RR Lyrae is a variable star

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...

 in the 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...

 constellation. It is the prototype of the RR Lyrae variable

RR Lyrae variable

RR Lyrae variables are periodic variable stars, 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....

 class of stars. It has a period of about 13 hours, and oscillates between apparent magnitude

Apparent magnitude

The apparent magnitude of a celestial body is a measure of its brightness as seen by an observer on Earth, adjusted to the value it would have in the absence of the atmosphere...

s 7 and 8. Its variable nature was discovered by the Scottish astronomer Williamina Fleming

Williamina Fleming

-External links:* * * * from the Astronomical Society of the Pacific- Obituaries :*...

 at Harvard Observatory in 1901. As RR Lyrae variables are important standard candles, knowing the precise distance of this star is necessary to determine its luminosity

Luminosity

Luminosity is a measurement of brightness.-In photometry and color imaging:In photometry, luminosity is sometimes incorrectly used to refer to luminance, which is the density of luminous intensity in a given direction. The SI unit for luminance is candela per square metre.The luminosity function...

, and hence that of other stars in its class.

Its distance long uncertain, 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...

's Fine Guidance Sensor

Fine Guidance Sensor

The Fine Guidance Sensor is an interferometric instrument on board the Hubble Space Telescope that provides high-precision pointing information as input to the observatory's attitude control systems....

 was used in 2002 to determine the distance of RR Lyrae within ~5%: 262 parsecs, or 854 light-years.