Beta Monocerotis (β Monocerotis A, B & C)
Coordinates, Hipparcos distance (680 ly), 7.1″ and 2.9″ triple Be-star arc, and telescope observing guide for Beta Monocerotis.
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) | 3.76 mag | ± 0.02 mag | Johnson UBV photometry (A = 4.62 mag, B = 5.00 mag, C = 5.32 mag) | definitive | Hipparcos New Reduction Catalog |
| Trigonometric parallax | 4.82 mas | ± 0.77 mas | Hipparcos New Reduction astrometric solution (~207 ± 33 pc / 680 ly) (J2000.0) | high-confidence | Hipparcos New Reduction Catalog |
| Visual A–B angular separation | 0.118 arcmin | ± 0.002 arcmin | WDS astrometry (A–B = 7.1″ at PA 133°; B–C = 2.9″ at PA 108°) | definitive | SIMBAD Astronomical Database (CDS) |
| Component effective temperatures | 18500 K | ± 1000 K | B3 Ve shell-star UV/optical spectroscopy across A, B, and C | high-confidence | SIMBAD Astronomical Database (CDS) |
J2000 Equatorial Sky Chart & Culmination Altitude
The chart below plots Beta Monocerotis (Herschel's Wonder Triple) at RA 06h 28m 49.07s, Dec -07° 01′ 59.0″ within Monoceros, 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
Beta Monocerotis (11 Monocerotis) is universally celebrated by visual astronomers as the finest true triple star in the heavens: when William Herschel discovered it on December 17, 1781, he wrote in his observing log that it is 'one of the most beautiful sights in the heavens.' Shining as a single magnitude 3.76 star to the unaided eye in western Monoceros (roughly midway between Sirius and Betelgeuse), Beta Monocerotis is actually the brightest star in the constellation Monoceros, slightly outshining 3.93-magnitude Alpha Monocerotis.
In a telescope, Beta Monocerotis resolves into a tight, gracefully curving arc of three matching blue-white gems located ~207 parsecs (680 light-years) from Earth: Beta Monocerotis A (magnitude 4.62) lies 7.1 arcseconds northwest of Beta Monocerotis B (magnitude 5.00), while Beta Monocerotis C (magnitude 5.32) sits just 2.9 arcseconds east-southeast of B along position angle 108°.
Astrophysically, all three components are coeval (~35-million-year-old) massive B3 Ve main-sequence stars (~6 to 7 solar masses each, effective temperatures ~18,500 K, luminosities ~1,500 to 3,200 solar luminosities) spinning so rapidly (~250–350 km/s) that they are surrounded by gaseous equatorial Be emission disks! A fourth co-moving visual component, 12.2-magnitude Beta Monocerotis D, sits 26.9 arcseconds away.
Practical Visual Observing Guide
- Find Beta Monocerotis by forming an equilateral triangle with Sirius (α CMa) and Saiph (κ Ori), or looking 9.5° northeast of Rigel.
- A 50–60 mm telescope at 50× splits A from the BC pair at 7.1 arcseconds, and increasing magnification to 100×–150× in any 70–100 mm telescope splits B and C (2.9″ apart), displaying three brilliant icy-white Airy disks arranged like a miniature curved Orion's Belt!
Catalog Cross-Identifications & Aliases
Common Aliases: β Monocerotis · HD 45725 / HD 45726 / HD 45727 · HIP 30867 · 11 Monocerotis
| Catalog Authority | Identifier |
|---|---|
| HIP | HIP 30867 |
| HD | HD 45725 |
| SIMBAD | * bet Mon |
| WIKIDATA | Q66477097 |
Related Objects, Comparisons & Guides
Sirius (Alpha Canis Majoris Binary)
Coordinates, Hipparcos parallax (379.21 mas, 8.60 ly), magnitude −1.46, Sirius B white dwarf orbit, and observing guide for Sirius.
Rigel (Beta Orionis Blue Supergiant)
Coordinates, Hipparcos distance (264 pc), luminosity (120,000 L☉), Rigel BC companion, and observing guide for Rigel in Orion.
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.
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.
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.
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