Altair
What is Altair?
Stars· Distance 16.7 ly · size 1.8× Sun (flattened by spin). The Cowherd star of the East Asian Qixi (Tanabata, Chilseok) legend, facing Vega across the Milky Way at the southern corner of the Summer Triangle. It spins once in under nine hours, so fast that its equator bulges.
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Distance and spectral type
Altair is about 17 light-years away, a white subgiant (spectral type A7IV-V), in the constellation Lyra. Its apparent magnitude is 0.8, visible to the naked eye.
Stellar figures
| Distance | 16.7 ly |
|---|---|
| Magnitude | 0.76 — twelfth brightest |
| Type | A7 white main-sequence |
| Rotation | ~7.8 h (the Sun: 25 days) |
Figures are representative approximations. Source: compiled from NASA and IAU public data.
Behind the name
Arabic "the flying eagle". It spins so fast it bulges at the equator; in East Asia it is the Cowherd star.
2007: Altair shows its face as the first ordinary star after the Sun
Stars are so far away that most look like points even through a telescope. But in 2007 the CHARA interferometer, run by Georgia State University in the United States, produced an image of Altair's surface. It combined the light of four widely spaced telescopes, using them like one giant telescope. Apart from the Sun, it was the first time the surface of an ordinary star like Altair, one still burning hydrogen in its core, had been imaged.
The Altair in the picture was not round but oval. The poles were bright and the equator dark. The temperature worked out to about 8,600 K at the poles and about 6,800 K at the equator. The prediction that fast spin flattens a star and cools its equator was confirmed in a picture. Because Altair's spin axis is tilted about 60 degrees as seen from Earth, the bright poles and the dark equator could be seen together in one view.
The star in depth
Altair is the brightest star in Aquila, the Eagle. At magnitude 0.76 it is the twelfth-brightest star in the night sky. At 16.7 light-years it is one of the nearer stars visible to the naked eye. It is a white star with about 1.8 times the Sun's mass and eleven times its brightness. In East Asia it is the Cowherd, the hero of the legend celebrated as Qixi in China, Tanabata in Japan and Chilseok in Korea, who meets the Weaver, Vega, across the Milky Way once a year.
Altair spins very fast. Its equator moves at about 286 km/s, so it turns once in under nine hours. The Sun takes about 25 days to turn once, which makes Altair more than sixty times faster. Spinning that fast pushes the equator outward. As a result, Altair is squashed, more than 20 percent wider across the equator than from pole to pole.
A squashed star does not have an even temperature. The bulging equator lies farther from the centre and is cooler, while the poles are closer to the centre and hotter. So Altair's poles are bright and its equator a little dim. This was not just worked out on paper. In 2007 astronomers combined the light of several telescopes and photographed Altair's surface, confirming it directly.
From July to September, Altair stands high in the eastern to southern sky in the evening. With Vega and Deneb it forms the Summer Triangle. A slightly fainter star sits on each side of it, making it easy to recognise. In the Sky view, choose an August night to see Altair shining white at the southern edge of the Milky Way.
Frequently asked
- How far is Altair from Earth?
- The distance of Altair is 16.7 ly.
- How bright is Altair (apparent magnitude)?
- The magnitude of Altair is 0.76 — twelfth brightest.
- What kind of object is Altair?
- Altair is a A7 white main-sequence.
- How long is a day on Altair?
- The rotation of Altair is ~7.8 h (the Sun: 25 days).
- Which constellation is Altair in?
- Altair lies in the constellation Lyra.
Related
Figures are representative approximations. Simulated positions use J2000 mean elements.