Altair

alpha aquilae

Altair, Alpha Aquilae (α Aql), is a white main sequence star located at a distance of 16.73 light years from Earth in the constellation Aquila, the Eagle. With an apparent magnitude of 0.76, it is the brightest star in Aquila and the 12th brightest star in the sky.  It shares the 12th place with Acrux in the constellation Crux. The two stars are slightly fainter than Betelgeuse in Orion and Hadar in Centaurus, but they (usually) outshine the variable Aldebaran in Taurus, Antares in Scorpius and Spica in Virgo constellation. At a distance of 16.73 light years, Altair is one of the nearest stars to Earth visible to the naked eye.

alpha aquilae
Altair (Alpha Aquilae), image: Wikisky

Altair is one of the vertices of the Summer Triangle, a prominent asterism also formed by the bright stars Vega in the constellation Lyra and Deneb in Cygnus. Altair is the nearest of the three stars. It is also the coolest and least luminous. It appears brighter than Deneb, but not quite as bright as Vega.

Altair is also part of the Shaft of Aquila, a line of stars that makes it easy to identify the star in the night sky in good conditions. The Shaft asterism consists of Altair, the orange bright giant Tarazed, Gamma Aquilae, and the binary star Alshain, Beta Aquilae. With apparent magnitudes of 3.87 and 2.7, Alshain and Tarazed are easily visible next to Altair on a clear night.

Stars of Aquila, with Altair, Tarazed and Alshain appearing in the left portion of the image, credit: J. Aleu (Flickr Commons)

Star type

Altair has the stellar classification A7 V, indicating a white star that is still fusing hydrogen to helium in its core. Altair has 1.79 times the Sun’s mass and an estimated radius between 1.63 and 2.03 times that of the Sun. With a temperature in the range between 6,900 K and 8,500 K, it is 10.6 times more luminous than the Sun.

Altair is a very fast spinner, with a rotational velocity of about 286 km/s, or 71.5 percent of its estimated breakup speed (400 km/s). Its rotational period is about 8.9 hours. For comparison, the Sun’s rotational period is a little more than 25 days.

Altair, image: NASA/JPL/Caltech/Steve Golden

Observations with the Palomar Testbed Interferometer (PTI) in 1999 and 2000 revealed that the star had an oblate photosphere as a result of its high rotation rate. Altair’s diameter at the equator is more than 20 percent greater than its diameter at the poles.  Altair was the first main sequence star for which rotational velocity was established from observations of the geometry of its photosphere. Published in 2001, the study gave an estimated rotational velocity (v sin i) of 210 ± 13 km s-1. Other observations in the infrared have confirmed Altair’s flattened shape.

Gravity darkening was confirmed for Altair in 2001 after measurements with the Navy Prototype Optical Interferometer. Gravity darkening is a phenomenon that occurs with rapidly rotating stars whose shape is oblate as a result of their high rotation rate. Because the surface is closer to the centre of mass at the poles, the star’s surface gravity and temperature are higher at the poles than at the equator. As a result, the poles are more luminous than the equator. Vega in Lyra and Achernar in Eridanus constellation are two other well-known examples of this.

In 1999, measurements with the Wide Field Infrared Explorer (WIRE) satellite revealed very slight fluctuations in Altair’s brightness, measured in thousandths of magnitude. In 2005, the star was classified as a Delta Scuti variable, a star that shows changes in luminosity as a result of both radial and non-radial pulsations of its surface. These stars are used as standard candles to measure distances to star clusters and nearby galaxies.

Facts 

Altair is one of the 58 stars selected for navigation. Navigational stars are some of the brightest and most identifiable stars in the sky. They have a special status in the field of celestial navigation.

Altair is currently located in the G-Cloud complex, an interstellar cloud that also contains Alpha Centauri. The G-Cloud is located next to the Local Interstellar Cloud (the Local Fluff), through which our solar system is currently moving. The Sun is believed to be either embedded in the LIC or in the area which is interacting with the G-Cloud, and it is moving in the direction of the G-Cloud.

