Okab

Zeta Aquilae (ζ Aquilae, abbreviated Zeta Aql, ζ Aql) is a spectroscopic binary star in the equatorial constellation of Aquila. It is readily visible with the naked eye, being of the third magnitude. Based on parallax measurements obtained during the Hipparcos mission, it is approximately 83 light-years (25 parsecs) distant from the Sun.

Zeta Aquilae's two components can be designated Zeta Aquilae A (officially named Okab /ˈoʊkæb/, the traditional name for the system) and B. Zeta Aquilae has a number of companions listed and together they are designated WDS J19054+1352. As the primary star of this group, Zeta Aquilae also bears the designation WDS J19054+1352A. The companions are then designated WDS J19054+1352B, C, D and E.

Zeta Aquilae has a combined stellar classification of A0 Vn, with the luminosity class 'V' indicating is a main sequence star that is generating energy through the nuclear fusion of hydrogen at its core. It has more than double the mass and twice the radius of the Sun, and is radiating more than 39 times the Sun's luminosity. The effective temperature of the star's outer envelope is about 9620 K, which gives it the white hue typical of A-type stars. The estimated age of this star is 50–150 million years.

This star is rotating rapidly, with a projected rotational velocity of 317 km s−1 giving a lower bound on the azimuthal velocity along the equator. As a result, it has a pronounced equatorial bulge, causing the star to assume an oblate spheroidal shape. The equatorial radius is about 30.7% greater than the polar radius. Because of the Doppler effect, this rapid rotation makes the absorption lines in the star's spectrum broaden and smear out, as indicated by the 'n' suffix in the stellar class.

Astronomers use Zeta Aquilae as a telluric standard star. That is, the spectrum of this star is used to correct for telluric contamination from the Earth's atmosphere when examining the spectra of neighboring stars. Observation of this star in the infrared band during the 2MASS survey appeared to reveal excess emission. However, the distribution of this emission couldn't be readily explained by a conjectured disk of circumstellar dust. Instead, the detection was later ascribed to errors caused by saturation of the near-infrared detectors.