RR Lyrae variable
Pulsating standard candles used for measuring galactic distances.
RR Lyrae variables are a type of star that brightens and dims on a regular schedule. They turn up most often in globular clusters and other ancient star groups within galaxies. Astronomers use them as standard candles to figure out distances across the galaxy and beyond, which helps build the cosmic distance ladder. The class takes its name from its brightest and first-known member, the star RR Lyrae. These stars belong to the horizontal branch, have spectral types A or F, and weigh roughly half as much as the Sun. They likely lost a lot of mass while passing through the red-giant branch phase, and they originally started out as stars of about 0.8 solar masses. Today, a relationship between their pulsation period and brightness makes them reliable standard candles for relatively close targets, especially inside the Milky Way and the Local Group. They also get a lot of attention in studies of globular clusters and the chemistry—and even the quantum mechanics—of older stars.
In the mid-1890s, surveys of globular clusters turned up many of these "cluster-type" variables, especially through the work of E. C. Pickering. The first star of this kind found outside a cluster was probably U Leporis, discovered by J. Kapteyn in 1890. The prototype, RR Lyrae itself, was found before 1899 by Williamina Fleming, and Pickering reported in 1900 that it was "indistinguishable from cluster-type variables." Between 1915 and the 1930s, astronomers increasingly recognized RR Lyrae stars as a distinct class separate from classical Cepheids, because they have shorter periods, different locations in the galaxy, and different chemical compositions. They are metal-poor, Population II stars. These stars are hard to spot in other galaxies because they are intrinsically faint. In fact, Walter Baade’s failure to find them in the Andromeda Galaxy made him think the galaxy was much farther away than previously believed, which led him to rethink Cepheid calibration and propose the concept of stellar populations. In the 1980s, using the Canada-France-Hawaii Telescope, Pritchet and Van Den Bergh finally found RR Lyrae stars in the halo of Andromeda. More recently, the Hubble Space Telescope has spotted them in Andromeda’s globular clusters.
Based on the shape of their light curves, S. I.
- Mass
- around half the Sun's
Lore & Background
In surveys of globular clusters, these 'cluster-type' variables were rapidly identified in the mid-1890s, especially by E. C. Pickering. 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, RR Lyrae variables became accepted as a class distinct from classical Cepheids due to their shorter periods, differing locations within the galaxy, and chemical differences; they are metal-poor, Population II stars. RR Lyrae stars are conventionally divided into three main types following classification by S.I. Bailey: RRab (most common, 91%, with steep rises in brightness), RRc (9%, shorter periods and more sinusoidal variation), and RRd (rare, double-mode pulsators). They pulse due to the κ-mechanism, when the opacity of ionised helium varies with temperature. Some RRab stars, including RR Lyrae itself, exhibit the Blazhko effect, a conspicuous phase and amplitude modulation. RR Lyrae stars were formerly called 'cluster variables' because of their strong association with globular clusters; over 80% of all variables known in globular clusters are RR Lyraes. They are found at all galactic latitudes, as opposed to classical Cepheids, 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, with some estimates of about 85,000 in the Milky Way.
Reader's Guide
RR Lyrae variables are significant as standard candles for relatively nearby targets, especially within the Milky Way and Local Group, due to a period-luminosity relation in the infrared K band. They are also frequent subjects in studies of globular clusters and the chemistry of older stars. Unlike Cepheid variables, they do not follow a strict period-luminosity relationship at visual wavelengths, but are analysed using a period-colour-relationship, such as a Wesenheit function. Difficulties include effects of metallicity, faintness, and blending, which can introduce systematic uncertainty into the cosmic distance ladder and may bias estimates of the age of the Universe and the Hubble constant.
Walter Baade's failure to find RR Lyrae variables in the Andromeda Galaxy led him to suspect the galaxy was much farther away than predicted, to reconsider the calibration of Cepheid variables, and to propose the concept of stellar populations. Using the Canada-France-Hawaii Telescope in the 1980s, Pritchet and Van Den Bergh found RR Lyrae variables in the Andromeda Galaxy's galactic halo. More recently, the Hubble Space Telescope identified several RR Lyrae candidates in globular clusters of the Andromeda Galaxy and measured the distance to RR Lyrae itself. The Kepler space telescope detected new phenomena such as period-doubling. The Gaia mission mapped 140,784 RR Lyrae stars, and the PanSTARRS1 3π survey identified ~45,000 RR Lyrae stars, representing the widest and deepest sample to date. The Dark Energy Survey identified ~6000 RR Lyrae candidates, and Feng et al. (2024) used deep imaging from the Next Generation Virgo Cluster Survey to identify 180 faint RR Lyrae candidates in the outer Milky Way halo.
Did You Know?
- RR Lyrae variables are named after the prototype star RR Lyrae, discovered prior to 1899 by Williamina Fleming.
- They are pulsating horizontal branch stars of spectral class A or F, with a mass of around half the Sun's.
- RRab variables make up 91% of all observed RR Lyrae, while RRd are rare double-mode pulsators.
- The Gaia mission mapped 140,784 RR Lyrae stars, of which 50,220 were not previously known to be variable.
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