Mu Cephei (Herschel's Garnet Star)
Coordinates, Cepheus OB2 distance (~940 pc / 3,060 ly), magnitude 4.08, ~970 R☉ radius, and visual color guide for Herschel's Garnet Star.
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) | 4.08 mag | ± 0.6 mag | AAVSO V-band monitoring (varies semiregularly between 3.43 and 5.10) | definitive | Hipparcos New Reduction Catalog |
| Distance (Cepheus OB2 association & interferometry) | 940 pc | ± 140 pc | Cepheus OB2 / IC 1396 cluster membership + NOEMA/CHARA modeling (~3,060 ± 450 ly) | model-dependent | ESA Gaia Archive — Data Release 3 |
| Photospheric radius | 972 R☉ | ± 228 R☉ | CHARA/FLUOR limb-darkened angular diameter (18.67 ± 0.35 mas) scaled to 940 pc (Montargès et al. 2019) | model-dependent | SIMBAD Astronomical Database (CDS) |
| Bolometric luminosity | 135000 L☉ | ± 35000 L☉ | Integrated spectrophotometry + Teff = 3,550 ± 100 K at 940 pc | model-dependent | SIMBAD Astronomical Database (CDS) |
| Primary semiregular pulsation period | 730 d | ± 50 d | Fourier analysis of 150-year visual light curve (secondary super-period ~4,400 d) | high-confidence | SIMBAD Astronomical Database (CDS) |
J2000 Equatorial Sky Chart & Culmination Altitude
The chart below plots Garnet Star (Mu Cephei Red Supergiant) at RA 21h 43m 30.46s, Dec +58° 46′ 48.2″ 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
Mu Cephei (officially named 'Garnet Star' by the IAU after Sir William Herschel's 1783 description of its 'very fine deep garnet colour, such as the periodical star ο Ceti') is one of the reddest and most intrinsically luminous stars visible to the unaided eye. Situated on the northern rim of the giant emission nebula IC 1396 (the Elephant's Trunk Nebula) in the Cepheus OB2 association ~940 parsecs (~3,060 light-years) from Earth, Mu Cephei varies semiregularly between apparent magnitude 3.43 and 5.10 with a primary period of ~730 days (2.0 years) and a long secondary period of ~4,400 days (12 years).
Serving since 1943 as the Morgan–Keenan spectral standard for class M2 Ia, Mu Cephei has a limb-darkened angular diameter of 18.67 ± 0.35 milliarcseconds measured by the CHARA interferometer—corresponding at 940 parsecs to a radius of ~972 solar radii (~4.5 AU, nearly reaching the orbit of Jupiter) and a bolometric luminosity of ~135,000 solar luminosities (or ~270,000 L☉ in older 1.2 kpc models).
High-resolution ALMA and NOEMA millimeter interferometry of carbon monoxide (CO) gas around Mu Cephei shows that, like Betelgeuse and VY Canis Majoris, it sheds mass not only in a steady spherical wind (~2 × 10^-6 solar masses per year) but also in dense, clumpy convective plumes that condense into water vapor and silicate dust shells.
Practical Visual Observing Guide
- Located 2.5° southeast of Alderamin (α Cephei), Mu Cephei's deep orange-crimson tint (color index B−V = +2.26) is striking to the naked eye and utterly breathtaking in 7×50 or 10×50 binoculars against the snowy blue-white background stars of the Cepheus Milky Way.
- When viewing Mu Cephei in a telescope or binoculars, avoid staring directly at it for more than a few seconds at a time (the Purkinje effect can bleach out its saturation) or defocus it slightly to fill your fovea with its glowing ruby-garnet hue.
Catalog Cross-Identifications & Aliases
Common Aliases: μ Cephei · Herschel's Garnet Star · Erakis · HD 206936 · HIP 107259
| Catalog Authority | Identifier |
|---|---|
| GAIADR3 | 2202630001603670016 |
| HIP | HIP 107259 |
| HD | HD 206936 |
| SIMBAD | * mu. Cep |
| WIKIDATA | Q13065 |
Related Objects, Comparisons & Guides
Betelgeuse (Alpha Orionis Red Supergiant)
Coordinates, competing optical vs radio distances (168 vs 222 pc), radius, Great Dimming physics, and observing guide for Betelgeuse.
UY Scuti (Pulsating Red Supergiant in Scutum)
Coordinates, revised Gaia DR3 distance (~1.55 kpc vs older 2.9 kpc), ~755–909 R☉ radius revision, 740-day pulsation, and guide for UY Scuti.
Delta Cephei (Cepheid Distance Ladder Anchor)
Coordinates, Gaia/HST distance (290 pc), 5.366-day pulsation period, Leavitt law calibration, and observing guide for Delta Cephei.
Stellar Spectral Classification (OBAFGKM)
Reference guide to Morgan-Keenan (MK) stellar classification (O B A F G K M), luminosity classes (I–V, WD), color index (B-V), and the H-R diagram.
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.
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
- Hipparcos New Reduction Catalog (2007 New Reduction): van Leeuwen, F. 2007, A&A, 474, 653 (Validation of the new Hipparcos reduction) — ESA / van Leeuwen (2007)
- SIMBAD Astronomical Database (CDS) (2026-10 release): Wenger, M., et al. 2000, A&AS, 143, 9 (The SIMBAD astronomical database) — Centre de Données astronomiques de Strasbourg