Altair as a

Altair's spectral type is A7 and its luminosity class is V—a main sequence star. It is the coolest of the three Summer Triangle stars, with a surface temperature

Altair's globe, imaged by CHARA inter ferometer.

of about 7,550 K. Altair's mass and its average diameter are both about 1.7 or 1.8 times that of our Sun. But notice the phrase "average diameter." Altair is decidedly oblate, its equatorial diameter about 14 percent greater than its polar diameter. Altair was the first of the bright stars found to have very fast rotation (by observation of the widening of its spetral lines). Its rotational speed at its equator is at least 219 km/sec (maybe greater depending on the tilt of Altair's axis). That's over one hundred times faster than the rotation of our Sun at its equator. The rotational period must at most be about ten hours but may be considerably less.

Altair lies at a distance of 16.7 light-years, making it summer's closest star plainly visible to the naked eye. Its distance is about 1 million times greater than the distance to our Sun. Altair is about: 4 times farther from us than Alpha Centauri, twice as far as Sirius, 1H times as far as Procyon, % as far as Vega, V2 as far as Pollux, and V3 as far as Castor.

Changed view of Orion and Taurus as seen from Altair.

Altair is moving relatively fast though space from our viewpoint. Its proper motion is changing its position by about 1° each 5,000 years. The star is very slightly variable in brightness. It is classified as a Delta Scuti variable with nine different periods of variation that range from 50 minutes to 9 hours.

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