Altair (Alpha Aquilae)
Coordinates, Hipparcos distance (16.73 ly), magnitude 0.76, 8.9-hour oblate spin imaged by CHARA, and observing guide for Altair in Aquila.
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) | 0.76 mag | ± 0.01 mag | Johnson UBV photoelectric photometry | definitive | Hipparcos New Reduction Catalog |
| Trigonometric parallax | 194.95 mas | ± 0.57 mas | Hipparcos New Reduction astrometric solution (J2000.0) | definitive | Hipparcos New Reduction Catalog |
| Distance in light-years | 16.73 ly | ± 0.05 ly | Derived from parallax 194.95 ± 0.57 mas (5.130 ± 0.015 pc) | definitive | Hipparcos New Reduction Catalog |
| Stellar mass | 1.86 M☉ | ± 0.03 M☉ | CHARA direct surface image + asteroseismology (Bouchaud et al. 2020) | high-confidence | SIMBAD Astronomical Database (CDS) |
| Effective temperature (polar to equatorial) | 7550 K | 6860 to 8450 K | CHARA/MIRC direct interferometric surface imaging | definitive | SIMBAD Astronomical Database (CDS) |
J2000 Equatorial Sky Chart & Culmination Altitude
The chart below plots Altair (Alpha Aquilae Rapid Rotator) at RA 19h 50m 47.00s, Dec +08° 52′ 06.0″ within Aquila, 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
Altair (Alpha Aquilae) is the twelfth-brightest star in the night sky (magnitude 0.76) and the closest vertex of the Summer Triangle, lying just 5.130 ± 0.015 parsecs (16.73 light-years) from Earth in Aquila. In 2007 astronomers using the Michigan Infrared Combiner (MIRC) on the CHARA optical interferometer produced the first direct resolved surface image of a main-sequence star beyond the Sun by mapping Altair's 3.4-milliarcsecond disk.
That landmark image proved directly what rotational Doppler broadening of Altair's A7 Vn spectral lines had long implied: Altair spins on its axis once every 8 hours 51 minutes at an equatorial velocity of ~285 km/s (~140 times faster than the Sun's 2 km/s equatorial speed). Centrifugal force flattens the star into an oblate spheroid whose equatorial diameter is 22% wider than its polar diameter.
Wire-Satellite (WIDE-field Infrared Explorer) space photometry also showed that Altair is the brightest Delta Scuti pulsating variable star in the sky, oscillating simultaneously across at least seven non-radial acoustic modes with periods between 50 minutes and 2.5 hours and micro-amplitudes below a thousandth of a magnitude.
Practical Visual Observing Guide
- Anchors the southern apex of the Summer Triangle, easily recognized by its two flanking guardian stars: yellow-orange Tarazed (γ Aquilae, mag 2.72) 2° to the northwest and subgiant Alshain (β Aquilae, mag 3.71) 2.5° to the southeast.
- Look just 1.5° west of Tarazed with 10×50 binoculars or a small telescope to see Barnard's E (B142 and B143), a famous pair of opaque dark dust nebulae silhouetted against the rich Aquila Milky Way.
Catalog Cross-Identifications & Aliases
Common Aliases: α Aquilae · HD 187642 · HIP 97649 · 53 Aquilae
| Catalog Authority | Identifier |
|---|---|
| HIP | HIP 97649 |
| HD | HD 187642 |
| SIMBAD | * alf Aql |
| WIKIDATA | Q12974 |
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.
Albireo (Beta Cygni Color Double Star)
Coordinates, Gaia DR3 parallaxes of Beta Cygni A & B, 34.6″ angular separation, color contrast, and observing guide for Albireo in Cygnus.
Dumbbell Nebula (Messier 27 in Vulpecula)
Coordinates, Gaia DR3 distance (389 pc), magnitude 7.4, central white dwarf, and observing guide for the Dumbbell Nebula (M27).
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.
Northern Summer Milky Way Observing Tour
Summer observing guide from the Summer Triangle (Vega, Altair, Deneb) and Hercules Keystone down the Milky Way rift to the Sagittarius Galactic Center.
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.
Bright, Multiple & Variable Stars Catalog
Cross-reference catalog of solar-neighborhood and navigation stars across Bayer, Henry Draper (HD), Hipparcos (HIP), WDS, and Gaia DR3 designations.
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)
- IAU Working Group on Star Names (WGSN) (2026 IAU WGSN Bulletin): Mamajek, E., et al. (IAU WGSN) 2016–2026, IAU Catalog of Star Names — International Astronomical Union
- 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