Altair is a yellow-white star of magnitude 0.76, the brightest star in Aquila, and the 13th brightest star in the sky. The Arabic name "Altair" comes from a phrase meaning "the flying eagle." Altair is also the southern end of "Summer Triangle" with Vega and Deneb.
The classic 1956 science fiction film, "Forbidden Planet", was set on the planet Altair IV.
Properties
At a distance of 16.8 light years, Altair is one of the closest bright stars. Due to its closeness, Altair is moving across the sky against the background of distant stars more quickly than most - a degree in only 5000 years.
Though the three stars the Summer Triangle are all white in color, they have individual differences. Altair is the coolest of the three, a class A1 V star with a temperature of 7550 K. Altair is also the least luminous. From its distance, we find Altair's luminosity to be 10.6 times the Sun's, as opposed to 50 times for Vega and an astounding 160,000 or so for much more distant Deneb. Like the Sun and Vega, Altair is a main-sequence dwarf, fusing hydrogen into helium in its core. Its mass is between 1.7 and 1.8 solar masses.
Altair is also a very rapid rotator. Its equatorial spin speed is an astonishing 210 km/sec (and may be greater, since its axial tilt is not known), as compared with the Sun's 2 km/sec. With a radius 1.8 times the Sun's, the star has a rotation period of at most only 10 hours, as opposed to nearly a month for our Sun. Altair's fast rotation has caused it to become distorted. Observation with an interferometer, from which the angular size of the star is measured, reveals a 14% oblateness. Altair is far from its rotational breakup speed of 450 km/sec, however.
In 2006, Altair became the first main-sequence star, apart from the Sun, to have its surface imaged directly. Theory predicts that Altair's rapid rotation should cause its surface gravity and effective temperature to be lower at the equator than the poles. This phenomenon, known as gravity darkening, was confirmed for Altair by these images, and by earlier interferometric measurements in 2001.
Altair has also recently been identified as a subtle "Delta Scuti" variable, the brightest in the sky, flickering by a few thousandths of a magnitude with nine different periods that range from 50 minutes to 9 hours.