Caption: Above: The globular cluster M5, located 25,000 light-years away in Serpens, is nearly 13 billion years old--plenty old enough for stars a little less massive than the Sun to have become fast-pulsing RR Lyrae variables.
RR Lyraes are extremely old, however, so they mark ancient populations of stars that lack any Cepheids--such as globular clusters and the Milky Way's outer halo.
The idea is to time when their maxima occur, in particular to follow strange irregularities that many RR Lyraes display known as Blazhko cycles.
The RR Lyrae
stars are moving targets -- their pulsations result in changes in their apparent velocity over the course of a day.
To illustrate the RR Lyrae stars, I modified my simple plotting program to read in two images of a single cluster and produce on one chart a combined H-R diagram.
The RR Lyrae stars are exactly where they should be--on the horizontal branch.
If RR Lyrae stars are intrinsically brighter than had been thought, then their globular clusters lie farther from Earth.
Because the extent of helium mixing is not yet known, Sweigart cautions that he cannot determine how much brighter RR Lyrae stars really are.
The detection of thousands of RR Lyrae stars in the LMC is particularly impressive, suggesting that the total number of old stars there may be about twice that previously estimated.
As with Cepheids, RR Lyraes can pulsate with two periods simultaneously.
All RR Lyrae variables have nearly the same absolute magnitude, so they are especially useful as distance indicators.
Additional nearby galaxies have recently revealed their RR Lyrae variables as well.