Fomalhaut (Alpha Piscis Austrini)
Coordinates, Hipparcos distance (25.13 ly), magnitude 1.16, JWST triple debris-belt system, and observing guide for Fomalhaut.
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) | 1.16 mag | ± 0.01 mag | Johnson UBV photoelectric standard star | definitive | Hipparcos New Reduction Catalog |
| Trigonometric parallax | 129.81 mas | ± 0.47 mas | Hipparcos New Reduction astrometric solution (7.704 ± 0.028 pc) (J2000.0) | definitive | Hipparcos New Reduction Catalog |
| Distance in light-years | 25.13 ly | ± 0.09 ly | Derived from Hipparcos parallax 129.81 ± 0.47 mas | definitive | Hipparcos New Reduction Catalog |
| Photospheric radius | 1.842 R☉ | ± 0.019 R☉ | VLTI/VINCI limb-darkened angular diameter (2.228 ± 0.020 mas) | definitive | SIMBAD Astronomical Database (CDS) |
| Stellar mass | 1.92 M☉ | ± 0.02 M☉ | Interferometric radius + 440 Myr evolutionary models (Mamajek 2012) | high-confidence | SIMBAD Astronomical Database (CDS) |
J2000 Equatorial Sky Chart & Culmination Altitude
The chart below plots Fomalhaut (Alpha Piscis Austrini Debris Disk) at RA 22h 57m 39.05s, Dec -29° 37′ 20.0″ within Piscis Austrinus, 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
Fomalhaut (Alpha Piscis Austrini, from the Arabic Fam al-Ḥūt meaning 'Mouth of the Southern Fish') is the eighteenth-brightest star in the night sky at magnitude 1.16. Located just 7.704 ± 0.028 parsecs (25.13 light-years) from Earth—virtually the exact same distance as Vega—Fomalhaut is a young (~440 million-year-old) A3 Va main-sequence star of 1.92 solar masses, 1.84 solar radii, and 16.6 solar luminosities.
Fomalhaut is the primary anchor of one of the widest gravitationally bound triple star systems known in the solar neighborhood: 2.0° (~0.28 parsecs or 57,000 AU) away orbits the K4 V orange dwarf TW Piscis Austrini (Fomalhaut B, magnitude 6.48), and 5.7° (~0.77 parsecs or 158,000 AU / 2.5 light-years) away orbits the M4 V red dwarf LP 876-10 (Fomalhaut C, magnitude 12.6), all sharing identical space velocities and ages.
Surrounded by a massive outer Kuiper-belt-like debris ring spanning 133 to 158 AU first resolved by Hubble and ALMA, Fomalhaut was imaged at mid-infrared wavelengths in 2023 by the James Webb Space Telescope's MIRI instrument, which uncovered three nested planetesimal belts—an inner asteroid belt, an intermediate dynamically sculpted dust gap, and the outer ring—plus a 'Great Dust Cloud' produced by colliding icy bodies.
Practical Visual Observing Guide
- Nicknamed 'the Lonely One' of the autumn sky because it shines as the sole first-magnitude star across a vast, dim expanse of southern Aquarius, Capricornus, and Piscis Austrinus.
- Draw a line southward through the two western stars of the Great Square of Pegasus (Scheat and Markab) and extend it 45° south to land directly on Fomalhaut.
- Use 7×50 binoculars to spot its physical K4 V companion TW Piscis Austrini (Fomalhaut B, magnitude 6.48) sitting 1.96° northwest of Fomalhaut A.
Catalog Cross-Identifications & Aliases
Common Aliases: α Piscis Austrini · Fomalhaut A · HD 216956 · HIP 113368 · 24 Piscis Austrini
| Catalog Authority | Identifier |
|---|---|
| HIP | HIP 113368 |
| HD | HD 216956 |
| SIMBAD | * alf PsA |
| WIKIDATA | Q13019 |
Related Objects, Comparisons & Guides
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.
Achernar (Alpha Eridani Oblate Be Star)
Coordinates, Hipparcos distance (139 ly), magnitude 0.46, extreme 1.35:1 equatorial flattening, and observing guide for Achernar in Eridanus.
Beta Pictoris b — Directly Imaged Gas Giant
Reference data for Beta Pictoris b: 11.9 Jupiter masses, 9.9 AU orbit inside an edge-on debris disk, rapid 8.1-hour rotation, and interferometry.
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