RR Lyrae variable
Encyclopedia
RR Lyrae variables are periodic variable star
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 Lyrae
in the constellation Lyra
.
RR Lyraes are pulsating horizontal branch
stars of spectral class A (and rarely 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 variable
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 magnitude
of an RR Lyrae is 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.
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.
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.
.
Probably the first star of definitely RR Lyrae type found outside a cluster was Mu 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.
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.) Using the Hubble Space Telescope
in the 1980s, Pritchet & van den Bergh found Lyraes in Andromeda's galactic halo and, more recently, in its globular clusters.
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 Gaia mission is expected to greatly improve knowledge of RR Lyraes by providing homogeneous spectrographic information of a large population of such stars.
Variable star
A star is classified as variable if its apparent magnitude 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 Lyrae
RR Lyrae
RR Lyrae is a variable star in the Lyra constellation. It is the prototype of the RR Lyrae variable class of stars. 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 Lyra
Lyra
Lyra is a small constellation. It is one of the 48 constellations listed by the 2nd century astronomer Ptolemy, and remains one of the 88 modern constellations recognized by the International Astronomical Union. Its principal star, Vega — a corner of the Summer Triangle — is one of the brightest...
.
RR Lyraes are pulsating horizontal branch
Horizontal branch
The 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...
stars of spectral class A (and rarely F), with a mass of around half the Sun
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...
'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 variable
Cepheid variable
A Cepheid is a member of a class of very luminous variable stars. The strong direct relationship between a Cepheid variable's luminosity and pulsation period, secures for Cepheids their status as important standard candles for establishing the Galactic and extragalactic distance scales.Cepheid...
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 magnitude
Absolute magnitude
Absolute magnitude is the measure of a celestial object's intrinsic brightness. it is also the apparent magnitude a star would have if it were 32.6 light years away from Earth...
of an RR Lyrae is 0.75, only 40 or 50 times brighter than our Sun
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...
. ) 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 candles
Standard Candles
Standard 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 cluster
Globular cluster
A 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
Edward 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...
.
Probably the first star of definitely RR Lyrae type found outside a cluster was Mu Leporis
Mu Leporis
Mu 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:*Jim Kaler's Stars, University of Illinois:...
, discovered by J. Kapteyn
Jacobus Kapteyn
Jacobus 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 Lyrae
RR Lyrae
RR Lyrae is a variable star in the Lyra constellation. It is the prototype of the RR Lyrae variable class of stars. 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 Fleming
Williamina Fleming
-External links:* * * * from the Astronomical Society of the Pacific- Obituaries :*...
, 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.
RR Lyraes have proven difficult to observe in external galaxies
Galaxy
A 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 word galaxy is derived from the Greek galaxias , literally "milky", a...
because of their intrinsic faintness. (In fact, Walter Baade
Walter Baade
Wilhelm Heinrich Walter Baade was a German astronomer who worked in the USA from 1931 to 1959.-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...
's failure to find them in the Andromeda galaxy
Andromeda Galaxy
The Andromeda Galaxy is a spiral galaxy approximately 2.5 million light-years from Earth in the constellation Andromeda. It is also known as Messier 31, M31, or NGC 224, and is often referred to as the Great Andromeda Nebula in older texts. Andromeda is the nearest spiral galaxy to the...
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.) Using the Hubble Space Telescope
Hubble Space Telescope
The Hubble Space Telescope is a space telescope that was carried into orbit by a Space Shuttle in 1990 and remains in operation. A 2.4 meter aperture telescope in low Earth orbit, Hubble's four main instruments observe in the near ultraviolet, visible, and near infrared...
in the 1980s, Pritchet & van den Bergh found Lyraes in Andromeda's galactic halo and, more recently, in its globular clusters.
Subtypes
The RR Lyrae stars are conventionally divided into three main types following a classification of S.I. Bailey based on the shape of the stars' brightness curves:- RRab, the majority type, which display steep rises in brightness of about 91%;
- RRc, a type with shorter periods and more sinusoidal variation of about 9%; and
- RRd, a rare type of double-mode pulsators.
New and upcoming developments
The Hubble Space TelescopeHubble Space Telescope
The Hubble Space Telescope is a space telescope that was carried into orbit by a Space Shuttle in 1990 and remains in operation. A 2.4 meter aperture telescope in low Earth orbit, Hubble's four main instruments observe in the near ultraviolet, visible, and near infrared...
has identified several RR Lyrae candidates in globular clusters of the Andromeda galaxy
Andromeda Galaxy
The Andromeda Galaxy is a spiral galaxy approximately 2.5 million light-years from Earth in the constellation Andromeda. It is also known as Messier 31, M31, or NGC 224, and is often referred to as the Great Andromeda Nebula in older texts. Andromeda is the nearest spiral galaxy to the...
and has measured the distance to the prototype star RR Lyrae.
The Gaia mission is expected to greatly improve knowledge of RR Lyraes by providing homogeneous spectrographic information of a large population of such stars.
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
- Animation with the variable stars RR Lyrae in the center area of the globular cluster M15
- RR Lyrae stars
- M3 variable stars 2 night animation RR Lyrae at the globular cluster M3, 10min intervals.