Star Facts: Altair

Altair Star
Image Credit: astropixels.com

Altair (Alpha Aquilae) is the brightest star in the constellation Aquila, and the 12th most luminous in the entire night sky. Although the star has been in the human consciousness since ancient Babylonian and Sumerian times, when it was known as “the eagle star”, the name Altair, meaning “the flying eagle” in Arabic, has only been in use by Western cultures since medieval times.

Quick Facts

• Constellation: Aquila
• Coordinates: RA 19h 50m 47s | Dec +8° 52′ 6?
• Distance: 16.73 light years
• Star Type: White Dwarf (A7 V)
• Mass: 1.79 sol
• Radius: 1.63 sol
• Apparent Magnitude: +0.76
• Luminosity: 10.6 sol
• Surface Temperature: 6,900K – 8,500K
• Rotational Velocity: 286 km/s
• Age: 1 billion years
• Other Designations: Atair, a Aquilae, a Aql, Alpha Aquilae, Alpha Aql, 53 Aquilae, 53 Aql, BD+08°4236, FK5 745

Visibility

Altair in Aquila can be seen from between latitudes of +90° and -75°, with August being the best month to see the bright star. It is also not difficult to view Altair with the naked eye, since it forms one of the vertices of the famous Summer Triangle asterism, with the other two points marked by the stars Deneb in Cygnus and Vega in Lyra. Furthermore, Altair is also a point in the equally famous line of stars known as the Family of Aquila, or sometimes, as the Shaft of Aquila, together with the stars Beta Aquilae and Gamma Aquilae.

Altair is also among the handful of stars, apart from the Sun, of which a direct image has been obtained. In 2006 and 2007, J.D. Monnier et al used more than 2,000 infrared images taken with the MIRC instrument on the CHARA array interferometer to produce a false color image that shows some detail on Altair’s surface. This image was published in 2007.

Physical Properties

Altair is located 16.73 light years from Earth in the G-cloud, a huge interstellar cloud of gas and dust. It is a normal, A-type, white main-sequence star which is around 1.63 bigger than the Sun, with 1.8 times its mass, and 10.6 times its brightness. It is also a rapid rotator, with its spin rate of 286 km/sec representing a significant fraction of the speed (400 km/sec) needed for the star to break apart. At its current rotational speed, Altair rotates once every nine hours, in contrast to the Sun that takes around 25 days to complete one revolution.

As one result of its high spin rate, Altair has been shown to be highly oblate, with its equatorial diameter being 20% bigger than its polar diameter. A further consequence of the stars’ high spin rate is the fact that due to a phenomenon called “gravity darkening”, Altair’s effective temperature and surface gravity is measurably lower along its equator, thus reducing the stars luminosity in its equatorial region. This effect was observed through measurements made by the Navy Prototype Optical Interferometer in 2001, as well as subsequent measurements taken by the VINCI instrument at the VLT (Very Large Telescope) in northern Chile.

Variable Star

Altair has also been shown to be a Delta Scuti-type variable star. Typically, Delta Scuti variable stars do not vary by large amounts, and in the case of Altair, its variations in brightness are measured in mere thousandths of a magnitude. However, Altair’s brightness variations span several periods that range from 0.8 to 1.5 hours, and as a result, the only way to approximate Altair’s light curve is to add up all the periodic variations as a series of sine waves.

Altair is also a weak source of coronal X-ray emissions, with the most active sources being arranged along its equator. Although investigations into the mechanisms that produce the low-level X-ray emission are still being investigated, most researchers believe that it is caused by convection cells that migrate from the hot, polar regions to the cooler equatorial latitudes.

History

Aquila ConstellationAltair has a long and storied history that spans several millennia, across diverse cultures. As mentioned the ancient civilizations of Sumer and Babylon knew it as “the eagle star”, with the tradition carried on by the ancient Greeks who called it Aetos (“the eagle”). Altair also figured in Western astrology, in which it has a rather bad reputation, since it portends ill-fortune and danger from snakes and other reptiles.

In old China, however, Altair was a cowherd, with the two stars on either side of it that complete the Family of Aquila representing his sons carried on a shoulder pole. Unfortunately, he was separated from his Weaving Girl wife, the star Vega, by a celestial river (Milky Way), but once a year they were able to meet, on the 7th day of the 7th lunar month, after magpies would form a bridge with their outspread wings.

In Australia, the Koori aboriginal people knew Altair as Bunjil, the wedge-tailed eagle, who had Beta and Gamma Aquilae as his wives, in the form of black swans. Other Australian aboriginal peoples, such as the people of the Murray River, knew Altair as Totyerguil, a hunter who once speared a giant Murray cod known as Otjout. However, instead of killing the fish, he only wounded it and in his attempts to get away from his murderer, Otjout left the river and churned a giant channel clear across southern Australia before ascending into the sky, where he can still be seen to this day as the constellation Delphinus.

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