RR Lyrae stars are variable stars that pulsate -- they
are photometrically variable because their
photospheres move in and out in a regular way. This class of
stars is named for the prototype RR Lyrae, a variable star in the
constellation Lyra.
RR Lyrae stars were first discovered in the late nineteenth century, where they
were found in great numbers in globular clusters. They
were first called ``cluster variables,'' a name which they retained for many
decades. In 1899, the field star RR Lyrae was found to be variable by
Willamina Fleming of the Harvard College Observatory, and the connection
between its pulsation behavior and that of the cluster variables was
established. In 1902, Solon I. Bailey, also of Harvard, grouped the
RR Lyrae stars into several classes according to the shapes of their light curves and their periods. In 1913, C. C. Kiess of the Lick Observatory
published a short paper in Publications of the Astronomical Society of the
Pacific (PASP 25, 121), relating the behavior of the cluster variables to
that of the Cepheid variables. The RR Lyrae were simply considered to be
short-period Cepheids, which while incorrect, is not far
wrong. Harlow Shapley, also studied the RR Lyrae stars in great detail,
publishing both a comprehensive analysis of RR Lyrae in 1916 (AstrophysicalJournal 43, 217), and a calibration of RR Lyrae
absolute magnitudes and a determination of globular cluster
distances in 1918 (Astrophysical
Journal 48, 89). It was the latter work which Shapley used as the basis
for his determination of the structure of the Milky Way.
Work continued on the cluster variables and field RR Lyrae for several decades.
In 1940, Martin Schwarzschild first showed that the RR Lyrae stars
in clusters are not true Cepheids, but are much more evolved stars
which lie on the horizontal branch. (The Cepheids are in the brief stage
of their lives between the main sequence and the red giant branch.) Now,
the ``cluster variables'' and field RR Lyrae stars are all known to be the
same kind of star. They are very evolved stars, with masses less than that
of our Sun. They are all metal-poor compared to the Sun (meaning their
chemical compositions contain fewer metallic elements than the Sun), and are
all very old. Although they are not true Cepheids, they lie on the
Cepheid instability strip, a region of the Hertzsprung-Russell Diagram
where stars are capable of sustaining pulsations with no external driving
mechanism. The RR Lyrae stars all have pulsation periods between
0.2 and 1.2 days, and thus have much shorter periods than most of the
Cepheids.
The RR Lyrae stars are divided into several Bailey types, mentioned
above, which are: RRa, RRb, RRab, RRc, and RRd. The RRa,
b, and ab stars are all fundamental mode pulsators,
meaning that the entire star ``breathes'' in and out at the same time. In
one dimension, the fundamental mode is analogous to the lowest tone of a
plucked guitar string, where there are no nodal points along the string.
The RRc stars are first overtone pulsators, which means that
at a certain radius from the center, there is a shell where the stellar
material does not move (called a ``node''), and waves propagate in and out
above and below that point. The RRc stars have shorter periods
than the RRa,b stars. The RRd pulsate in both the fundamental
and first overtone mode at the same time.
The RRa stars have very asymmetric
light curves, almost like a sawtooth wave (just like the ascii art there,
except mirror it left-to-right). The RRa stars have very large
amplitudes, sometimes greater than one magnitude. This large
amplitude corresponds to very large changes in radial velocity, and the
high radial velocity amplitudes in turn cause a shock wave. The shock is
strong enough in these stars to actually cause ionization in the
stellar photosphere, generating emission lines of
hydrogen during maximum light. The RRab and RRb stars have
similar light curves, but with lower amplitude, and they do not generate
emission lines during the pulsation cycle. The RRc stars have very
rounded light curves, almost like a pure sine wave. The reason the RR
Lyrae stars are divided into these classes is due to their location within
the Cepheid instability strip. Stars on the hot (blue) side of the instability
strip will be RRc stars, while stars on the cool (red) side of the
instability strip will be RRab stars.
In his 1916 paper, Shapley was also the first to note the Blazhko effect, a long-period modulation in the light curves of some RR Lyrae stars.
RR Lyrae shows a small modulation with a period of 41 days superimposed upon
the dominant period of 0.56 days. The reason for the Blazhko effect is not
known. One possibility is that the dominant period may excite
non-radial or sectoral mode pulsations with similar oscillation
periods. These modes interfere with the dominant mode, creating a Blazhko
modulation at the beat period of the two modes. A paper
on this theory was recently published by R. Nowakowski and W. Dziembowski
(Acta Astronomica 51, 5), though there is currently no consensus as to
the correct answer.
The RR Lyrae stars are very important in astrophysics, because they
are used for determining the distances to
globular clusters and nearby galaxies. There are
also many field RR Lyrae stars in the halo of the Milky Way -- since we
have a way of measuring their distances, we can use them to determine the
shape and size of our Galaxy's halo. They are still important targets of
study, because their exact luminosity calibration has yet to be
determined -- their properties seem to be influenced heavily by their
chemical composition. Their high amplitudes make them good
targets for amateur astronomers interested in studying
variable stars. In fact, the American Association of Variable Star
Observers or AAVSO has ongoing observing campaigns on many RR Lyrae
stars which backyard astronomers can contribute to.
I made use of the Encyclopedia of Astronomy and Astrophysics recently
published by the Institute of Physics, as well as the NASA ADS Abstract Service
at http://adsabs.harvard.edu. The AAVSO maintains a webpage at
http://www.aavso.org.