Kaus Australis (Epsilon Sagittarii)
Coordinates, Hipparcos distance (143 ly), magnitude 1.85, brightest star in Sagittarius, and observing guide for Kaus Australis.
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) | 1.85 mag | ± 0.01 mag | Johnson UBV photoelectric photometry | definitive | Hipparcos New Reduction Catalog |
| Trigonometric parallax | 22.76 mas | ± 0.24 mas | Hipparcos New Reduction astrometric solution (43.94 ± 0.46 pc) (J2000.0) | definitive | Hipparcos New Reduction Catalog |
| Distance in light-years | 143.3 ly | ± 1.5 ly | Derived from Hipparcos parallax 22.76 ± 0.24 mas | definitive | Hipparcos New Reduction Catalog |
| Photospheric radius | 6.8 R☉ | ± 0.3 R☉ | Narrabri intensity interferometry angular diameter (1.44 ± 0.06 mas) at 43.94 pc | high-confidence | SIMBAD Astronomical Database (CDS) |
| Bolometric luminosity | 363 L☉ | ± 20 L☉ | UV/optical spectrophotometry + Hipparcos distance | definitive | SIMBAD Astronomical Database (CDS) |
J2000 Equatorial Sky Chart & Culmination Altitude
The chart below plots Kaus Australis (Epsilon Sagittarii B9.5 Giant) at RA 18h 24m 10.32s, Dec -34° 23′ 04.6″ within Sagittarius, 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
Kaus Australis (Epsilon Sagittarii, blending the Arabic al-Qaws for 'the Bow' with the Latin australis for 'southern') is the brightest star in the zodiacal constellation Sagittarius at apparent visual magnitude 1.85, far outshining 4.0-magnitude Alpha Sagittarii (Rukbat). Anchoring the bottom-right corner of the iconic 'Teapot' asterism (where the base of the spout joins the pot) and the southern tip of the Archer's bow, Kaus Australis lies 43.94 ± 0.46 parsecs (143.3 light-years) from Earth.
Classified as a B9.5 III blue-white giant of ~3.5 solar masses and ~230 million years age, Kaus Australis has exhausted its core hydrogen and expanded to 6.8 solar radii, shining with 363 solar luminosities at an effective temperature of 9,960 K. Like Vega and Altair, it spins rapidly with a projected rotational velocity of 236 km/s and exhibits an infrared thermal excess detected by IRAS and Spitzer, indicating a cold circumstellar debris disk at ~155 AU.
High-contrast adaptive-optics imaging and X-ray observations also resolved a close physical companion, Kaus Australis B, a young low-mass star (~0.9 solar masses) orbiting at a separation of 2.39 arcseconds (~106 AU) that accounts for the system's coronal X-ray emission, plus a wider 14th-magnitude companion at 32.3 arcseconds.
Practical Visual Observing Guide
- Forms the southern base of the Sagittarius Teapot alongside Kaus Media (δ Sgr) to the north and Alnasl (γ² Sgr) at the spout tip, framed directly against the brightest star clouds of the Galactic bulge.
- Use Kaus Australis to star-hop 2.4° southeast to the bright 6.1-magnitude globular cluster Messier 69, followed by M70 and M54 along the southern edge of the Teapot.
Catalog Cross-Identifications & Aliases
Common Aliases: ε Sagittarii · HD 169022 · HIP 90185 · 20 Sagittarii
| Catalog Authority | Identifier |
|---|---|
| HIP | HIP 90185 |
| HD | HD 169022 |
| SIMBAD | * eps Sgr |
| WIKIDATA | Q13048 |
Related Objects, Comparisons & Guides
Sagittarius A* (Galactic Center Black Hole)
Coordinates, GRAVITY S2 stellar orbit distance (8,277 pc) & mass (4.297 × 10^6 M☉), EHT 51.8 μas shadow, and Galactic Center guide for Sgr A*.
Shaula (Lambda Scorpii Triple System)
Coordinates, interferometric distance (570 ly), magnitude 1.62, eclipsing pre-main-sequence companion, and observing guide for Shaula.
Vega (Alpha Lyrae Zero-Point Star)
Coordinates, distance (25.04 ly), magnitude 0.03 photometric zero point, rapid oblate rotation, and observing guide for Vega in Lyra.
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.
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.
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