Variability class · RRab · RRc · RRd

RR Lyrae

Old, metal-poor pulsators — the standard candles of the halo.

RR Lyrae are evolved, low-mass stars on the horizontal branch, pulsating radially as they cross the instability strip. Their near-constant luminosity makes them rungs on the distance ladder and superb tracers of the Galaxy's ancient halo.

61,244 active stars RRab · RRc · RRd

Phase φ 0.00 · Δm 0.00

The fundamental-mode sawtooth: a slow decline to minimum, then an abrupt rise back to maximum light. Rich in harmonics — the whole cycle is the period.

What it is

A horizontal-branch giant burning helium in its core, thermally unstable in the classical instability strip so that it pulsates radially — physically breathing in and out — with periods of roughly 0.2 to 1 day. Because the absolute magnitude of an RR Lyrae is nearly fixed (and tightly correlated with metallicity and infrared period), measuring its apparent brightness yields a distance.

The physics

The pulsation is driven by the κ-mechanism: a layer of partially ionized helium acts as a valve, storing and releasing radiation each cycle. Fundamental-mode pulsators (RRab) show the characteristic asymmetric sawtooth — a steep rise rich in harmonics; first-overtone pulsators (RRc) are nearly sinusoidal. A slow modulation of amplitude and phase over tens to hundreds of cycles is the still-unexplained Blazhko effect.

The families within

Subtypes

Fundamental mode

RRAB

Asymmetric, steep-rising sawtooth; amplitudes up to ~1.5 mag; periods 0.4–0.9 d.

First overtone

RRC

Near-sinusoidal, lower amplitude, shorter period (0.2–0.45 d) — easy to mistake for an EW or δ Scuti.

Double-mode

RRD

Pulsating simultaneously in the fundamental and first overtone; two interleaved periodicities.

The varchive method

Finding the period

The same science code runs for every star. Here is how it behaves for this class — and where it can be fooled.

RR Lyrae are pulsators, so adoption ignores BLS and trusts the GLS peak. The sawtooth is harmonic-rich, which actually helps: the true period is the full cycle, not a sub-multiple. The main hazard is the cadence — at periods near half a day the one-day sidereal aliases sit close to the real peak, so the alias lattice is checked before a period is adopted.

What to watch for

Near-sinusoidal RRc stars overlap EW contact binaries and δ Scuti in period — the fold shape and amplitude break the tie.
Extra structure flanking the main periodogram peak is usually the Blazhko modulation, not a second star.
Periods around 0.5 d are a stone's throw from the sidereal alias at 1.0027 d⁻¹; the top peak is not automatically the right one.

From the archive

Worked examples

Gaia DR3 5785219693863430016

RRab · 0.57-day fundamental-mode sawtooth

#5267 →

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