RR Lyrae is a

variable star A variable star is a star whose brightness as seen from Earth (its apparent magnitude) changes systematically with time. This variation may be caused by a change in emitted light or by something partly blocking the light, so variable stars are ...

in the

Lyra , from ; pronounced: ) is a small constellation. It is one of the 48 listed by the 2nd century astronomer Ptolemy, and is one of the modern 88 constellations recognized by the International Astronomical Union. Lyra was often represented on star ...

constellation, figuring in its west near to Cygnus. As the brightest star in its class, it became the

eponym An eponym is a noun after which or for which someone or something is, or is believed to be, named. Adjectives derived from the word ''eponym'' include ''eponymous'' and ''eponymic''. Eponyms are commonly used for time periods, places, innovati ...

for the

RR Lyrae variable RR Lyrae variables are periodic variable stars, commonly found in globular clusters. They are used as standard candles to measure (extra) galactic distances, assisting with the cosmic distance ladder. This class is named after the prototype a ...

class of stars and it has been extensively studied by astronomers. RR Lyrae variables serve as important

standard candle 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 ...

s that are used to measure astronomical distances. The period of pulsation of an RR Lyrae variable depends on its mass, luminosity and temperature, while the difference between the measured luminosity and the actual luminosity allows its distance to be determined via the

inverse-square law In science, an inverse-square law is any scientific law stating that the observed "intensity" of a specified physical quantity is inversely proportional to the square of the distance from the source of that physical quantity. The fundamental ca ...

. Hence, understanding the

period-luminosity relation In astronomy, a period-luminosity relation is a relationship linking the luminosity of pulsating variable stars with their pulsation period. The best-known relation is the direct proportionality law holding for Classical Cepheid variables, somet ...

for a local set of such stars allows the distance of more distant stars of this type to be determined.

History

The variable nature of RR Lyrae was discovered by the Scottish astronomer

Williamina Fleming Williamina Paton Stevens Fleming (15 May 1857 – 21 May 1911) was a pioneering Scottish astronomer, who made significant contributions to the field despite facing gender biases. She was a single mother hired by the director of the Harvard Co ...

Harvard Observatory The Harvard College Observatory (HCO) is an institution managing a complex of buildings and multiple instruments used for astronomical research by the Harvard University Department of Astronomy. It is located in Cambridge, Massachusetts, United St ...

in 1901. The distance of RR Lyrae remained uncertain until 2002 when the

Hubble Space Telescope The Hubble Space Telescope (HST or Hubble) is a space telescope that was launched into low Earth orbit in 1990 and remains in operation. It was not the Orbiting Solar Observatory, first space telescope, but it is one of the largest and most ...

's fine guidance sensor was used to determine the distance of RR Lyrae within a 5%

margin of error The margin of error is a statistic expressing the amount of random sampling error in the results of a Statistical survey, survey. The larger the margin of error, the less confidence one should have that a poll result would reflect the result of ...

, yielding a value of . When combined with measurements from the

Hipparcos ''Hipparcos'' was a scientific satellite of the European Space Agency (ESA), launched in 1989 and operated until 1993. It was the first space experiment devoted to precision astrometry, the accurate measurement of the positions and distances of ...

satellite and other sources, the result is a distance estimate of .

Variable star class

This type of low-mass star has consumed the hydrogen at its core,

evolved Evolution is the change in the heritable Phenotypic trait, characteristics of biological populations over successive generations. It occurs when evolutionary processes such as natural selection and genetic drift act on genetic variation, re ...

away from the

main sequence In astronomy, the main sequence is a classification of stars which appear on plots of stellar color index, color versus absolute magnitude, brightness as a continuous and distinctive band. Stars on this band are known as main-sequence stars or d ...

, and passed through the

red giant A red giant is a luminous giant star of low or intermediate mass (roughly 0.3–8 solar masses ()) in a late phase of stellar evolution. The stellar atmosphere, outer atmosphere is inflated and tenuous, making the radius large and the surface t ...

stage. Energy is now being produced by the

thermonuclear fusion Nuclear fusion is a reaction in which two or more atomic nuclei combine to form a larger nuclei, nuclei/neutron by-products. The difference in mass between the reactants and products is manifested as either the release or absorption of ener ...

of helium at its core, and the star has entered an evolutionary stage called the

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 proc ...

