Altair is most notable for its extremely rapid rotation; by measuring the width of its spectral lines, it was determined that its equator does a complete rotation in about 6½ hours (various other sources give 9 hours, or 10.4 hours). For comparison, our Sun requires just over 25 days for a complete rotation. As a result of its rapid rotation, Altair is oblate: its equatorial diameter is at least 22 percent greater than its polar diameter. Recently, images of the surface of Altair have been captured. This is the first visual verification of gravity darkening.
Altair is one of the few stars for which a direct image has been obtained. With an angular size of roughly 3.2 millarcseconds, the MIRC instrument on the CHARA Array was able to construct this image of the star. In this image, north is up and east is left; overlaid are lines of latitude and longitude, and the rotation axis. The whiter spot at the pole and the deeper blue at the equatorial regions are manifestations of the polar temperature increase & equatorial temperature decrease due to Altair's rapid rotation. This is known in astronomy as the von Zeipel effect. The equatorial radius of the star is 2.02 solar radii, and the polar radius is 1.66 solar radii - a 20% increase of the stellar radius from pole to equator, solely due to its rapid rotation.
The star's location in the constellation of Aquila is shown in the starmap.
In Chinese, it is officially known as 河鼓二 (Hégǔ'èr, the Second Star of the Drum at the River, or more literally, Riverdrum II). However, it is more famously known as its other names: 牵牛星 or 牛郎星 (the Star of the Cowherd), after the Cowherd in the Chinese mythology: the Cowherd and the Weaver Girl; the star's Japanese name of Hikoboshi derives from this same myth.
The NASA Constellation Program announced "Altair" as the name of the Lunar Surface Access Module (LSAM) on December 13, 2007.
The Russian made Beriev Be-200 Altair seaplane is also named after the star.
|NAME||Right ascension||Declination||Apparent magnitude (V)||Spectral type||Database references|
|ADS 13009 B (BD+08 4236B)||19h 50m 40.5s||+08° 52' 13||9.6||Simbad|
|ADS 13009 C (BD+08 4238)||19h 51m 00.8s||+08° 50' 58||10.1||Simbad|