Variability class

Cepheids

Cepheids are bright, yellow supergiants pulsating as they loop across the instability strip. Their period-luminosity relation — longer period means intrinsically brighter — is the calibration that built the extragalactic distance scale.

17,055 starsDCEP · CW

Phaseφ 0.00 · Δm 0.00

the star at this phase

A smoother rise than RR Lyrae. Longer period means higher luminosity.

What it is

A massive star (several to ten-plus solar masses) caught crossing the instability strip as it evolves, pulsating radially with periods of days to months. Classical Cepheids are young Population I stars confined to the thin disk; the Population II cousins (W Virginis / type-II Cepheids) are old, lower-mass stars following a fainter, parallel relation.

The physics

Same κ-mechanism valve as RR Lyrae, but in a far more luminous star, so the periods are longer and the period-luminosity (Leavitt) relation is tight enough to use as a ruler. The light curve rises a little more gently than an RR Lyrae sawtooth, and at intermediate periods a secondary "bump" marches across the cycle (the Hertzsprung progression) — a clue to the star's exact mode and mass.

VSX types

Subtypes

Classical (δ Cep)

DCEP

Young, Population I, fundamental mode; the Leavitt-law calibrators. Periods ~1–100 d.

First overtone

DCEPS

Shorter-period, lower-amplitude, more sinusoidal classical Cepheids.

Type II (W Vir)

Old, metal-poor, lower-mass pulsators ~1.5 mag fainter than classical Cepheids at the same period.

Method

Finding the period

Pulsators: the archive uses GLS and ignores BLS. Cepheids are among the easiest stars to phase: large amplitude, smooth shape, periods comfortably longer than the daily cadence. The period-luminosity sequence is exactly the sloped locus you can isolate by brushing Cepheids in the population explorer.

Pitfalls

The brightest Cepheids run into saturation above g ≈ 11; their amplitudes and means flatten out and must be read with care.
Classical and type-II Cepheids share the period axis but sit ~1.5 mag apart in luminosity — don't mix the two relations.
Long-period (tens of days) Cepheids need a baseline spanning many cycles before the fold sharpens.

Archive stars