Alnitak (Zeta Orionis)
Coordinates, distance (~1,260 ly), magnitude 1.77, brightest O-type star in the sky, triple system, and observing guide for Alnitak.
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 |
|---|---|---|---|---|---|
| Combined apparent visual magnitude (V) | 1.77 mag | ± 0.02 mag | Johnson UBV photometry (Aa = 2.08 mag, Ab = 4.28 mag, B = 3.70 mag) | definitive | Hipparcos New Reduction Catalog |
| Distance (interferometric orbit & Orion OB1b) | 387 pc | ± 54 pc | NPOI interferometric orbit of Aa–Ab (Hummel et al. 2013: ~387 ± 54 pc / 1,260 ly; Hipparcos = 226 ± 33 pc) | disputed | SIMBAD Astronomical Database (CDS) |
| Primary bolometric luminosity (Alnitak Aa) | 250000 L☉ | ± 50000 L☉ | O9.5 Iab UV/optical quantitative spectroscopy at Orion OB1b distance | model-dependent | SIMBAD Astronomical Database (CDS) |
| Primary effective temperature (Alnitak Aa) | 29500 K | ± 1000 K | Non-LTE O9.5 Iab stellar-wind spectroscopy | definitive | SIMBAD Astronomical Database (CDS) |
| Primary supergiant mass (Alnitak Aa) | 33 M☉ | ± 6 M☉ | NPOI 7.3-year interferometric + radial-velocity orbit (Ab = 14 ± 3 M☉) | high-confidence | SIMBAD Astronomical Database (CDS) |
J2000 Equatorial Sky Chart & Culmination Altitude
The chart below plots Alnitak (Zeta Orionis O-Type Supergiant) at RA 05h 40m 45.53s, Dec -01° 56′ 33.3″ 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
Alnitak (Zeta Orionis, from the Arabic al-Niṭāq meaning 'the Girdle') anchors the eastern (left-hand) end of Orion's Belt, shining at combined visual magnitude 1.77. Its primary star, Alnitak Aa (magnitude 2.08), has a spectral class of O9.5 Iab and an effective surface temperature of 29,500 K—making it the brightest O-type star in the entire night sky! Only ~6 million years old and weighing ~33 solar masses, this scorching blue supergiant spans 20 solar radii and radiates roughly 250,000 times the bolometric power of the Sun.
Alnitak is a close hierarchical triple system of massive OB stars: in 1998 astronomers using the Navy Prototype Optical Interferometer (NPOI) resolved Alnitak A into a 40-milliarcsecond binary in which the O9.5 Iab supergiant (Aa) is orbited every 7.3 years by a 4.3-magnitude B1 IV subgiant (Alnitak Ab, ~14 solar masses). At a wider separation of 2.4 arcseconds (~900 AU) orbits magnitude 3.70 Alnitak B, a fast-spinning B0 III giant completing one revolution every ~1,500 years.
In deep-sky astrophotography and visual observing, Alnitak sits at one of the richest intersections in the sky: its intense ultraviolet light directly ionizes both the Flame Nebula (NGC 2024) 15 arcminutes to its northeast and the glowing red hydrogen curtain IC 434 against which the famous dark Horsehead Nebula (Barnard 33) is silhouetted 30 arcminutes to the south.
Practical Visual Observing Guide
- Splitting magnitude 3.70 Alnitak B at 2.4 arcseconds separation (position angle ~166°) from magnitude 1.9 Alnitak A is a classic, rewarding double-star test for an 80–120 mm telescope at 150×–220×.
- When hunting the adjacent Flame Nebula (NGC 2024) or Horsehead Nebula (B33) visually in a 200–400 mm telescope with an H-beta filter, move dazzling Alnitak just outside the edge of the eyepiece field so its glare does not overwhelm the faint nebulosity.
Catalog Cross-Identifications & Aliases
Common Aliases: ζ Orionis · HD 37742 · HIP 26727 · 50 Orionis
| Catalog Authority | Identifier |
|---|---|
| HIP | HIP 26727 |
| HD | HD 37742 |
| SIMBAD | * zet Ori |
| WIKIDATA | Q13044 |
Related Objects, Comparisons & Guides
Alnilam (Epsilon Orionis Belt Supergiant)
Coordinates, distance (~1,340–2,000 ly), magnitude 1.69, 837,000 L☉ blue supergiant luminosity, and observing guide for Alnilam in Orion.
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.
Trapezium Cluster (Theta1 Orionis OB System)
Coordinates, VLBA/Gaia distance (1,344 ly), 40-M☉ O6Vp ionizing star θ1 Ori C, A–B–C–D–E–F split guide, and Trapezium physics.
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.
Angular Size, Separation & Field of View
How angular diameter (degrees, arcminutes, arcseconds), spherical angular separation, and small-angle physical diameter relate to telescope field of view.
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