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Astro Bob: Northern Cross makes last stand as the Great Bear leaves his den

As the Northern Cross stands on the western horizon, Ursa Major claws its way up the eastern sky accompanied by a gazelle.

The Big Dipper in January
The Big Dipper, standing on its handle, brings cheer to a bitter-cold January night. The famous, seven-starred asterism comes back into clear view during mid-winter, standing on its handle in the northeastern sky around 8 p.m. local time.
Contributed / Bob King
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A few nights ago, I braved 25-below temperatures so I could look at a couple of evening comets. But more than that, I just wanted to get away from all the light pollution and see what a dark sky looked like. It had been a while.

Winter in my snowy part of the world is the worst season for observing. Despite the glitter of Orion and his stellar entourage, the increased intensity and spread of LED lighting has become an obstacle to appreciating the stars in recent years. And no more so than during the winter months, when snow reflects much of that light up into the sky.

I love snow, and a certain amount of sky brightening is to be expected, but the broad spread of city and home-lighting has exacerbated the problem. If you love seeing the stars, do yourself a favor and turn off your yard lights when they're not needed. Close your curtains, too. Each small effort adds up to more darkness.

Orion and the winter Milky Way
The winter Milky Way competes against light pollution from the city of Duluth, lower right. Orion and his three belt stars are visible just to the right of center.
Contributed / Bob King

Bundled in multiple layers of feathers and wool, with chemical hand warmers tucked in each palm, I stood in cold silence and looked up. How wonderful to see the Milky Way again. The softly luminescent band reached from Sirius in the southeastern sky overhead to Auriga and down through the W of Cassiopeia all the way to the Northern Cross.

Northern Cross stands tall.
You can still catch sight of the Northern Cross (Cygnus) in late January if you look to the northwest as soon as it gets dark. Jupiter stands off to the left. Deneb, its brightest star, is more than 2,600 light-years from the Earth and nearly 200,000 times as luminous as the sun.
Contributed / Stellarium

I shouldn't have been but was surprised to see the Cross, formally known as Cygnus the Swan, still visible. The constellation is associated with the summer and early fall, when it stands high in the south at nightfall. But here it was, lingering into the coldest month of the year. When Cygnus first appears in the east in April-May, it rises on its side. Now, more than a half-year later, it's finally upright. Catch it now before it sinks out of view in a couple weeks.

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The winter half of the Milky Way — from the Orion region in the southern sky to Perseus overhead — isn't nearly as bright and showy as the summer half. That's because we face outward toward the galaxy's edge this time of year, where the stars gradually thin out and give way to virtually empty intergalactic space.

Milky Way face on edge on NASA Richard Powell_ANNO S2.jpg
Here are two views of our spiral galaxy, the Milky Way: face-on, left, and edge-on. The sun is located about halfway from center to edge. In summer, we face toward the galaxy's center; in winter, we look toward the edge. These different perspectives cause the difference in brightness and width of the Milky Way band from summer to winter.
Contributed / NASA, JPL-Caltech, R. Hurt (SSC/Caltech) with additions by Bob King

In summer, we face inward toward the center, where the majority of stars congregate. And while cosmic dust from supernovae and the like block the view of the innermost core, the sheer number of stars in this direction bloats and brightens the band of the Milky Way.

From Deneb and the Northern Cross I turned my attention to seven bright, familiar stars rising in the northeast — the Big Dipper. Like the Northern Cross, it's also an asterism, the easy-to-recognize, bright star pattern within a constellation. The Dipper is a part of Ursa Major the Great Bear. Since all seven stars are justifiably famous each has a proper name: Alkaid (AL-kade); Mizar (MY-zahr); Alioth (AL-ee-oth); Megrez (ME-grez); Phecda (FEK-da); Merak (ME-rak) and Dubhe (DOO-bee).

The Big Dipper's Back!
During fall and early winter, the Big Dipper is very low in the northern sky and often obscured by the surroundings. Come late January however, it returns to view in the northeastern sky. You can use the Dipper to find the North Star (Polaris) by shooting a line through Merak and Dubhe. To the right of the figure, look for three fainter pairs of stars that represent the Three Leaps of the Gazelle.
Contributed / Stellarium

All shine at 2nd magnitude except for Megrez, the faintest of the seven at 3rd magnitude. If you've ever wondered what a one-magnitude difference looks like, compare Megrez to Merak, located at the end of the Dipper's Bowl.

What we see as the handle of the Big Dipper is actually the tail of the Great Bear, while the Bowl represents the bear's back and hind quarters. Pay special attention to Mizar . If you glance sideways at the star, peering off to the side, you'll spot fainter Alcor, Mizar's brightest companion star. Together the pair is one of the few true, naked-eye double stars, not just a chance alignment.

Back in 1869, English astronomer Richard Proctor determined that all the stars in the Dipper except for Dubhe and Alkaid shared the same motion across the sky and the same distance, about 80 light-years. In fact, they form the core, along with about eight other stars, of the Ursa Major Moving Group , a loose assemblage of suns that form the core of a star cluster born some 500 million years ago.

Great Bear figure
The Big Dipper is part of the larger constellation and mythological figure, Ursa Major the Great Bear.
Contributed / Stellarium

On the map I've also circled three pairs of stars to the right of the Dipper. These represent three of the bear's paws, but they're better known as the Three Leaps of the Gazelle. In Arabic culture they portray three hoof prints of a gazelle that was startled by the swishing sound of a lion's tail and leaped across the sky, leaving tracks in the heavens. The constellation Coma Berenices , which we know as Queen Berenice's hair, was pictured instead as the furry lion's tail, located just behind the constellation of Leo the Lion.

After an hour and a half of comets, constellations and the Milky Way it was time to warm up. The human body, standing at a telescope, doesn't generate much fire. I packed everything up and then gazed a while longer. Half the galaxy hovered above me, but the only thing I heard was my pulse.

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"Astro" Bob King is a freelance writer for the Duluth News Tribune.

Related Topics: SCIENCE AND NATURE
"Astro" Bob King is a freelance writer and retired photographer for the Duluth News Tribune.
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