Variability class

Long-period variables

Long-period variables are cool, luminous, evolved giants and supergiants, mostly on the asymptotic giant 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.

2,345,508 starsM · SR · L

Phaseφ 0.00 · Δm 0.00

the star at this phase

Near-sinusoidal, with periods of hundreds of days; a fold needs many cycles.

What it is

Mostly asymptotic-giant-branch stars — low- or intermediate-mass stars in their final pulsational phase, bloated and cool, shedding mass into a dusty wind — with red-giant-branch stars and red supergiants among the semiregular and irregular types. 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.

VSX types

Subtypes

Mira

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

Semiregular

Smaller amplitude, periods from ~20 d to over 1,000 d, periodicity that comes and goes.

Slow irregular

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

Method

Finding the period

Smooth shapes are easy for GLS — once the baseline spans several cycles. The catch is that long periods sit near the observing windows: the archive checks the ~1-year (1/365.25 d⁻¹) and ~29.5-day lunar neighborhoods, and flags a period that falls there as near a window period rather than trusting it blindly. A Mira found at twice or half its VSX period is converted to VSX's period, though another survey's own period can still outrank it; for the other long-period variables a factor of two is flagged, not corrected.

Pitfalls

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.

Archive stars

Examples

SR · a ~188-day semiregular giant

#10000000044869 →

Gaia DR3 4048392501987999104

L · a slow irregular giant that barely keeps a period

#10000006811475 →

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