In 2006 and 2007, a direct image of Altair was obtained from infrared observations with the Michigan Infrared Combiner (MIRC) on the Center for High Angular Resolution Astronomy (CHARA) array interferometer on Mount Wilson in California. An international team led by University of Michigan assistant professor of astronomy John Monnier and graduate student Ming Zhao used optical interferometry and four telescopes separated by almost 300 yards to obtain an image revealing details of the star’s  surface. This made Altair the first main sequence star other than the Sun to have its surface imaged. Altair’s equatorial radius was calculated to be about 2.03 solar radii and its polar radius 1.63 times solar, 25 percent smaller. The star’s polar axis is inclined by about 60 degrees to our line of sight.

altair surface image
Image of the rapidly rotating star Altair, made with the MIRC imager on the CHARA Array on Mt. Wilson, credit: Ming Zhao, John Monnier

The CHARA image was the first ever image of a star that provided visual confirmation of gravity darkening. The image showed even more darkening at the equator than scientists had predicted using standard models, highlighting potential flaws in those models.

Altair is listed in the Washington Double Star Catalog (WDS) as WDS 19508+0852A and its three dim visual companions are listed as WDS 19508+0852B, C and D. WDS 19508+0852B (mag. 9.82) lies at a separation of 192.1″, WDS 19508+0852C is (mag. 10.3) at a separation of 189.6″, and WDS 19508+0852D (mag. 11.9) at 31.7″ from Altair.

Altair is moving relatively fast against the background of stars and will move by as much as a full degree within the next 5,000 years.

Like other bright stars, Altair has been used and mentioned in countless works of fiction. Some of the best known ones include Douglas Adams’ The Hitchhiker’s Guide to the Galaxy (1978), Stephen King’s The Tommyknockers (1987), Anne McCaffrey’s The Rowan (1990), and Arthur C. Clarke and Stephen Baxter’s Sunstorm (2005).

altair,vega,deneb
The Summer Triangle with Vega (top left), Altair (lower middle) and Deneb (far left), image: NASA, ESA. Credit: A. Fujii

Name  

The name Altair comes from the Arabic phrase an-nasr aṭ-ṭāʼir, which means “the flying eagle.” The name was officially approved by the International Astronomical Union’s (IAU) Working Group on Star Names (WGSN) on June 30, 2016.

The star’s Arabic name appeared in Egyptian astronomer Al Achsasi al Mouakket’s star catalogue, written in 1650, and was translated into Latin as Vultur Volans. The Arabic name was also used for the asterism Altair forms with Alshain (Beta Aquilae) and Tarazed (Gamma Aquilae).

In India, Altair, Tarazed and Alshain were known as the footprints of Vishnu.

shaft of aquila
Altair, Tarazed and Alshain, image: Wikisky

Altair’s Chinese name is 河鼓二 (Hé Gǔ èr), meaning “the second star of the drum at the river” or “river drum two,” referring to the Chinese asterism known as “the river drum” (河鼓). The asterism consists of Altair, Tarazed and Alshain.

In Chinese culture, Altair is better known for its association with the myth of the cowherd and the weaver girl, in which Niulang, the cowherd (represented by Altair) and his two children (represented by Tarazed and Alshain) are separated by a large river (the Milky Way) from Zhinu, the weaver girl (represented by Vega), their wife and mother. They can only meet once a year when magpies create a bridge to allow them to cross the celestial river. Altair is known as “the cowherd star” (Niú Láng Xīng, 牛郎星).

The meeting of the star-crossed lovers is celebrated during the annual Qixi Festival, held on the seventh day of the seventh month on the Chinese lunar calendar. The tale also inspired the Tanabata festival in Japan, which celebrates the meeting of Orihime (Vega) and Hikoboshi (Altair), and the Chilseok festival in Korea, associated with the legend of Jiknyeo (the weaver girl) and Gyeonwu (the cowherd).

