ALTAIR (Alpha Aquilae). First magnitude (0.77) Altair, the 13th brightest star in the sky and the Alpha star of Aquila the Eagle, is also the southern anchor of the famed Summer Triangle, which it makes with Vega and Deneb. The Arabic name "Altair," reflective of the constellation itself, comes from a phrase meaning "the flying eagle." Though the constellation does not look much like its name, Altair itself is flanked by a pair of stars (the Beta and Gamma stars Alshain and Tarazed) that really do remind the sky- gazer of a bird with outstretched wings. The trio of stars has in fact been taken for an airplane with wing lights slowly flying across the sky. Though three of the stars of the Summer Triangle are all white in color and hotter than the Sun, all are also individuals. A class A (A7) hydrogen-fusing dwarf with a temperature of 7550 degrees Kelvin, Altair is the coolest of the three (with Vega and Deneb warmer at 9500 and 8400 respectively). Altair is also the least luminous. From its distance of 16.7 light years, we find it to be 10.6 times brighter than the Sun, 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 "on the main sequence" of stars, fusing hydrogen into helium in its core, its mass falling between 1.7 and 1.8 solar. 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 5000 years. Altair is also a very rapid rotator. Its equatorial spin speed, while certainly not a record, is still an astonishing 210 kilometers per second (and may be greater, since the axial tilt is not known), as compared with the Sun's 2 kilometers per second. With a radius 1.8 times that of the Sun (confirmed by direct angular diameter measures, which give 1.7), the star has a rotation period of at most only 10 hours, as opposed to nearly a month for our ponderously spinning Sun. Altair's high speed has even caused it to become distorted. Observation with a sophisticated interferometer, from which the angular size of the star is measured, reveals a 14% oblateness. Even with its high rotational velocity, however, Altair is far from its rotational breakup speed of 450 kilometers per second. Altair has also recently been identified as a subtle "Delta Scuti) variable, the brightest in the sky, the star flickering by a few thousandths of a magnitude with nine different periods that range from 50 minutes to 9 hours.
Over a period of 18 hours on November 5, 1999, Altair varied by a few thousandths of a magnitude (millimagnitudes, mmag, on the vertical axis), showing it to be Delta Scuti star that varies with multiple periods. The observations were made in the visual part of the spectrum by the star camera on the sensitive Wide Field Infrared Explorer satellite. The horizontal axis is the Julian Day number, a running count of dates beginning on January 1, 4713 BC of the Julian calendar. (From an article in the Astrophysical Journal by D. L. Buzasi, H. Bruntt, T. R. Bedding, A. Retter, H. Kjeldsen, H. L. Preston, W. J. Mandeville, J. C. Suarez. J. Catanzarite, T. Conrow, and R. Laher, vol. 619, p. 1072, 2005)
Written by Jim Kaler. Return to STARS.