Alnilam (Epsilon Orionis)
Coordinates, distance (~1,340–2,000 ly), magnitude 1.69, 837,000 L☉ blue supergiant luminosity, and observing guide for Alnilam in Orion.
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 |
|---|---|---|---|---|---|
| Apparent visual magnitude (V) | 1.69 mag | ± 0.04 mag | Johnson UBV photometry (Alpha Cygni micro-pulsations between 1.64 and 1.74) | definitive | Hipparcos New Reduction Catalog |
| Distance (Orion OB1b / Hipparcos) | 412 pc | ± 65 pc | Orion OB1b association membership (~412 pc / 1,340 ly; Hipparcos Revision = 606 ± 140 pc) | disputed | Hipparcos New Reduction Catalog |
| Bolometric luminosity | 537000 L☉ | ± 150000 L☉ | Non-LTE B0 Ia wind-atmosphere modeling (275,000 L☉ at 412 pc up to 832,000 L☉ at 606 pc) | model-dependent | SIMBAD Astronomical Database (CDS) |
| Effective temperature | 27500 K | ± 500 K | Quantitative UV/optical B0 Ia spectroscopy (Puebla et al. 2016) | definitive | SIMBAD Astronomical Database (CDS) |
J2000 Equatorial Sky Chart & Culmination Altitude
The chart below plots Alnilam (Epsilon Orionis Belt Supergiant) at RA 05h 36m 12.81s, Dec -01° 12′ 06.9″ within Orion, 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
Alnilam (Epsilon Orionis, from the Arabic al-Niẓām meaning 'the String of Pearls') is the central jewel of Orion's Belt and the brightest of the three belt stars at apparent visual magnitude 1.69 (fourth-brightest star in Orion and twenty-ninth in the night sky). Even though it appears slightly dimmer than flanked Rigel and Betelgeuse, Alnilam lies much farther away (~1,340 to 1,980 light-years), making it the most intrinsically luminous star among the 30 brightest stars in Earth's sky!
Classified as a B0 Ia blue supergiant with an effective surface temperature of 27,500 K, a radius ~32 to 42 times that of the Sun, and a mass of ~30–40 solar masses, Alnilam radiates between 275,000 and 832,000 solar luminosities depending on the adopted distance. Its fierce ultraviolet radiation pressure drives a clumpy stellar wind screaming outward at 1,900 km/s, shedding roughly two-millionths of a solar mass every year—twenty million times the mass-loss rate of our Sun.
Only ~5.7 million years old, Alnilam illuminates the surrounding molecular cloud NGC 1990 and serves as a premier background continuum source for ultraviolet interstellar absorption spectroscopy because its clean B0 Ia spectrum slices across multiple foreground gas clouds.
Practical Visual Observing Guide
- Sits squarely in the middle of the 2.7°-long three-star line of Orion's Belt, flanked symmetrically by Alnitak (ζ Ori) 1.36° to the southeast and Mintaka (δ Ori) 1.37° to the northwest.
- Sweep Orion's Belt with 7×50 or 10×50 binoculars to reveal the rich Cr 70 (Collinder 70) / Orion OB1b star association—an S-shaped stream of more than a hundred 6th-to-9th magnitude blue-white stars looping around Alnilam.
Catalog Cross-Identifications & Aliases
Common Aliases: ε Orionis · HD 37128 · HIP 26311 · 46 Orionis
| Catalog Authority | Identifier |
|---|---|
| HIP | HIP 26311 |
| HD | HD 37128 |
| SIMBAD | * eps Ori |
| WIKIDATA | Q13039 |
Related Objects, Comparisons & Guides
Alnitak (Zeta Orionis O-Type Supergiant)
Coordinates, distance (~1,260 ly), magnitude 1.77, brightest O-type star in the sky, triple system, and observing guide for Alnitak.
Mintaka (Delta Orionis Eclipsing Multiple)
Coordinates, distance (~1,200 ly), magnitude 2.23, 5.73-day O-star eclipsing binary, interstellar Ca II discovery, and guide for Mintaka.
Rigel (Beta Orionis Blue Supergiant)
Coordinates, Hipparcos distance (264 pc), luminosity (120,000 L☉), Rigel BC companion, and observing guide for Rigel in Orion.
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.
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
- 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