Variability class

RR Lyrae stars

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.

316,977 starsRRab · RRc · RRd

Phaseφ 0.00 · Δm 0.00

the star at this phase

Fundamental-mode sawtooth: slow decline, abrupt rise.

What it is

A horizontal-branch star burning helium in its core and sitting in the classical instability strip, where it is pulsationally unstable (the κ-mechanism in the He II ionization zone drives radial pulsation): it physically breathes in and out, with periods of roughly 0.2 to 1 day. Because the absolute magnitude of an RR Lyrae is nearly fixed (in V it depends mainly on metallicity; in the infrared it follows a tight period-luminosity relation), 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.

VSX types

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.

Method

Finding the period

RR Lyrae are pulsators, so the archive ignores BLS for them 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: a period near 0.5 d is almost commensurate with a once-a-night visit (2 d⁻¹ ≈ 2 × 1.0027 d⁻¹), so each night samples nearly the same phase and the daily aliases crowd the true peak. Every candidate is checked against those aliases before a period is adopted.

Pitfalls

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 pulsation mode.
Periods near 0.5 d (2 d⁻¹) sit next to twice the sidereal frequency (2.0055 d⁻¹), where the daily aliases pile up; the top peak is not automatically the right one.

Archive stars

Examples

DL Com

RRab · a 0.43-day fundamental-mode sawtooth, ~1.3 mag amplitude

#10000000009709 →

ASAS J121553-5157.4

RRc · a 0.32-day first-overtone pulsator, near-sinusoidal

#10000000084319 →

XX Crv

RRd · double-mode — fundamental and first overtone together

#10000000119727 →

Browse RR Lyrae stars

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