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