# Okab (Zeta Aquilae)

Okab (Zeta Aquilae) is a star in the constellation Aquila, magnitude 3.0.

**The tail of the Eagle: a white star spinning so fast it is hotter at its poles than at its equator.**

Okab spins at more than 300 kilometres a second at its equator, and the spin has reshaped it. Its equator bulges to about 2.8 times the Sun’s radius while its poles reach only 2.2. The spin also sorts the heat: the poles, closer to the star’s centre, glow at about 11,000 kelvins, while the stretched equator cools to about 8,700. We see this white [star](https://astropilot.space/glossary/star) almost exactly edge-on.

Close in, something glows in the infrared. Interferometers in 2008–2011 picked up emission within an Earth–Sun distance of the star, and a 2017 study proposed hot dust — perhaps debris from a collision between orbiting bodies, or a giant comet vaporising in the heat.

Its name comes from Deneb el Okab, Arabic for ‘the tail of the eagle’, a title it once shared with Epsilon Aquilae. In Chinese star lore the same two stars stood for the ancient states of Wu and Yue.

Okab (Zeta Aquilae) is 83 light-years away: the light we see from it set off in 1943, 13 years after Pluto was discovered.

## Did you know?

- **A standard for Earth’s air.** Astronomers use Okab as a telluric standard: its spectrum lets them measure, and remove, the fingerprints Earth’s own atmosphere leaves on starlight.
- **A false alarm.** The 2MASS survey seemed to show extra infrared light around Okab, hinting at a dusty disc. In 2009 the signal was traced to overloaded detectors.
- **Wall of the Heavenly Market.** In Chinese astronomy Okab is the sixth star of the Left Wall of the Heavenly Market Enclosure, a chain of eleven stars, each standing for an old state of China.

A white 3rd-magnitude star about twelve degrees west-north-west of Altair; with nearby Epsilon Aquilae it marks the Eagle’s tail. Its 12th-magnitude companion, 7 arcseconds away, is a test for a mid-sized telescope.

## When and where to see it

It is best placed in the evening sky around August, when it crosses the meridian at about 10 pm local time (around midnight in July). It can be seen from everywhere north of 76°S and never sets north of 76°N.

At its highest it stands 54° up from 50°N, 69° up from 35°N, 76° up from the equator, 41° up from 35°S.

## Facts

- Type: Star
- Constellation: Aquila
- Designations: Zeta Aquilae, ζ Aql, 17 Aql, HR 7235, HD 177724, HIP 93747
- Magnitude: 3.0
- Position: RA 19h 5.4m, Dec +13° 52′ (J2000)
- Spectral type: A0IV-Vnn
- Colour: white (B−V 0.01)
- Distance: 83 light-years
- Discovered: Known since antiquity

Open AstroPilot to see when Okab (Zeta Aquilae) is highest tonight from your location, how easy it is with your telescope or binoculars, and an eyepiece view. https://astropilot.space/object/okab