Variable Zoo · The Zoo · RR Lyrae
RRab · horizontal branchOld, metal-poor pulsators — the sawtooth heartbeat of the galactic halo, and a rung on the cosmic distance ladder.
The animal
An RR Lyrae star is an old, low-mass, metal-poor star that has reached the horizontal branch, fusing helium in its core after its red-giant phase. It sits in the instability strip and pulsates radially, brightening and fading in well under a day.
They crowd the globular clusters and the galactic halo, tracing the oldest populations of the Milky Way. Because their average luminosity is nearly fixed (about 40–50 times the Sun's), they serve as standard candles for distances out to a few hundred thousand light-years.
RR Lyrae pulsate through the κ-mechanism: a layer of partially ionised helium acts as a valve, growing more opaque when compressed, trapping heat, and driving the envelope to expand and contract. The result is the characteristic fast rise and slow decline of the RRab light curve.
Fundamental-mode pulsators (RRab) show that sharp sawtooth; first-overtone pulsators (RRc) are more sinusoidal and lower in amplitude; a rare few (RRd) pulsate in both modes at once.
How it works
Pulsating stars are heat engines. A layer of ionizing helium deep in the envelope turns opaque when compressed — trapping heat like a closed valve — then clears as it expands and releases it. That cycle, the κ-mechanism, drives the whole star to swell and shrink; an RR Lyrae runs through a full breath in well under a day.
A real light curve
A real RR Lyrae star from Gaia DR3 epoch photometry, folded on the period Gaia measured. The steep rise and gradual decline — the RRab sawtooth — is textbook.
Source: Gaia DR3
From our survey
In the catalog
VSX lists 317,558 RR Lyrae stars — most pulsating in the fundamental mode (RRab, the sawtooth kind, 191,915) or the first overtone (RRc, more sinusoidal, 99,611).
Try it yourself
An RR Lyrae runs through its entire cycle in well under a day, so a single clear night of continuous imaging can capture a full light curve. Green/V frames every few minutes for a few hours, folded on the catalog period, reveal the sharp sawtooth rise — as in our own M3 harvest.
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