Delta Cephei (δ Cephei)
Coordinates, Gaia/HST distance (290 pc), 5.366-day pulsation period, Leavitt law calibration, and observing guide for Delta Cephei.
Provenance-Backed Scientific Measurements
Every measurement below preserves its unit, uncertainty interval, observational method, reference epoch, quality status, and primary catalog source. See Uncertainty & Provenance for how competing measurements are handled.
| Parameter | Value | Uncertainty | Method & Epoch | Quality Status | Source Archive |
|---|---|---|---|---|---|
| Visual magnitude range (V) | 3.95 mag | 3.48 to 4.37 mag | Photoelectric V-band light curve (0.89-mag amplitude) | definitive | General Catalogue of Variable Stars (GCVS) & AAVSO VSX |
| Fundamental radial pulsation period | 5.36627 d | ± 0.00001 d | Two centuries of visual & photoelectric phase monitoring | definitive | General Catalogue of Variable Stars (GCVS) & AAVSO VSX |
| Trigonometric distance (HST FGS + Gaia DR3) | 290 pc | ± 8 pc | HST Fine Guidance Sensor + Gaia DR3 parallax of δ Cep A & B (~946 ly) (J2016.0) | definitive | ESA Gaia Archive — Data Release 3 |
| Mean photospheric radius | 44.5 R☉ | 41.5 to 46.5 R☉ | CHARA interferometric Baade-Wesselink angular diameter oscillation | definitive | SIMBAD Astronomical Database (CDS) |
| Mean bolometric luminosity | 2000 L☉ | ± 150 L☉ | Period-luminosity zero point + trigonometric parallax | definitive | SH0ES Local Distance Ladder Hubble Constant |
J2000 Equatorial Sky Chart & Culmination Altitude
The chart below plots Delta Cephei (Cepheid Distance Ladder Anchor) at RA 22h 29m 10.27s, Dec +58° 24′ 54.7″ within Cepheus, alongside peak meridian transit altitudes for 40°N and 30°S observers. Use the RA/Dec Coordinate Converter to compute exact culmination altitude for your latitude.
Astrophysical Context & Structure
Delta Cephei (δ Cephei) is one of the most consequential stars in the history of science: discovered to vary in brightness by John Goodricke in October 1784 (just two years after his discovery of Algol's periodicity), it became the prototype of the Classical Cepheid variable stars. When Henrietta Swan Leavitt discovered in 1908–1912 that a Cepheid's pulsation period is tightly proportional to its intrinsic luminosity, Cepheids became the master 'standard candles' that Edwin Hubble used in 1924 to prove the Andromeda Nebula is an external galaxy and in 1929 to discover the expansion of the universe.
Located 290 ± 8 parsecs (946 light-years) away in southern Cepheus, Delta Cephei oscillates with clockwork precision every 5.36627 days (5 days 8 hours 47 minutes) between apparent magnitude 3.48 at maximum and 4.37 at minimum. Driven by the Eddington valve (kappa-mechanism) opacity cycle of doubly ionized helium in its envelope, the star's radius physically expands and contracts between 41.5 and 46.5 solar radii at speeds up to 38 km/s, while its spectral class shifts from hot yellow-white F5 Ib at peak brightness to cooler G1 Ib at minimum.
Delta Cephei is also a physical visual double star: 40.8 arcseconds directly south sits magnitude 6.31 Delta Cephei C (HD 213307), a hot blue-white B7-8 III-IV star sharing the exact same Gaia DR3 parallax and proper motion—providing both a stunning color contrast in small telescopes and an independent astrometric parallax check free of Cepheid photocenter pulsations.
Practical Visual Observing Guide
- Delta Cephei forms the southern apex of a compact naked-eye right triangle with Zeta Cephei (mag 3.35) and Epsilon Cephei (mag 4.19)—giving you two ideal comparison stars right in the same field of view!
- Over 1.5 days Delta Cephei surges rapidly up to match Zeta Cephei (3.4), then glides down over 3.9 days to match Epsilon Cephei (4.2).
- Point any 50–80 mm telescope at 30×–60× at Delta Cephei to split its 40.8-arcsecond companion HD 213307 (mag 6.3), resembling a subtle miniature Albireo in gold and blue.
Catalog Cross-Identifications & Aliases
Common Aliases: δ Cephei · HD 213306 · HIP 110991 · 27 Cephei
| Catalog Authority | Identifier |
|---|---|
| GAIADR3 | 2200153425475938048 |
| HIP | HIP 110991 |
| HD | HD 213306 |
| SIMBAD | * del Cep |
| WIKIDATA | Q13024 |
Related Objects, Comparisons & Guides
Polaris (Alpha Ursae Minoris North Star)
Coordinates, Gaia/HST distance (137 pc), 3.97-day Cepheid pulsation, Polaris B companion, and pole-alignment guide for Polaris.
Andromeda Galaxy (Messier 31 / NGC 224)
Coordinates, Cepheid/TRGB distance (765 kpc / 2.50 Mly), magnitude 3.44, blueshift (−300 km/s), and observing guide for Andromeda (M31).
Algol (Beta Persei Eclipsing Binary)
Coordinates, Hipparcos distance (90 ly), 2.8673-day eclipse cycle, Algol Paradox mass transfer, and visual timing guide for Algol.
Mira (Omicron Ceti Pulsating Giant)
Coordinates, Hipparcos distance (100 pc), 332-day pulsation cycle (mag 2.0 to 10.1), white dwarf companion, and observing guide for Mira.
Tracking Bright Variable Stars Observing Guide
How to estimate visual magnitudes and light curves of naked-eye and binocular variable stars: Algol, Delta Cephei, Betelgeuse, Mira, and Eta Carinae.
Splitting Bright Double & Multiple Stars Guide
How to resolve showcase binary and multiple star systems—Albireo, Mizar & Alcor, Epsilon Lyrae, Castor, and Sirius B—and test telescope Dawes limit.
Apparent vs Absolute Magnitude Explained
Understand the logarithmic Pogson astronomical magnitude scale, apparent visual magnitude (m), absolute magnitude (M at 10 parsecs), and bolometric corrections.
Trigonometric Parallax, Parsecs & Gaia DR3
How trigonometric parallax (d_pc = 1 / p_arcsec), the distance modulus equation, Hipparcos, and Gaia DR3 microarcsecond astrometry measure cosmic distances.
Measurement Uncertainty & Optics in Astronomy
How random vs systematic errors, asymmetric error bars, model dependencies, and optical aperture/exit-pupil limits shape astronomical measurements.
Scientific Sources & Dataset Provenance
- ESA Gaia Archive — Data Release 3 (DR3 (2022-06-13)): Gaia Collaboration, Vallenari, A., et al. 2023, A&A, 674, A1 (Gaia Data Release 3) — European Space Agency / Gaia DPAC
- General Catalogue of Variable Stars (GCVS) & AAVSO VSX (v5.1 (2026 update)): Samus, N. N., et al. 2017, Astronomy Reports, 61, 80 (General Catalogue of Variable Stars: Version GCVS 5.1) — Sternberg Astronomical Institute / AAVSO
- SH0ES Local Distance Ladder Hubble Constant (2022 Baseline Cepheid + SN Ia): Riess, A. G., et al. 2022, ApJL, 934, L7 (A Comprehensive Measurement of the Local Value of the Hubble Constant) — SH0ES Collaboration (HST / JWST)