RR Lyrae Variables
RR Lyrae variables are another type of pulsating variable star that
may be used to determine distance.
This class of variables is named after
the first such star discovered, RR Lyrae, an approximate apparent magnitude 7 variable star
in the constellation Lyra.
RR Lyrae Characteristics
The RR Lyrae variables, also sometimes termed cluster variables because
they are most commonly found in globular clusters,
are of much shorter period than the Cepheid variables
(typically 0.5 - 1 day). They are Pop II blue giants with spectral class around
F0, and
all are of approximately the same luminosity (near absolute magnitude 0.5).
Thus, they are confined to a
small region of a period-luminosity diagram, as indicated in the above right figure.
Distances from RR Lyrae Variables
Because all RR Lyrae stars have about the same luminosity,
the apparent brightness of an RR Lyrae
variable indicates rather directly its distance through the inverse square intensity relation.
The figure shown above illustrates the
relationship between period and absolute magnitude for RR Lyrae variables, as well as
for Cepheid variables.