RR Lyrae Variable
Variable stars with periods of 12-24 hours, common in some globular clusters.
S ...
RR Lyrae variable star: Variable star with a period of 12 to 24 hours; common in some globular clusters.
Sagittarius A*: The powerful radio source located at the core of the Milky Way Galaxy.
RR Lyrae variable - (n.)
A member of a class of pulsating variable stars named after the prototype star, RR Lyrae.
RR Lyrae Variables: A variable star that has a regularly varying luminosity. These stars all have about the same luminosity making them suitable for obtaining distances.
Many RR Lyrae variables are found in globular clusters. Early in the twentieth century the American astronomer Harlow Shapley used observations of RR Lyrae stars to make two very important discoveries about the Galactic globular cluster system.
Short-period Cepheids, also called cluster-type variables, or RR Lyrae variables, all have periods of less than one day and show no dependable relationship between period and luminosity; ...
Instead, distance indicators whose origins are in an older stellar population (like novae and RR Lyrae variables) must be used.
Cepheids, along with RR Lyrae variables, occupy an area on the H-R Diagram known as the instability strip. This lies in the top right, between the main sequence and the long period variables.
The absolute magnitude of all RR Lyrae variables seems to be nearly constant at <MV = 0.75 ± 0.1. There may be some poorly-determined metallicity dependence.
A sub-class of Bailey type RR Lyrae variables, having asymmetric lioght curves of large amplitude. [H76]
AB Magnitude System ...
The second group has a slightly longer period of RR Lyrae variable stars. Both groups have weak lines of metallic elements. But the lines in the stars of Oosterhoff type I (OoI) cluster are not quite as weak as those in type II (OoII).
Harlow Shapley mostly used RR Lyrae variables, but in any case he made use of the period-luminosity relationship and he found that the globular cluster systems were far away and centered, not around the sun, ...
Also called Population II stars because of their later discovery, this group also includes RR Lyrae variables with periods greater than 12 hours, subdwarfs and other extremely metal-poor stars, and some red giants.
He continued to research and publish papers right up until his death, his last paper on RR Lyrae variable stars (another kind of standard candle) appearing in the Astrophysical Journal in June.
The first hints of an unusually high density of stars in the direction of Virgo were made in 2001 by the QUEST survey, which used a 1-meter telescope in Venezuela to study a class of variable stars called RR Lyrae variables.
M4 (NGC 6121) is a rather near globular cluster (6000-10,000 light years) but without a large telescope it will not appear very spectacular. There may be as many as fifty RR Lyrae variables in the cluster.
 See also: Distance, Star, RR Lyrae, Light, Sun
  
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