All RR Lyrae
variables of the type considered here have nearly the same absolute visual magnitude, +0.75.
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.
Moreover, those younger clusters contain the longer-period RR Lyrae
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.
Sweigart used computer models to analyze a set of stars known as RR Lyrae
. They all have the same intrinsic luminosity, like light bulbs of a single wattage.
Zinn (Yale University) also found clumping in their survey of RR Lyrae
variables in the halo.
Dozens of projects here await the willing amateur, such as discovering new variables, monitoring cataclysmic variables for outbursts, and refining the light curves (graphs of periodic brightness change) of eclipsing binaries, recurrent novae, rotating spotted stars, and RR Lyrae
, Mira, and Be stars.
The recognition that the long-period Cepheids were distinct from their RR Lyrae
cousins, and that they came in two varieties, was a watershed.
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.
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.
Judging from the RR Lyrae
distances, no galaxy-like clump of stars is located between the Milky Way and the LMC.
Among those luminaries are the RR Lyraes
. RR Lyrae
stars are pulsators, like Cepheids, but they are older, which means they can be found in globular clusters.
stars (again, named for the prototype) are another breed, with short periods ranging from six to 18 hours.
For example, even the RR Lyrae
method -- used by Harlow Shapley at the turn of the century to measure the Milky Way's size -- has shown considerable progress recently.