The Milky Way Is Hiding A Fat Slice Of The Universe

The Milky Way (hazy band) and the bright stars of familiar winter constellations like Orion, Gemini and Taurus shine in this photo taken around 8:30 p.m. on January 20. Sirius is the brightest star near the bottom. The glow at right is light pollution from the city of Duluth, Minn. Details: 12mm lens, f/2.8, ISO 2000, 30-second exposure. Bob King

Hungry for starlight I drove 25 miles north of my light-polluted city to where the Milky Way still glows with its ancient vigor. A throng of stars twinkled above the cold, snowy Earth. I looked up and wondered out loud “where have you been all these nights?” Gleaming with a fire-like intensity, they looked like the suns they truly are.

Many of us have to contend with light trespass from stores, gas stations and insidious “security lights” that rob the night sky of its stars. Add in snow cover, which reflects stray light into the sky, and winter nights in my part of the world are now pale and anemic compared to just a decade ago.

Face-on (left) and edge-on views of the Milky Way, a spiral galaxy about 105,000 light years across but only 10,000 light years thick at the sun’s location. The solar system lies in the galaxy’s flat plain. Looking through the plane, the stars pile up to form the Milky Way band. In summer, we face toward the richer, denser core, which makes for a relatively bright band.  In winter we look out toward the edge and into empty space, the reason the Milky Way band looks fainter. NASA with annotations by the author

While the winter Milky Way may be next to impossible to see from town, here in the countryside it rose straight up from the southeastern horizon like a plume of chimney smoke on a calm night. A host of bright stars lined either side of the luminous haze like a crowd of admirers. Appearances can be deceptive. The Milky Way band seems tenuous, but it’s jammed with at least 250 billion suns along with thousands of star clusters, glowing gas clouds called nebulae and enormous clouds of interstellar dust and gas.

In this telephoto view of a section of the summertime Milky you can see a web of dark dust clouds (in silhouette) blotting out the view of more distant stars. Other knots and fuzzy spots are star clusters and nebulae. Saturn is the bright dot at right. Bob King

The whole of it looks like a glittering pizza from afar — a slightly warped but generally flat disk 105,000 light years across with a thicker bulge in the center where the stars are more concentrated. The sun and solar system occupy a minor arm or spur called the Orion Arm situated between the enormous whorls of the Perseus and Scutum-Centaurus arms about halfway between the galaxy’s center and edge.

When we look through the plane of our galaxy the stars stack up across many light years into a fuzzy band across the sky we call the Milky Way. Few individual stars are seen in the band with the naked eye because most are so faint and far away they blur into a haze. But a telescope will show them in droves!

When you look away from the band you’re looking above or below the galaxy’s plane, where the stars thin out rapidly. That’s why the Milky Way looks like a band in the first place and not just a bunch of stars spread evenly across the sky. Remember, you’re looking through a thin but star-packed disk.

This map shows the entire sky on a January night from mid-northern latitudes. I circled Orion so you can get your bearings. The blue band is the Milky Way. Dust clouds within the galaxy block the view of galaxies (red ovals) beyond our own. Stellarium

Living in the Milky Way has its perks. From here we can study a galaxy and its parts up close. It doesn’t take much equipment either. Your eyes are a good start, followed by a pair of binoculars. And if you want to go deeper and see the best and brightest nebulae and star clusters a modest telescope will get you there.

But there are disadvantages, too. All those goodies get in the way of the rest of the universe. Vast amounts of interstellar dust created and scattered in the wake of supernova explosions not only absorbs the light of billions of more distant stars but also blocks other galaxies from view. The Milky Way obscures about 20 percent of the night sky. Call it “dark pollution.”

When we look through the plane of the Milky Way we only see the local stuff like the lovely clusters and glowing gas clouds I mentioned earlier. Nearly everything else within and along the band of the Milky Way is obscured from view. It’s like being stuck in the middle of the forest where the trees block the view of the open field beyond. In fact, astronomers even have a name for the dusty plane of the Milky Way — the Zone of Avoidance.

In spring, the Milky Way curls around the horizon so the entire overhead sky opens into the depths of intergalactic space with no Milky Way to interfere. Look at all those galaxies! Stellarium

However. If you gaze above or below the plane of the galaxy — very easy to do, just look a few fists to the left or right of the Milky Way band — the dust and stars thin out rapidly, allowing a view of the deepest, most distance objects in the cosmos. Did you know there are some 2 trillion galaxies out there?

To get a sense of how the our galaxy obscures a slice of the cosmos take a look at the two  fisheye-view maps of entire night sky for January and May. In winter, when the Milky Way stares us in the face we see its nebulae and clusters (shown in green and yellow) but hardly a single external galaxy (red ovals).

But let your eye wander from the Milky Way band and galaxies swarm the sky like clouds of mosquitos on a summer night. By May, when the Milky Way encircles the horizon, we’re facing into the void, and galaxies are mostly what we see in the night sky. Along with scattered nearby stars that belong to our galaxy. Any amateur astronomer will tell you that spring serves up a heaping helping of galaxies for telescopic observers.

One group of clever astronomers figured out a way to “see” what’s behind our galaxy using a radio telescope equipped with a special receiver that can see through the dust. Hundreds of otherwise invisible galaxies have recently been discovered with the setup.

So I encourage you to take a drive to the countryside before the moon returns to the evening sky. Commune with the shimmering stars, appreciate the meek splendor of the winter Milky Way and think about all the galaxies that are hiding behind that band of misty light.