(HB). The

effective temperature The effective temperature of a body such as a star or planet is the temperature of a black body that would emit the same total amount of electromagnetic radiation. Effective temperature is often used as an estimate of a body's surface temperature ...

of an HB star's outer envelope will gradually increase over time. When its resulting

stellar classification In astronomy, stellar classification is the classification of stars based on their stellar spectrum, spectral characteristics. Electromagnetic radiation from the star is analyzed by splitting it with a Prism (optics), prism or diffraction gratin ...

enters a range known as the

instability strip The unqualified term instability strip usually refers to a region of the Hertzsprung–Russell diagram largely occupied by several related classes of pulsating variable stars: Delta Scuti variables, SX Phoenicis variables, and rapidly oscillat ...

—typically at stellar class A—the outer envelope can begin to pulsate. RR Lyrae shows just such a regular pattern of pulsation, which is causing its

apparent magnitude Apparent magnitude () is a measure of the Irradiance, brightness of a star, astronomical object or other celestial objects like artificial satellites. Its value depends on its intrinsic luminosity, its distance, and any extinction (astronomy), ...

to vary between 7.06 and 8.12 over a short cycle lasting 0.567 days (13 hours, 36 minutes). Each radial pulsation causes the radius of the star to vary between 5.1 and 5.6 times the

Sun's radius Solar radius is a unit of distance used to express the size of objects in astronomy relative to the Sun. The solar radius is usually defined as the radius to the layer in the Sun's photosphere where the optical depth equals 2/3: 1\,R_ = 6.957\ti ...

. This star belongs to a subset of RR Lyrae-type variables that show a characteristic behavior called the

Blazhko effect The Blazhko effect, also known as the Tseraskaya–Blazhko effect, and which is sometimes called long-period modulation, is a variation in period and amplitude in RR Lyrae type variable stars. Sergey Blazhko first reported its observation by Lid ...

, named after Russian astronomer

Sergey Blazhko Sergey Nikolayevich Blazhko (; November 17, 1870, Khotsimsk – February 11, 1956, Moscow) was a Russian and Soviet astronomer, a corresponding member of the Academy of Sciences of the Soviet Union (1929). He was a graduate of Moscow State Unive ...

. This effect is observed as a periodic modulation of a variable star's pulsation strength or phase; sometimes both. It causes the

light curve In astronomy, a light curve is a graph (discrete mathematics), graph of the Radiance, light intensity of a celestial object or region as a function of time, typically with the magnitude (astronomy), magnitude of light received on the ''y''-axis ...

of RR Lyrae to change from cycle to cycle. In 2014, Time-series photometric observations demonstrated the physical origin of this effect.

Other stellar classifications

As with other RR Lyrae-type variables, RR Lyrae itself has a low abundance of elements other than hydrogen and helium – what astronomers term its

metallicity In astronomy, metallicity is the Abundance of the chemical elements, abundance of Chemical element, elements present in an object that are heavier than hydrogen and helium. Most of the normal currently detectable (i.e. non-Dark matter, dark) matt ...

: It belongs to the

Population II In 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 observed th ...

category of stars that formed during the early period of the Universe when there was a lower abundance of metals in star-forming regions. The trajectory of this star is carrying it along an orbit that is close to the plane of the Milky Way, taking it no more than above or below this plane. The Blazhko period for RR Lyrae is . The orbit has a high

eccentricity Eccentricity or eccentric may refer to: * Eccentricity (behavior), odd behavior on the part of a person, as opposed to being "normal" Mathematics, science and technology Mathematics * Off-Centre (geometry), center, in geometry * Eccentricity (g ...

, bringing RR Lyrae as close as to the

Galactic Center The Galactic Center is the barycenter of the Milky Way and a corresponding point on the rotational axis of the galaxy. Its central massive object is a supermassive black hole of about 4 million solar masses, which is called Sagittarius A*, a ...

periapsis An apsis (; ) is the farthest or nearest point in the orbit of a planetary body about its primary body. The line of apsides (also called apse line, or major axis of the orbit) is the line connecting the two extreme values. Apsides perta ...

, and taking it as far as at apapsis.

References

External links

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image RR Lyrae
{{DEFAULTSORT:Rr Lyrae 182989 Lyra RR Lyrae variables F-type giants Lyrae, RR A-type giants 095497 J19252793+4247040 BD+42 3338 TIC objects Horizontal-branch stars