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RR Lyrae variables are periodic variable stars, commonly found in
globular clusters A globular cluster is a spheroidal conglomeration of stars. Globular clusters are bound together by gravity, with a higher concentration of stars towards their centers. They can contain anywhere from tens of thousands to many millions of memb ...
. They are used as standard candles to measure (extra) galactic distances, assisting with the
cosmic distance ladder The cosmic distance ladder (also known as the extragalactic distance scale) is the succession of methods by which astronomers determine the distances to celestial objects. A ''direct'' distance measurement of an astronomical object is possible o ...
. This class is named after the prototype and brightest example,
RR Lyrae RR Lyrae is a variable star in the Lyra constellation, figuring in its west near to Cygnus. As the brightest star in its class, it became the eponym for the RR Lyrae variable class of stars and it has been extensively studied by astr ...
. They are pulsating
horizontal branch The horizontal branch (HB) is a stage of stellar evolution that immediately follows the red-giant branch in stars whose masses are similar to the Sun's. Horizontal-branch stars are powered by helium fusion in the core (via the triple-alpha proce ...
stars of spectral class A or F, with a mass of around half the Sun's. They are thought to have shed mass during the red-giant branch phase, and were once stars at around 0.8 solar masses. In contemporary astronomy, a period-luminosity relation makes them good
standard candles The cosmic distance ladder (also known as the extragalactic distance scale) is the succession of methods by which astronomers determine the distances to celestial objects. A ''direct'' distance measurement of an astronomical object is possible o ...
for relatively nearby targets, especially within the Milky Way and Local Group. They are also frequent subjects in the studies of globular clusters and the chemistry (and quantum mechanics) of older stars.


Discovery and recognition

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 definitely of RR Lyrae type found outside a cluster was U Leporis, discovered by J. Kapteyn in 1890. The prototype star
RR Lyrae RR Lyrae is a variable star in the Lyra constellation, figuring in its west near to Cygnus. As the brightest star in its class, it became the eponym for the RR Lyrae variable class of stars and it has been extensively studied by astr ...
was discovered prior to 1899 by
Williamina Fleming (15 May 1857 – 21 May 1911) was a Scottish-American astronomer. She was a single mother, hired by the director of the Harvard College Observatory to help in the photographic classification of stellar spectra. She helped develop a common ...
, 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
classical Cepheids Classical Cepheids (also known as Population I Cepheids, Type I Cepheids, or Delta Cepheid variables) are a type of Cepheid variable star. They are population I variable stars that exhibit regular radial pulsations with periods of a few days to a ...
, due to their shorter periods, differing locations within the galaxy, and chemical differences. RR Lyrae variables are metal-poor, Population II stars. 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 A Cepheid variable () is a type of star that pulsates radially, varying in both diameter and temperature and producing changes in brightness with a well-defined stable period and amplitude. A strong direct relationship between a Cepheid varia ...
, and to propose the concept of
stellar populations During 1944, Walter Baade categorized groups of stars within the Milky Way into stellar populations. In the abstract of the article by Baade, he recognizes that Jan Oort originally conceived this type of classification in 1926: Baade noticed t ...
.) Using the Canada-France-Hawaii Telescope in the 1980s, Pritchet & van den Bergh found RR Lyraes in Andromeda's galactic halo and, more recently, in its globular clusters.


Classification

The RR Lyrae stars are conventionally divided into three main types, 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% of RR Lyrae in a system, and are double-mode pulsators, unlike RRab and RRc


Distribution

RR Lyrae stars were formerly called "cluster variables" because of their strong (but not exclusive) association with
globular clusters A globular cluster is a spheroidal conglomeration of stars. Globular clusters are bound together by gravity, with a higher concentration of stars towards their centers. They can contain anywhere from tens of thousands to many millions of memb ...
; conversely, over 80% of all variables known in globular clusters are RR Lyraes. RR Lyrae stars are found at all galactic latitudes, as opposed to
classical Cepheids Classical Cepheids (also known as Population I Cepheids, Type I Cepheids, or Delta Cepheid variables) are a type of Cepheid variable star. They are population I variable stars that exhibit regular radial pulsations with periods of a few days to a ...
, 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. 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. Though
binary star A binary star is a system of two stars that are gravitationally bound to and in orbit around each other. Binary stars in the night sky that are seen as a single object to the naked eye are often resolved using a telescope as separate stars, in ...
systems are common for typical stars, RR Lyrae are very rarely observed in pairs.


Properties

RR Lyrae stars pulse in a manner similar to Cepheid variables, but the nature and histories of these stars is thought to be rather different. Like all variables on the Cepheid instability strip, pulsations are caused by the κ-mechanism, when the opacity of ionised helium varies with its temperature. RR Lyraes are old, relatively low mass, Population II stars, in common with W Virginis and BL Herculis variables, the type II Cepheids. Classical Cepheid variables are higher mass population I stars. RR Lyrae variables are much more common than Cepheids, but also much less luminous. The average
absolute magnitude Absolute magnitude () is a measure of the luminosity of a celestial object on an inverse logarithmic astronomical magnitude scale. An object's absolute magnitude is defined to be equal to the apparent magnitude that the object would have if it ...
of an RR Lyrae star is about +0.75, only 40 or 50 times brighter than our Sun. Their period is shorter, typically less than one day, sometimes ranging down to seven hours. Some RRab stars, including RR Lyrae itself, exhibit the Blazhko effect in which there is a conspicuous phase and amplitude modulation.


Period-luminosity relationships

Unlike Cepheid variables, RR Lyrae variables do not follow a strict period-luminosity relationship at visual wavelengths, although they do in the infrared K band. They are normally analysed using a period-colour-relationship, for example using a Wesenheit function. In this way, they can be used as standard 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 the
cosmic distance ladder The cosmic distance ladder (also known as the extragalactic distance scale) is the succession of methods by which astronomers determine the distances to celestial objects. A ''direct'' distance measurement of an astronomical object is possible o ...
, and may bias the estimated age of the Universe and the
Hubble constant Hubble's law, also known as the Hubble–Lemaître law, is the observation in physical cosmology that galaxies are moving away from Earth at speeds proportional to their distance. In other words, the farther they are, the faster they are moving ...
.


Recent developments

The
Hubble Space Telescope The Hubble Space Telescope (often referred to as HST or Hubble) is a space telescope that was launched into low Earth orbit in 1990 and remains in operation. It was not the first space telescope, but it is one of the largest and most vers ...
has identified several RR Lyrae candidates in globular clusters of the Andromeda Galaxy and has measured the distance to the prototype star RR Lyrae. The
Kepler Johannes Kepler (; ; 27 December 1571 – 15 November 1630) was a German astronomer, mathematician, astrologer, natural philosopher and writer on music. He is a key figure in the 17th-century Scientific Revolution, best known for his laws o ...
space telescope provided accurate photometric 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. The Gaia mission mapped 140,784 RR Lyrae stars, of which 50,220 were not previously known to be variable, and for which 54,272 interstellar absorption estimates are available.


References


External links


APOD ''M3: Inconstant Star Cluster''
four-frame animation of RR Lyrae variables in globular cluster M3
Animation of RR Lyrae-Variables in globular cluster M15
* ttp://www.daviddarling.info/encyclopedia/R/RR_Lyrae_star.html RR Lyrae starsbr>AAVSO Variable Star of the Season - RR Lyrae
{{DEFAULTSORT:Rr Lyrae Variable Standard candles Variable stars