RR Lyrae stars

Typical light curves of RR Lyrae stars in subgroups RRab, RRcclick for a larger image
Typical light curves of RR Lyrae stars in subgroups RRab, RRc

RR Lyrae stars

(lÿ -ree) A large group of pulsating variables that are old giant stars (halo and disk population II stars) and are found principally in globular clusters. They usually have periods of less than one day, light variations of 0.2–2 magnitudes, and median spectral types in the range A7 to F5. They were discovered in 1895 by Solon I. Bailey, the group being named after RR Lyrae, discovered in 1899. Available evidence indicates that all RR Lyrae stars have about the same mean absolute magnitude (about +0.6); they can therefore be used as distance indicators up to about 200 kiloparsecs (see distance modulus).

RR Lyrae stars are of a mixed nature. They were divided by Bailey into three groups – RRa, RRb, and RRc – depending on period and the asymmetry of their light curves (see illustration). Groups a and b are often combined today as RRab stars. Other groupings have been made according to period. Many RR Lyrae stars show a periodic variation in both period and shape of light curve – the Blazhko effect. In addition the periods of some RR Lyrae stars are slowly changing at a constant rate, as predicted by evolutionary theory, while others show abrupt changes in period. The pulsations of these stars are very complex and can be subdivided into one group (RRab) oscillating in fundamental mode (see pulsating variables) and a second group (RRc) oscillating in first harmonic mode. See also horizontal branch.

RR Lyrae stars

[¦är¦är ′lī·rē ‚stärz]
(astronomy)
Pulsating variable stars with a period of 0.05-1.2 days in the halo population of the Milky Way Galaxy; color is white, and they are mostly stars of spectral class A. Also known as cluster cepheids; cluster variables.
References in periodicals archive ?
For this reason, RR Lyrae stars are known as standard candles -- objects of known luminosity whose distance and position can be used to help us understand more about vast celestial distances and the scale of the cosmos.
If you ever took an astronomy course, you probably learned about the role of the pulsing RR Lyrae stars in astronomers' long and heroic construction of the cosmic distance scale.
Luminosities of SX Phoenicis, Large Amplitude Delta Scuti, and RR Lyrae stars. PASP.
The puzzle has to do with RR Lyrae stars, bright stars of the Milky Way that pulsate every few hours.
In addition, the atlas includes measurements of 599 Cepheid stars (43 newly discovered) and 2,595 RR Lyrae stars (343 new) around the south ecliptic pole.
But to the astronomers' astonishment, the RR Lyrae stars do not follow football-shaped orbits, but have large random motions more consistent with their having formed at a great distance from the centre of the Milky Way.
Another team, which includes Zeljko Ivezic of Princeton University, used the Sloan survey to examine a second group of elderly objects, known as RR Lyrae stars. In the northern part of the halo, the astronomers found the same pattern of clumping that Newberg's team identified.
But the RR Lyrae stars are variable and therefore can be expected to have slightly different brightnesses when imaged at two different times (even if taken just hours or days apart).
Applying the Cepheid results to our own galaxy, the astronomers recalibrated the brightness of RR Lyrae stars, another type of variable star.
Furthermore, the fact that the Ursa Minor dwarf's variable-star population consists exclusively of RR Lyrae stars and anomalous Cepheids implies that the galaxy is populated by very old stars, as old as the oldest globular clusters.
Astronomers use the brightness of RR Lyrae stars to infer the ages of the globular clusters in which they reside.
The huge globular clusters of our galaxy's halo contain Cepheid-like (but much lower-mass) RR Lyrae stars, which pulse with periods of a day or less.