On the next clear night, go outside around 8 p.m. and look east.

There you’ll see a bright, red-orange star: Aldebaran, the eye of Taurus, the Bull. Aldebaran derives its name from Arabic for “the follower,” since it follows the Pleiades (the tiny “Seven Sisters” star cluster) on their nightly journey. An easy way to find it is to trace a line upward through Orion’s famous belt (which currently rises around 9 p.m.).

Since it lies along the zodiac, Aldebaran can occasionally be hidden by the moon. How quickly Aldebaran disappears behind the moon’s edge during such “lunar occultations” provides one of the best ways to measure the star’s angular diameter which, combined with its distance, allows a calculation of its true diameter. Not counting the sun, Aldebaran is the 14th brightest star in the sky, and the 10th brightest in Idaho’s skies. Its brightness is owed to its proximity (65 light years away), and the fact that it’s a giant star.

Unlike most stars, powered by hydrogen fusing into helium in their cores, giants fuse hydrogen in a shell surrounding their ultra-dense, helium “ash” cores. Because this shell occupies a greater volume than a typical stellar core, it generates more energy, causing the star to swell to enormous size: Aldebaran, with only 116% of the sun’s mass, spans more than 40 times its diameter.

With such a vast surface area to heat, Aldebaran is about 3,600 degrees cooler than the sun, which gives it its more reddish hue.

On moonless nights, Aldebaran appears at one end of a V-shaped grouping of fainter stars known as the Hyades star cluster, which represents Taurus’s head. The Hyades’ brightest stars are giants, too; their relative faintness belies their greater distance, more than twice as far away as Aldebaran.

Next column: Last stop before black hole.

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Chris Anderson manages the College of Southern Idaho’s Centennial Observatory in Twin Falls. He can be reached at 732-6663 or canderson@csi.edu.


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