On the next clear night, go outside around 7:30 PM and look toward the west about one outstretched hand-span above the horizon. There you’ll see a bright, bluish-white star, called Altair.
Altair is the brightest star in the constellation of Aquila, the Eagle, from which it derives its name: “al-nesr al-ta’ir” is Arabic for “the Flying Eagle.” While it ranks as the twelfth brightest star visible from Earth (not counting the sun), it is more remarkable for being one of the few stars whose size and shape has been directly measured.
Because of their enormous distances, only a few stars have been imaged as larger than tiny specks. In 2007, using the CHARA (Center for High Angular Resolution Astronomy) telescope array on Mt. Wilson, California—whose light is combined through the technique of infrared interferometry to achieve the equivalent power of an 820-foot-wide instrument—astronomers were able to image Altair. At Altair’s distance of 100 trillion miles, this is the equivalent of photographing a penny as far away as Tucson, Arizona from Twin Falls.
Unlike our sun, a nearly-perfect sphere which rotates on its axis in about a month, Altair rotates in less than nine hours, causing its equator to bulge out 25 percent farther than its poles. With such extreme rotation (640,000 mph at the equator, 70 percent of the speed at which it would fly apart) it resembles a beach ball under an elephant’s foot.
Altair’s bluish color reveals its temperature: Compared to the sun’s all-over 10,000° F, Altair is 12,000° F at its poles and 15,000° F at its equator. The CHARA image reveals this as well, showing a darker
(cooler) equator (further from the nuclear furnace at the star’s center) and the brighter (hotter) poles.
Next column: New Year’s resolutions for astronomy buffs.
Sky calendar through December 27th:
One hour after sunset:
No planets visible.
One hour before sunrise:
Mercury: ESE, extremely low
Jupiter: SE, low
Mars: SSE, low
Moon: Close to Jupiter 12/14. New moon 11:30 p.m. 12/17. First quarter 2:20 a.m. 12/26.
Other data: Winter Solstice (shortest day, longest night of 2017) 9:28 a.m. 12/21
Chris Anderson manages the College of Southern Idaho’s Centennial Observatory in Twin Falls. He can be reached at 732-6663 or firstname.lastname@example.org.