page_title: RR Lyrae
section_title: Variable star class
hierarchical_section_title: RR Lyrae / Variable star class
caption_attribution_description: English: This image, captured with the VISTA infrared survey telescope, as part of the Variables in the Via Lactea (VVV) ESO public survey, shows the central part of the Milky Way. While normally hidden behind obscuring dust, the infrared capabilities of VISTA allow to study the stars close to the galactic centre. Within this field of view astronomers detected several ancient stars, of a type known as RR Lyrae. As RR Lyrae stars typically reside in ancient stellar populations over 10 billion years old, this discovery suggests that the bulging centre of the Milky Way likely grew through the merging of primordial star clusters.
context_page_description: 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 astronomers. RR Lyrae variables serve as important standard candles 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. Hence, understanding the period-luminosity relation for a local set of such stars allows the distance of more distant stars of this type to be determined.
context_section_description: This type of low-mass star has consumed the hydrogen at its core, evolved away from the main sequence, and passed through the red giant stage. Energy is now being produced by the thermonuclear fusion of helium at its core, and the star has entered an evolutionary stage called the horizontal branch (HB). The effective temperature of an HB star's outer envelope will gradually increase over time. When its resulting stellar classification enters a range known as the instability strip—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 to vary between 7.06–8.12 over a short cycle lasting 0.56686776 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.
  This star belongs to a subset of RR Lyrae-type variables that show a characteristic behavior called the Blazhko effect, named after Russian astronomer Sergey Blazhko. This effect is observed as a periodic modulation of a variable star's pulsation strength or phase; sometimes both. It causes the light curve of RR Lyrae to change from cycle to cycle. As of 2009, the cause of this effect is not yet fully understood. The Blazhko period for RR Lyrae is 39.1 ± 0.3 days.
caption: RR Lyrae-type variable stars (not RR Lyr itself) close to the galactic center from the VVV ESO public survey
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