RR Lyrae variables are periodic
variable starA star is classified as variable if its apparent brightness as seen from Earth changes over time, whether the changes are due to variations in the star's actual luminosity, or to variations in the amount of the star's light that is blocked from reaching Earth...
s, 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 LyraeRR Lyrae is a variable star in the Lyra constellation. It is the prototype of the RR Lyrae variables star class. It has a period of about 13 hours, and oscillates between apparent magnitudes 7 and 8. Its variable nature was discovered by the Scottish astronomer Williamina Fleming at Harvard...
in the constellation
LyraLyra is a constellation. Its name derived from the lyre, a stringed musical instrument well known for its use in classical antiquity and later. Lyra was one of the 48 constellations listed by the 2nd century astronomer Ptolemy, and it remains one of the 88 modern constellations recognized by the...
.
RR Lyraes are pulsating
horizontal branchThe horizontal branch is a stage of stellar evolution which 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 and by hydrogen fusion in a shell surrounding the core...
stars of spectral class A (and rarely F), with a mass of around half the
SunThe Sun is the star at the center of the Solar System. The Earth and other matter orbit the Sun, which by itself accounts for about 99.86% of the Solar System's mass....
'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 variableA Cepheid is a member of a class of pulsating variable stars. The relationship between a Cepheid variable's luminosity and pulsation period is quite precise, securing Cepheids as viable standard candles and the foundation of the Extragalactic Distance Scale.Typical classical Cepheids pulsate with...
s, 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.
RR Lyrae variables are periodic
variable starA star is classified as variable if its apparent brightness as seen from Earth changes over time, whether the changes are due to variations in the star's actual luminosity, or to variations in the amount of the star's light that is blocked from reaching Earth...
s, 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 LyraeRR Lyrae is a variable star in the Lyra constellation. It is the prototype of the RR Lyrae variables star class. It has a period of about 13 hours, and oscillates between apparent magnitudes 7 and 8. Its variable nature was discovered by the Scottish astronomer Williamina Fleming at Harvard...
in the constellation
LyraLyra is a constellation. Its name derived from the lyre, a stringed musical instrument well known for its use in classical antiquity and later. Lyra was one of the 48 constellations listed by the 2nd century astronomer Ptolemy, and it remains one of the 88 modern constellations recognized by the...
.
RR Lyraes are pulsating
horizontal branchThe horizontal branch is a stage of stellar evolution which 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 and by hydrogen fusion in a shell surrounding the core...
stars of spectral class A (and rarely F), with a mass of around half the
SunThe Sun is the star at the center of the Solar System. The Earth and other matter orbit the Sun, which by itself accounts for about 99.86% of the Solar System's mass....
'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 variableA Cepheid is a member of a class of pulsating variable stars. The relationship between a Cepheid variable's luminosity and pulsation period is quite precise, securing Cepheids as viable standard candles and the foundation of the Extragalactic Distance Scale.Typical classical Cepheids pulsate with...
s, 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 magnitudeIn astronomy, absolute magnitude measures a celestial object's intrinsic brightness. To derive the absolute magnitude from the observed apparent magnitude of a celestial object its value is corrected for distance to the observer...
of an RR Lyrae is 0.75, only 40 or 50 times brighter than our
SunThe Sun is the star at the center of the Solar System. The Earth and other matter orbit the Sun, which by itself accounts for about 99.86% of the Solar System's mass....
.) Their period is shorter, typically less than one day, sometimes ranging down to seven hours.
The relationship between pulsation period and absolute magnitude of RR Lyraes makes them good
standard candlesStandard Candles is a compilation of short stories by American science fiction author Jack McDevitt. The sixteen stories in the anthology were originally published in various magazines from 1982 to 1996...
for relatively near objects, especially within the Milky Way. They are extensively used in
globular clusterA globular cluster is a spherical collection of stars that orbits a galactic core as a satellite. Globular clusters are very tightly bound by gravity, which gives them their spherical shapes and relatively high stellar densities toward their centers. The name of this category of star cluster is...
studies, and also used to study chemical properties of older stars.
Abundances and distribution
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.
