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Northern Rockies Skies for February

January 27, 2011

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.

Before spring arrives, take some time this month to study and admire Saturn, one of the most colorful and interesting planets, and Orion, the most recognizable winter constellation.

Saturn rises in the east around 11 p.m. and can be seen throughout the night. Its rings are a spectacular sight, but you need binoculars to appreciate them.

Orion, the Hunter, is available at sunset and most of the night with its bright stars Rigel and Betelguese. The sword of Orion, seen as a group of three stars just south of Orion's three bright belt stars, is a good target for modest binoculars. The middle "star" in the sword is actually a nebula, a region of current star formation, filled with dust and gas.

Sirius and Canis Major, the Big Dog constellation, still dominate the mid-winter skies and are located southeast of Orion. The shield of Orion, an arc of faint stars to its northwest, protects it from the angry Taurus, the Bull, a V-shaped constellation of stars, the most prominent being Aldebaran, a bright orangish-colored star that makes up the eye of the Bull.

Located to the northwest of Taurus is a small, concentrated group of whitish and bluish stars called the Pleiades (Subaru in Japanese), or the Seven Sisters. Jupiter, one of the brightest objects in the February night sky, sets three hours after sunset in the southwest. Venus, the brightest of the planets, rises low in the southeast two hours before sunrise.

Northern Rockies Skies February 2011 Interest: The Milky Way Galaxy III: Timescales and Motions (best URL:

Several cosmic chronometers, or clocks, give fairly concordant values for the age of the Milky Way. Stars are born from contracting clouds of interstellar gas. As they age, massive stars go supernova, exploding and depositing back into the interstellar gas all the elements heavier than hydrogen and helium that were "cooked up" by fusion inside the stars.

The oldest generations of stars -- having been through only one or two cycles of stellar birth and death -- have the least amounts of the heavier elements. Via spectroscopy, astronomers assay the ages of radioactive metals in these oldest stars, using such isotopes as thorium-232 and uranium-238, which have half-lives of billions of years.

These "radio-clocks" yield an age of 13 billion years for the oldest stars in the galaxy. Independent modeling techniques for stellar evolution yield similar ages for the oldest groups of stars in the galaxy, called globular clusters. While the age estimates for the galaxy have uncertainties of one billion years, they are close to the age of the universe, 13.7 billion years. Thus our galaxy was formed not too long after the big bang itself.

In comparison the same dating techniques yield an age for the sun and its solar system of 4.6 billion years, about one third the galaxy's age.

The sun resides in the galaxy's disk, rotating around the galactic center in 225-250 million years, and so has completed 20 orbits since its birth.

Unlike a solid body, the galactic disk rotates differentially: Stars closer to the center orbit faster than the sun; those farther from the center orbit slower than the sun.

The globular clusters were formed at nearly the same time as the galaxy and have orbits extending far from the disk into the galaxy's halo. The interstellar gas that formed the disk collapsed from the more spheroidal shape of the overall galaxy about nine billion years ago. The dynamics of this collapse are only partially understood, and appear to require the agency of an invisible component of the Universe, "dark matter," inferred to exist only by its gravitational influence.

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

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