Northern Rockies Skies for January

December 21, 2010

A monthly look at the night skies of the northern Rocky Mountains, written by astronomers Ron Canterna, University of Wyoming; Jay Norris, Challis Idaho Observatory; and Daryl Macomb, Boise State University.

The cold, crisp January nights are ripe for spectacular viewing of our stellar heavens. Around 8 p.m., directly overhead, the V-shaped constellation Taurus the Bull, with its orange eye Aldebaran, is prominent. Go north, about 10 degrees, where Auriga the Charioteer, and its brightest star Capella, accompanies Taurus.

Castor and Pollux, the twin stars of Gemini, lie to the east of Taurus by some 20 degrees. To the west of the V-shaped Taurus are the seven sisters, the constellation Pleiades, a compact group of bright-bluish stars.

To the south of Taurus lies Orion the Hunter, our winter beacon. Its prominent belt is surrounded by four very bright stars: Betelgeuse, a bright red supergiant northeast of the belt; the bluish Bellatrix, the female warrior to the northwest; Saiph, the blue "sword" which is southeast of the belt; and the left leg of Orion, Rigel, a white supergiant southwest of the belt.

From a very dark site you can easily see the Milky Way as it runs from the southeast horizon, through the zenith then toward the northwest skyline.

For you planet watchers look for Jupiter in Pisces right after sunset, just past the meridian. Saturn will rise around 2 a.m., and the morning star, Venus, rises at 5 a.m.

The Quadranitid meteor shower, which may be spectacular this year, peaks on Jan. 3-4.

January 2011 Interest: The Milky Way Galaxy II: Dimensions and Morphology (best URL: )

The Milky Way galaxy -- a disk-shaped, gravitationally stable assemblage of stars and gas -- is roughly 100,000 light-years in diameter (one light-year is about six trillion miles). The luminous parts of the galaxy, stars and glowing nebulae, are more concentrated toward the center regions, dropping off in numbers in the outer reaches of the disk.

The disk thickness varies according to component and age. The stellar disk is about 1,000 light-years thick with younger stars inhabiting the inner disk near its central plane and older stars less concentrated. Thus the stellar thickness-to-diameter ratio is about 1/100 -- very thin and slightly warped like those old phonograph records. The gas disk is much thicker, about 10,000 light-years.

In the outer reaches of the galaxy, at heights up to 150,000 light-years from the disk, is a tenuous population of very old stars that comprises the Galaxy's "halo." Near the halo’s edge are our two satellite galaxies, the Magellanic Clouds, visible from more southern latitudes.

The Milky Way has a spiral form -- possibly just two arms that wrap around more than once -- with a bar across the central bulge. Spiral galaxies are in the majority (60 percent), with elliptical galaxies constituting about one third, and irregular galaxies a small minority. The Magellanic Clouds are classified as irregular/barred spiral. Their gravitational influence is a probable cause of the Milky Way's warp.

To see this month's sky chart, click here.

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