Discovery and recognition
In surveys of globular clusters, these "cluster-type" variables were being rapidly identified in the mid 1890s (especially by
E. C. PickeringEdward Charles Pickering was an American astronomer and physicist, brother of William Henry Pickering.Along with Carl Vogel, Pickering discovered the first spectroscopic binary stars. He wrote Elements of Physical Manipulations .Pickering attended Boston Latin School, and received his B.S. from...
).
The first star of definitely RR Lyrae type outside a cluster was probably
Mu LeporisMu Leporis is a Class B9III, third-magnitude star in the constellation Lepus.This star is a blue giant, located about 185 light-years from Earth.-External links:*...
discovered by
J. KapteynJacobus Cornelius Kapteyn, was a Dutch astronomer, best known for his extensive studies of the Milky Way and as the first discoverer of evidence for galactic rotation....
in 1890.
The prototype star
RR LyraeRR Lyrae is a variable star in the Lyra constellation. It is the prototype of the RR Lyrae variables star class. It has a period of about 13 hours, and oscillates between apparent magnitudes 7 and 8. Its variable nature was discovered by the Scottish astronomer Williamina Fleming at Harvard...
was discovered prior to 1899 by
Williamina FlemingWilliamina Paton Stevens Fleming , astronomer, was born in Dundee, Scotland, to Robert Stevens and Mary Walker Stevens. She attended public schools in Dundee, and at the age of 14, she became a pupil-teacher. She married James Orr Fleming, and they moved to the U.S. and settled in Boston,...
, and reported by Pickering in 1900 as "indistinguishable from cluster-type variables".
From 1915 to the 1930s, the RR Lyraes became more accepted as a distinct class of star from classical Cepheids, on account of their shorter periods, different location within the galaxy, and finally, they are chemically different from classical Cepheids, being mostly metal-poor, Population II stars.
RR Lyraes have proven difficult to observe in external
galaxiesA galaxy is a massive, gravitationally bound system that consists of stars and stellar remnants, an interstellar medium of gas and dust, and an important but poorly understood component tentatively dubbed dark matter. The name is from the Greek root galaxias [γαλαξίας], meaning "milky," a reference...
, because of their intrinsic faintness. (In fact,
Walter BaadeWilhelm Heinrich Walter Baade was a German astronomer who emigrated to the USA in 1931.- Biography :He took advantage of wartime blackout conditions during World War II, which reduced light pollution at Mount Wilson Observatory, to resolve stars in the center of the Andromeda galaxy for the first...
's
failure to find them in the
Andromeda galaxyThe Andromeda Galaxy is a spiral galaxy approximately away in the constellation Andromeda. It is the nearest spiral galaxy to our own, the Milky Way...
led him to suspect that the galaxy was much farther away than predicted, and to re-consider the calibration of Cepheid variables and to propose stellar populations.) They were finally found in the 1980s by Pritchet & van den Bergh in the halo of the Andromeda galaxy, and more recently in its globular clusters by the
Hubble Space TelescopeThe Hubble Space Telescope is a space telescope that was carried into orbit by the space shuttle in April 1990. It is named after the American astronomer Edwin Hubble. Although not the first space telescope, the Hubble is one of the largest and most versatile, and is well-known as both a vital...
.
Subtypes
The RR Lyrae stars are conventionally divided into three main types, following a classification of
S.I. BaileySolon Irving Bailey was an American astronomer.He joined the staff of Harvard College Observatory in 1887....
based on the shape of the stars' brighness curve:
- RRab — the majority, with steep rise in brightness (about 91%)
- RRc — having shorter periods, more sinusoidal variation (about 9%).
- RRd — rare double-mode pulsators.
New and upcoming developments
The
Hubble Space TelescopeThe Hubble Space Telescope is a space telescope that was carried into orbit by the space shuttle in April 1990. It is named after the American astronomer Edwin Hubble. Although not the first space telescope, the Hubble is one of the largest and most versatile, and is well-known as both a vital...
has identified several RR Lyrae "candidates" in globular clusters of the
Andromeda galaxyThe Andromeda Galaxy is a spiral galaxy approximately away in the constellation Andromeda. It is the nearest spiral galaxy to our own, the Milky Way...
.
The Gaia mission is expected to greatly improve knowledge of RR Lyraes, by providing a homogeneous spectrographic information of a large population of such stars.
External links