August nights are some of the best of the year to see the Milky Way. With no moon in the sky in the coming week, try to get out of town and treat yourself and your family to one of nature’s most perspective-altering sights: the sight of our own galaxy arching high overhead. From a dark sky, the hazy band of the Milky Way looks grainy, a sign of its true nature as a flattened disk made of stars. Billions of them. Estimates range from 100 billion on the low end to 400 billion on the high. Throw in some billions of planets, trillions of comets and asteroids, star clusters, nebulae, and giant interstellar gas clouds and you’ve got yourself a galaxy.
The Milky Way galaxy, shaped something like an egg-over-easy — a thick, yolky middle surrounded by a thinner disk of egg white — measures 100,000 light years in diameter and about 10,000 light years thick where we live. The center, called the bulge, is some 25,000 light years thick. Because the sun and planets orbit within the disk of the galaxy, when we look through it, whether that’s toward the center or edge, the stars pile up across our line of sight to form a dense haze of starlight. The reason it looks like a band instead of an amorphous haze of stars is because the disk is thin. When our line of sight passes above or below the disk, the stars thin out quickly, which gives the band an edge or border.
We see our galaxy forever “edge-on” because of our location inside the disk. But if we had the technology, might launch a rocket at high speed straight up and out of the Milky Way, above it all! We’d then be able to see the Milky Way galaxy “face-on” as it truly is: a giant disk of pinpoint stars, some billions of which are gathered into several graceful spiral arms that unwind from a dense, bright bulge of stars concentrated in the galaxy’s center.
Looking more closely, we’d notice that the disk stars look bluer (younger) than the more ancient stars that glow a warmer yellow. Rosy-hued bits of fuzz would fleck the spiral arms like fuzz balls on your favorite sweater. These are clouds of gas fluorescing in the light of young, hot suns in newly-birthed star clusters. Even though we stuck where we are in the disk, astronomers have probed the shape and extent of the galaxy using visible light, radio waves and infrared light to reveal the shape of the core, the position and extent of the spiral arms and other details that are hidden within that magnificent arch of milky light that spans the sky during August and September.
Since we can’t shoot that rocket up and out of our galaxy just yet, wouldn’t it be nice if we could find galaxy that resembles ours? Just to help us see what we’re missing? The best so far is NGC 6744 in Pavo the Peacock some 30 million light years away. Click here for a full-screen view. Compare its photo to the face-on model and you’ll see strong similarities: bar-shaped center, multiple arms, pink nebulae and more. What a great image to contemplate before you step outside and look up the next clear night.