, and to propose the concept of stellar populations.[2]) Using the Canada-France-Hawaii Telescope in the 1980s, Pritchet & van den Bergh[4] found Lyraes in Andromeda's galactic halo and, more recently, in its globular clusters.


The RR Lyrae stars are conventionally divided into three main types,[2] following classification by S.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%[5] of RR Lyrae in a system, and are double-mode pulsators, unlike RRab and RRc

New and upcoming developments[edit]

The Hubble Space Telescope has identified several RR Lyrae candidates in globular clusters of the Andromeda galaxy[6] and has measured the distance to the prototype star RR Lyrae.

Kepler space telescope provided constant coverage of a single field with accurate photometric data. In addition, RR Lyrae itself was in Kepler field of view.[7]

The Gaia mission is expected to greatly improve knowledge of RR Lyraes by providing homogeneous spectrographic information of a large population of such stars.[8]

See also[edit]


  1. ^ Layden, A. C.; Hanson, Robert B.; Hawley, Suzanne L.; Klemola, Arnold R.; Hanley, Christopher J. (August 1996). "The Absolute Magnitude and Kinematics of RR Lyrae Stars via Statistical Parallax". Astron. J. 112: 2110–2131. arXiv:astro-ph/9608108. Bibcode:1996AJ....112.2110L. doi:10.1086/118167. 
  2. ^ a b c d Smith, Horace A., RR Lyrae Stars, Cambridge (2004)
  3. ^ Hajdu, G.; Catelan, M.; Jurcsik, J.; Dékány, I.; Drake, A.J.; Marquette, B. "New RR Lyrae variables in binary systems". Monthly Notices of the Royal Astronomical Society 449 (1): L113–L117. arXiv:1502.01318. Bibcode:2015MNRAS.449L.113H. doi:10.1093/mnrasl/slv024. 
  4. ^ Pritchet, C. J., & van den Bergh, S., "Observations of RR Lyrae stars in the halo of M31", Astrophysical Journal, 316, 517 (1987)
  5. ^ Christensen-Dalsgaard, J.; Balona, L. A.; Garrido, R.; Suárez, J.C. (Oct 20, 2012). "Stellar Pulsations: Impact of New Instrumentation and New Insights". Science. ISBN 9783642296307. Retrieved 17 October 2014. 
  6. ^ G. Clementini, L. Federici, C. Corsi, C. Cacciari, M. Bellazzini, and H. A. Smith, "RR Lyrae Variables in the Globular Clusters of M31: A First Detection of Likely Candidates", The Astrophysical Journal, 559:L109-L112 (2001)
  7. ^ Kinemuchi, K. (2011). "RR Lyrae Research with the Kepler Mission". arXiv:1107.0297 [astro-ph.SR]. 
  8. ^ Bono, G., "The Cepheid and RR Lyrae instability strip with GAIA", GAIA Spectroscopy: Science and Technology, ASP Conference Proceedings, Vol. 298 (2002)

External links[edit]