In Micronesia, Altair is known as Mai-lapa, or “big/old breadfruit.” The Māori people of New Zealand knew the star as Poutu-te-rangi, or “the pillar of heaven,” while the Koori people in Victoria in Australia called it Bunjil, the wedge-tailed eagle (Australia’s largest predatory bird), and Tarazed and Alshain were known as black swans, his two wives. The people of the Murray River in Australia associated the star with the story of the river’s formation and called it Totyerguil, after a mythical hunter who speared a giant cod. In local lore, the wounded cod created a channel across southern Australia and moved to the sky as the constellation Delphinus.

Location 

Altair is easy to identify because it is part of the Summer Triangle and has two relatively bright stars – Alshain and Tarazed – on either side. The three stars in Aquila are part of the larger eagle-shaped asterism that flies across the sky opposite Cygnus, the Swan. Altair marks the Eagle’s head or neck and it is facing the small but distinctive constellation Delphinus.

summer triangle,northern cross,deneb,altair,vega
The Summer Triangle and the Northern Cross, image: Wikisky

The Summer Triangle is a large and prominent asterism visible high overhead during the northern hemisphere summer. It is formed by Altair with the bright stars Vega in Lyra constellation and Deneb in Cygnus.

Constellation

Altair is the luminary of the constellation Aquila, the Eagle. Aquila is one of the Greek constellations, first listed by the Greco-Roman astronomer Ptolemy in the 2nd century CE. Occupying an area of 652 square degrees, it is the 22nd constellation in size. It is visible in its entirety from all locations except those south of the latitude 75°S.

Aquila constellation, image: Roberto Mura

Aquila contains several notable deep sky objects. These include the planetary nebulae NGC 6781, NGC 6778, NGC 6751, also known as the Glowing Eye Nebula, and NGC 6741, nicknamed the Phantom Streak Nebula, the open clusters NGC 6709 and NGC 6755, and the globular cluster NGC 6760.

The best time of year to observe the stars and deep sky objects of Aquila is during the month of August, when the Eagle flies high overhead in the evening for northern observers.

The 10 brightest stars in Aquila are Altair (Alpha Aql, mag. 0.76), Tarazed (Gamma Aql, mag. 2.712), Okab (Zeta Aql, mag. 2.983), Theta Aquilae (mag. 3.26), Delta Aquilae (mag. 3.365), Lambda Aquilae (mag. 3.43), Alshain (Beta Aql, mag. 3.87), Eta Aquilae (mag. 3.87), Epsilon Aquilae (mag. 4.02), and 12 Aquilae (mag. 4.02).

Altair – Alpha Aquilae 

Spectral classA7 V
Variable typeDelta Scuti
U-B colour index+0.09
B-V colour index+0.22
V-R colour index+0.14
R-I colour index+0.13
Apparent magnitude0.76
Absolute magnitude2.22
Distance16.73 ± 0.05 light years (5.13 ± 0.01 parsecs)
Parallax194.95 ± 0.57 mas
Radial velocity−26.1 ± 0.9 km/s
Proper motionRA: +536.23 mas/yr
Dec.: +285.29 mas/yr
Mass1.79 ± 0.018 M
Luminosity10.6 L
Radius1.63 (polar) – 2.03 (equatorial) R
Temperature6,900 – 8,500 K
Age1.2 billion years
Surface gravity4.29 cgs
Rotational velocity286 km/s
Metallicity-0.2 dex
ConstellationAquila
Right ascension19h 50m 46.99855s
Declination+08° 52′ 05.9563”
DesignationsAltair, Alpha Aquilae, α Aql, 53 Aquilae, HIP 97649, HD 187642, HR 7557, SAO 125122, LTT 15795, LHS 3490, BD+08°4236, GCTP 4665.00, WDS 19508+0852A, GJ 768, LFT 1499, NLTT 48314, TYC 1058-3399-1, GCRV 12193, 2MASS J19504698+0852060, IRAS 19483+0844, IDS 19459+0836 A, WDS 19508+0852A