Hadar (Beta Centauri / Agena)
Coordinates, interferometric distance (390 ly), magnitude 0.61, twin 10.7-M☉ Beta Cephei binary orbit, and observing guide for Hadar.
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) | 0.61 mag | ± 0.02 mag | Johnson UBV photometry (Beta Cephei pulsations of ±0.03 mag) | definitive | Hipparcos New Reduction Catalog |
| Dynamical orbital parallax distance | 119.8 pc | ± 5.5 pc | Combined SUSI optical interferometry & spectroscopic orbit (390.7 ± 18.0 ly) | high-confidence | SIMBAD Astronomical Database (CDS) |
| Inner binary orbital period (Aa–Ab) | 356.915 d | ± 0.015 d | Double-lined radial velocity + interferometry (eccentricity e = 0.8245) | definitive | SIMBAD Astronomical Database (CDS) |
| Primary component mass (Hadar Aa) | 12.02 M☉ | ± 0.13 M☉ | Pigulski et al. 2016 BRITE + orbit modeling (Ab = 10.58 ± 0.18 M☉) | definitive | SIMBAD Astronomical Database (CDS) |
J2000 Equatorial Sky Chart & Culmination Altitude
The chart below plots Hadar (Beta Centauri Triple B-Giant System) at RA 14h 03m 49.41s, Dec -60° 22′ 22.9″ within Centaurus, 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
Hadar (Beta Centauri, traditionally also called Agena) is the eleventh-brightest star in Earth's night sky at apparent magnitude 0.61. Paired on the southern sky just 4.4° west of Alpha Centauri, Hadar serves as the inner 'Southern Pointer' guiding observers toward Acrux and the Southern Cross. While Alpha Centauri lies a mere 4.37 light-years away, Hadar is a luminous background heavyweight located 119.8 ± 5.5 parsecs (391 light-years) from Earth in the Scorpius–Centaurus OB association.
High-resolution spectroscopy and optical interferometry reveal that Hadar is a hierarchical triple system of hot blue-white B-type stars. Its primary component (Hadar A) is a double-lined spectroscopic and interferometric binary composed of two nearly identical B1 III giants—Hadar Aa (12.02 solar masses) and Hadar Ab (10.58 solar masses)—whipping around each other every 356.92 days along a highly eccentric orbit (e = 0.825) that brings them within 0.45 AU at periastron.
Observations with the BRITE-Constellation nanosatellites proved that both massive B1 giants are multi-periodic Beta Cephei pulsating variables oscillating with periods of 3 to 5 hours, radiating a combined 55,000 solar luminosities at 25,000 K. A third star, magnitude 3.95 Hadar B, orbits the inner pair at an angular separation of ~0.5 to 1.3 arcseconds with a period of ~170–220 years.
Practical Visual Observing Guide
- Form a prominent 4.4° pair with golden Alpha Centauri in the rich southern Milky Way; notice the striking naked-eye color difference between 25,000 K blue-white Hadar and 5,790 K yellow Alpha Centauri.
- Splitting 4.0-magnitude Hadar B from 0.6-magnitude Hadar A (sub-arcsecond to 1.2″ separation depending on orbital phase) requires a 200–300 mm telescope at 300×+ under steady southern seeing.
Catalog Cross-Identifications & Aliases
Common Aliases: β Centauri · Agena · HD 122451 · HIP 68702
| Catalog Authority | Identifier |
|---|---|
| HIP | HIP 68702 |
| HD | HD 122451 |
| SIMBAD | * bet Cen |
| WIKIDATA | Q12183 |
Related Objects, Comparisons & Guides
Proxima Centauri (Nearest Star to Sun)
Coordinates, Gaia DR3 parallax (768.07 mas, 4.2465 ly), magnitude 11.13, flare activity, and planetary system guide for Proxima Centauri.
Acrux (Alpha Crucis Southern Cross Multiple)
Coordinates, Hipparcos distance (321 ly), magnitude 0.76, 4.0″ B-star visual binary + α Cru C triple, and observing guide for Acrux.
Spica (Alpha Virginis Ellipsoidal Binary)
Coordinates, Hipparcos distance (250 ly), 4.01-day close B-star binary, precession discovery history, and observing guide for Spica.
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.
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