Variability class · DCEP · CW

Cepheids

Luminous pulsators that measure the universe.

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.

9,871 active stars DCEP · CW

Phase φ 0.00 · Δm 0.00

A smoother, more rounded rise than RR Lyrae. The longer the period, the more luminous the star — the rung of the distance ladder you can brush in the explorer.

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.

The cosmic distance ladder

The period–luminosity law

A Cepheid's pulsation period is set by its size, and its size sets its luminosity — so the period alone predicts the star's intrinsic brightness. Henrietta Leavitt's 1908 discovery is what turns a light curve into a distance, and it underpins the whole extragalactic distance scale. Here it is in our own Cepheids: each point is one star at its absolute magnitude, and the longer the period, the brighter the star. The dashed line is the relation M_G = −1.3 − 2.8·log₁₀P.

The families within

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.

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.

Pulsators — adoption 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.

What to watch for

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.

From the archive

Worked examples

Gaia DR3 6701299805592611072

DCEP · a 5.4-day classical Cepheid

#7741 →

Keep exploring

Browse the Cepheids population

Browse 9,871 stars → See them in the explorer