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Starwatch December 2011

November 21, 2011


Horsehead nebula in Orion.

The Horsehead Nebula in Orion is just one of the many glories in this constellation.

By Deane Morrison

The onset of winter finds Venus, Mars and Saturn busily moving into prime viewing positions.

An “evening star,” Venus climbs rapidly higher above the western horizon. As it ascends, our sister planet escapes the sun’s afterglow and lingers longer in the evening sky. By New Year’s Eve it will shine for more than two and a half hours after sunset.

Mars rises in late evening, followed by Saturn several hours later. They’ll be best in the predawn sky, when they are well up in the south. Mars is higher, below the hindquarters of Leo, while Saturn appears just east of the bright star Spica, in Virgo. Both planets are brightening as Earth gains on them in the orbital race. Even better, Saturn’s rings have opened to about 15 degrees from horizontal, so amateur astronomers with small telescopes will have a good reason to get out of bed early.

On the morning of the 20th, a crescent moon visits Saturn and Spica. Together, the three objects form a face with a wry smile, or, depending on your point of view, a cosmic smirk.

Our old friends the bright winter constellations come out in the south after nightfall. Orion’s hourglass form is unmistakable, and the region of his belt is of special interest to astronomers because it’s a cauldron of new star formation. Tagging at his heels is Sirius, the Dog Star, which ranks as the brightest of stars thanks to its proximity—only about nine light-years from us.

High in the west, the Andromeda Galaxy appears as a fuzzy oval between the Great Square of Pegasus and W- or M-shaped Cassiopeia. With skies at their darkest, this is a great time to view our nearest large neighbor galaxy through binoculars or a small telescope.

Hanging east of Andromeda and just above the Pleiades cluster, the stars of Perseus offer a rich trove of treasures. Pointing toward Cassiopeia is the hero’s helmet, and just beyond its tip lies a beautiful double star cluster. To see it, wait for a moonless night and use binoculars or a small telescope.

Also in Perseus is Algol, the Demon Star, a longtime object of fascination. Its name comes from Arab astronomers, who called it ra's al-ghul, or “head of the ghoul.” It represents the head of Medusa, the snake-haired Gorgon slain by Perseus. But don’t confuse Algol with Mirfak, the brightest star in Perseus; Algol is second brightest and closer to the Pleiades.

Algol’s notoriety arises from its habit of regularly “winking,” or dimming and then re-brightening. Today, astronomers know that Algol is a three-star system, with a big, bright main star and two dimmer ones. One of the dimmer stars orbits the main star every two days, 20 hours and 49 minutes. For 10 hours in each revolution, the dimmer star eclipses the main star as seen from Earth, causing the dip in luminosity.

December’s full “cold” moon arrives the morning of the 10th. Unfortunately for us, it sets as it undergoes a total lunar eclipse. The Twin Cities will scarcely notice the faint shading of the Earth's outer shadow (the penumbra) on the setting moon, but in general, points farther west and north will see a later stage of the eclipse at moonset.

Winter arrives officially at 11:30 p.m. on the 21st, when the sun reaches a point directly over the Tropic of Capricorn and the Northern Hemisphere experiences the longest night of the year.  

The University of Minnesota offers public viewings of the night sky at its Duluth and Twin Cities campuses. For more information and viewing schedules, see:

Duluth, Marshall W. Alworth Planetarium: www.d.umn.edu/planet

Twin Cities, Minnesota Institute for Astrophysics (during fall and spring semesters): www.astro.umn.edu/outreach/pubnight

9/22/11 Contact: Deane Morrison, University Relations, (612) 624-2346, morri029@umn.edu

Find U of M astronomers and links to the world of astronomy at http://www.astro.umn.edu.

Tags: College of Science and Engineering

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