Not many people seek out dark skies on winter nights. But those who do get a treat: a nice display of the Milky Way. It arches high overhead in early to mid evening right now. It's anchored in the southeast by Canis Major, which is home to Sirius, the night sky's brightest star. The Milky Way climbs from Sirius to the "horns" of Taurus high overhead, then drops toward M-shaped Cassiopeia in the north, and the tail of Cygnus, the swan, in the northwest.
The winter Milky Way is thinner than that of summer because we're looking away from the crowded center of the galaxy and toward its sparsely populated outer precincts. In fact, the point that's directly opposite the galaxy's center is at the tip of one of the horns of Taurus, near a star called El Nath.
The Milky Way's visible disk spans about a hundred thousand light-years. We're a bit more than half-way from the center to the edge, so it's about 25,000 light-years to the edge.
Yet the galaxy doesn't end there. In fact, there's more stuff outside the disk than inside. Up to 90 percent of the Milky Way consists of dark matter -- material that produces no detectable energy, but that exerts a gravitational pull on the visible matter around it. Most of the dark matter forms a "halo" around the disk -- a dark zone that spans hundreds of thousands of light-years.
So while the stars may fizzle out as you look beyond the Milky Way's disk, the galaxy itself most certainly does not.
Script by Damond Benningfield, Copyright 2010
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