Definitions

Altair

Altair

[al-tair, -tahyuhr, al-tair, -tahyuhr]
Altair, brightest star in the constellation Aquila (Eagle); Bayer designation α Aquilae; 1992 position R.A. 19h50.5m, Dec. +8°51'. Its apparent magnitude is 0.74, making it one of the 20 brightest stars in the sky, and it is of spectral class A7 IV,V. Altair is one of the nearest bright stars, its distance being 16.8 light-years.
Altair (α Aql / α Aquilae / Alpha Aquilae / Atair) is the brightest star in the constellation Aquila and the twelfth brightest star in the nighttime sky, at visual magnitude 0.77. A groundbreaking study with the Palomar Testbed Interferometer revealed that Altair is not spherical, but is rather flattened at the poles due to its high rate of rotation. Synthetic aperture techniques with multiple optical telescopes have imaged this phenomenon.

Description

Altair is a vertex of the Summer Triangle. It is an "A" type or white star located away from Earth, one of the closest stars visible to the naked eye.

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, along with Beta Aquilae and Gamma Aquilae, form the well-known line of stars sometimes referred to as the shaft of Aquila.

Appearance

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.

Location

The star's location in the constellation of Aquila is shown in the starmap.

Etymology

The name "Altaïr" is Arabic for "the bird", from the phrase الطير. The spelling "Atair" is also used frequently. The name was given by Arabic astronomers and adopted by Western astronomers.

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.

Altair in mythology and culture

In Chinese mythology, there is a love story of Qi Xi in which Niu Lang (Altair) and his two children (Aquila -β and -γ) are separated forever from their mother Zhi Nü (Vega) who is on the far side of the Silvery River. The Japanese Tanabata festival is also based on this legend.

In astrology, the star Altair was ill-omened, portending danger from reptiles.

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.

See also

References

External links

Components

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

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