Altair, whose name comes from an Arabic expression meaning “the flying eagle,” is the brightest star in the constellation Aquila. Best known as one corner of the Summer Triangle, it is also one of the fastest-spinning bright stars visible in the night sky.
Easy to Find in the Summer Triangle
Altair is the southernmost of the three bright stars forming the Summer Triangle, with Vega to its northwest and Deneb to the northeast. From mid-northern latitudes, the trio is especially prominent on summer and early-fall evenings, with the Milky Way’s glowing band appearing between Altair and Vega. Altair can be distinguished by the two fainter stars positioned on either side of it: Tarazed and Alshain. Together, they form a short, nearly straight line representing part of Aquila the Eagle, with Altair shining conspicuously in the middle.
The 12th-Brightest Star in the Night Sky
Altair has an apparent magnitude of about 0.76, making it the 12th-brightest star in the night sky. Its prominence is partly explained by its proximity: at just 16.7 light-years away, Altair is one of our nearest bright stellar neighbors. It is a white A-type main-sequence star with approximately 1.9 times the Sun’s mass and about 11 times its luminosity. Modern models suggest that Altair is also relatively young, probably around 100 million years old.
Spins Once in Under Eight Hours
Altair completes one rotation in approximately 7 hours and 46 minutes, compared with about 25 days for the Sun at its equator. Its equatorial surface races around at more than 300 kilometers (190 miles) per second, making Altair one of the most rapidly rotating bright stars visible in the night sky.
Roughly One-Quarter Wider at Its Equator
Altair’s extreme rotation prevents it from maintaining a spherical shape. Centrifugal effects push material outward around its equator, making its equatorial radius approximately 28% greater than its polar radius. Altair is therefore roughly one-quarter wider across its equator than it is from pole to pole. By comparison, slowly rotating stars such as the Sun are almost perfectly spherical.
Its Poles Are Hotter and Brighter
Altair’s temperature varies considerably across its surface. Its poles reach approximately 8,600 kelvins, while its equator is closer to 6,800 kelvins—a difference of about 1,800 degrees. Even Altair’s cooler equator is hotter than the Sun’s roughly 5,800-kelvin surface. This variation is caused by gravity darkening: stronger effective gravity makes Altair’s poles hotter and brighter, while the star’s bulging equator experiences weaker effective gravity and remains cooler and dimmer.
First Main-Sequence Star Imaged Beyond the Sun
In 2007, astronomers published the first resolved image of the surface of a main-sequence star other than the Sun. The image was created from infrared observations of Altair made with the CHARA Array, which combines light collected by several telescopes to achieve exceptionally fine resolution. Rather than showing a conventional photograph, it reconstructed Altair’s surface and revealed its flattened shape, along with the uneven brightness produced by its hotter poles and cooler equator.
Altair Pulsates in Several Different Way
Altair is a Delta Scuti variable star, meaning that it pulsates in several different patterns. Some of its most prominent pulsations repeat over roughly an hour, but the resulting changes in brightness are only a few thousandths of a magnitude and cannot be seen with the naked eye. Astronomers have detected 15 separate oscillation modes, which provide clues about Altair’s internal structure in much the same way that earthquakes help scientists study Earth’s interior.
Altair and Vega Are Separated by the Milky Way
The apparent separation of Altair and Vega by the Milky Way inspired one of East Asia’s best-known star legends. In the Chinese legend, Altair represents the cowherd Niulang, while Vega represents his wife Zhinü, the weaver girl. Although the stars remain visibly apart, the story says that once each year magpies form a bridge across the Milky Way, allowing the lovers to reunite. The tale is celebrated during China’s Qixi Festival and Japan’s related Tanabata tradition.