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Winter: this season boasts the sky's brightest stars, clusters, and nebulae.

A QUICK GLANCE at a clear, wintry night sky will show you that this season offers a spectacular parade of bright stars. Two of these radiant gems belong to the constellation Gemini, the Twins. They mark the heads of the mythological brothers Pollux and Castor, and bear their names.

Pollux and Castor were sons of Leda, who was wife of the mortal king Tyndareus and lover of Zeus, the chief god. According to one legend, Pollux was immortal while his brother Castor was mortal. And indeed, Pollux is the brighter of the two stars and has a golden yellow glow, fitting features for a demigod, while Castor is a bit dimmer. You may also be able to detect a color difference, with Castor shining plain white.

Astronomically, Castor is more intriguing. At a magnification of 100x, a telescope splits Castor into two close components. The primary (brighter) star is called Castor A, while the somewhat fainter secondary star is Castor B.

Stars that look close together on the sky may simply be a line-of-sight coincidence, with one star more distant than the other. But 18th-century observers noticed that the spacing and orientation of Castor A and B change in a way that indicates orbital motion. Castor thus became the first star pair proved to be physically related and the first object beyond our solar system seen to be governed by the force of gravity. A full orbit takes 445 years, but a noticeable change takes just a few decades.

As a bonus, Castor has a faint component (C) much farther from the primary star than Castor B. In a 4-inch scope, its pale orange speck is the closest visible star to the bright pair.

Each of Castor's visible stars is actually a tight pair that can't be resolved through a telescope--three sets of secret twins disclosed only by telltale signs in the light they emit. Castor C's stars alternately pass in front of each other, so the pair dims every 9 3/4 hours as one star hides behind the other. Altogether, Castor is an incredible system of six suns locked in a complex gravitational dance.

Let's shift from Castor's head to his left foot, shown by an arc of three stars. The magnificent star cluster Messier 35 is cradled by this foot, so some observers call it the Shoe-Buckle Cluster.

At a dark site, M35 is a small patch of mist to the unaided eye, while binoculars will reveal it in all but the worst skies. A 4-inch telescope at 50x exposes dozens of stars in a lovely group covering as much sky as the Full Moon. Don't use high powers to view this great celestial jewel box. Its starry splendor stands out better when some of the surrounding sky is visible.

At low magnification, you may also see the star cluster NGC 2158 in the same field of view with M35. While M35 looks big and splashy, NGC 2158 is seen as a small tenuous vapor--like the fog of breath upon a window. Boosting the power to 150x pries many extremely faint points of light out of the haze. NGC 2158 is much dimmer than its showy neighbor, and it may remain invisible if your sky is brightened by outdoor lighting. This cluster's stars appear fainter than those of M35 mostly because they're six times farther away from us. This also accounts for the apparent difference in size between the two clusters. If they were equally distant, they would look equally large.

Our final target is the remarkable Crab Nebula (Messier 1), in the constellation Taurus, the Bull. It's the only supernova remnant in Charles Messier's catalog. In 1054 A.D., Chinese astronomers gazing toward the sky saw an object so bright that it was even visible during daylight. Today we can turn out backyard telescopes toward that same spot and view the tattered shell of material ejected by a dying star during a supernova explosion witnessed long ago.

Through a telescope at a power of 40x or less, the nebula fits in the same field of view with the star Zeta ([zeta]) Tauri, the tip of the Bull's southern horn. At around 90x, the Crab is a faint oval of light. It appears 1 1/2 times longer than wide and fades around the edges. You can admire it through a small telescope in a suburban sky, but you'll need a larger one in an urban area.

The Crab Nebula is still expanding from the force of the explosion that gave it birth. Rushing outward at more than 600 miles per second, the tendrils that give the Crab Nebula its name are moving with respect to foreground stars, as can be seen on photos taken some decades apart. Be sure to check out these photographs and others at SkyandTelescope.com/sw12.
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Title Annotation:Visual Observing: Deep-Sky Delights; constellation Gemini, the Twins, star cluster NGC 2158 and Crab Nebula
Publication:SkyWatch
Geographic Code:1USA
Date:Jan 1, 2012
Words:804
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