Variability class · M · SR · L

Long-period

Red giants breathing over months — Miras and semiregulars.

Long-period variables are cool, luminous, evolved giants on the asymptotic branch, pulsating slowly and enormously as they near the end of their lives. A Mira can swing by several magnitudes over the better part of a year.

286,540 active stars M · SR · L

Phase φ 0.00 · Δm 0.00

Smooth and near-sinusoidal in shape, but the period runs to hundreds of days — so the fold only makes sense once the baseline covers many cycles.

What it is

An asymptotic-giant-branch star — a low- or intermediate-mass star in its final pulsational phase, bloated and cool, shedding mass into a dusty wind. Miras are the large-amplitude, long-period, fairly regular end of the family; semiregulars (SR) pulse with smaller amplitude and looser coherence; slow irregulars (L) barely keep a period at all.

The physics

The pulsation is again driven from the envelope, but the huge visual amplitude of a Mira is mostly an illusion of chemistry: as the star cools through minimum, titanium-oxide bands blanket the visible spectrum and dump the flux into the infrared. The mean magnitude and even the period can drift over time, so these stars are only quasi-periodic.

The families within

Subtypes

Mira

Large-amplitude (>2.5 mag visual), long-period (~150–500 d), reasonably regular AGB pulsators.

Semiregular

Smaller amplitude, periods ~30–150 d, periodicity that comes and goes.

Slow irregular

Late-type giants with no well-defined period — variability without a clock.

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.

Smooth shapes are easy for GLS — once the baseline spans several cycles. The catch is that hundreds-of-day periods land right on top of the seasonal and lunar windows: the alias lattice explicitly checks the ~1-year (1/365.25 d⁻¹) and ~29.5-day neighbourhoods, and a period that falls there earns a near_window_period flag rather than blind trust.

What to watch for

A short baseline simply cannot pin a 300-day period; the fold stays a smear until enough cycles accumulate.
Seasonal gaps and the lunar cycle inject aliases at exactly the periods these stars occupy.
The mean brightness and the period itself wander, so a single adopted period is an approximation, not a constant of nature.
The brightest Miras can saturate near maximum light while disappearing below the limit near minimum — both ends bias the statistics.

From the archive

Worked examples

ASAS J132500−7953.4

SR · a 64-day semiregular, flagged near a window period

#8841 →

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