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Altair: One of the Summer Triangle Stars

Doorstep Astronomy: The Summer Triangle
The Summer Triangle at about 10 p.m. local time in July from mid-northern latitudes.

Altair is a bright star in the northern summer sky that is one of the three stars that form an astronomical asterism called the Summer Triangle. The star, which is in the constellation Aquila (the Eagle), is just 16.7 light-years from Earth, making it one of the closest naked-eye stars visible in the sky. It is the 12th brightest star in the night sky with an apparent magnitude of 0.77. [The Brightest Stars in the Sky: A Starry Countdown]

The star is also rotating so rapidly — at nearly 185 miles (300 kilometers) a second at the equator — that it is at about 90 percent of the speed it would take to rotate fast enough to blow apart. One rotation takes about 9 to 10 hours.

Locating Altair

The Summer Triangle is an easy-to-spot asterism in the northern summer sky. The trio of stars — Altair, Vega and Deneb — are among the brightest in the night sky. Altair's position is:

  • Right ascension: 19h 50m 47.0s
  • Declination: 08 degrees 52 minutes 06 seconds

Vega is the brightest of the three. Next is Altair, while Deneb appears much dimmer than its companions. However, this view is deceptive and illustrates the difference between magnitude and luminosity.

Altair is closer to us than Vega, but appears a bit dimmer. That is because Vega puts out more light; it is more luminous. Also, while Altair is 1½ times larger than Earth's sun, Vega is more than three times larger.

Altair
Altair is a rapidly spinning star. The diameter at its equator is at least 14 percent greater than at its poles.
Credit: NASA/JPL/Caltech/Steve Golden

Deneb appears only one-third as bright as Vega, but it is, in fact, one of the greatest supergiant stars known. It shines about 80,000 times brighter than the sun, but it is 1,467 light-years away, so it does not appear as bright.

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Altair in culture

In science fiction, Altair 4 (a fictional planet of the star) was where the movie "Forbidden Planet" took place.

The star was also an important personal symbol for an astronaut in James Michener's fictional book "Space," which followed the early American space program from Mercury's development through to the space shuttle.

Altair's bright appearance drew attention in many ancient cultures. While the name is Arabic (meaning Eagle), it also was noticed in areas such as India, where Altair and two nearby stars were sometimes referred to as footprints of the god Vishnu.

Chinese legend has a story involving the stars Altair and Vega. Every year on July 7, the story goes, Vega (a weaving girl) and Altair (a cowherd) meet each other after crossing a bridge — the Milky Way. This is celebrated in an event called the Qixi Festival.

The name Altair has also been used in the space industry, perhaps most notably with the proposed NASA Altair lunar lander. The name was released in 2007 for a vehicle being developed under the now-defunct Constellation human moon-to-Mars program.

"In Latin, 'Aquila' means Eagle, tying our new lander to the historic Apollo 11 Eagle," stated Lauri Hansen, NASA's Altair project manager, in an e-mail to other NASA officials later published on collectSPACE. Hansen was referring to the first spacecraft to land on the moon.

Recent observations of Altair

One of Altair's most interesting astronomical features is its variability, although it wouldn't be possible to see with the naked eye. Observations with the Wide-Field Infrared Explorer (WIRE) satellite published in 2004 showed fluctuations of less than a thousandth in magnitude.

Additionally, Altair's movements relative to other stars have astonished some astronomers: "Though seemingly ordinary, the star is not without its own striking characteristics. It is moving across the sky against the background of distant stars more quickly than most, and will displace itself by as much as a degree in only 5,000 years," wrote Jim Kaler, a professor emeritus of astronomy at the University of Illinois.

In 2006, astronomers were able to resolve flattening at the poles using long-baseline interferometry, or networking several telescopes together to collectively obtain an image of the star.

— Elizabeth Howell, SPACE.com Contributor

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