Twilight's end brings the return of the summer Milky Way to the eastern sky. We unravel the anatomy of our home galaxy by teasing out the hidden structures within that glowing band.
Stars, a Golden Guide written by Herbert Zim and Robert Baker, was the go-to night-sky book for kids back when I was one. I still have my copy, a 12th printing dated 1960, that's in better shape than I am. One of the book's best illustrations, by James Gordon Irving, depicts a man and woman standing together under the constellations of the spring sky, a hazy blue Milky Way rising along the eastern horizon. Irving captured the enchantment of a shared starlit evening and the anticipation of that great band of starry light returning to view.
I fell in love with that picture, and it still stirs up good feelings, as if a great adventure lies ahead. Maybe that's why I'm eager to see the Moon depart the evening sky this week and watch the Milky Way climb the eastern sky, so we can revel again in its sumptuous stars.
We all enjoy a starry night, but the Milky Way provides a sense of scale like no other naked-eye sight. Just the sweep of it! You know you're looking at something really, really big — and grand — like a mountain range at sunset but on a cosmic scale. It forces you to stop and take stock. To remind us there's beauty all around.
All that good stuff is back again. You only need a dark sky to the east and the will to stay up past your normal bedtime, a familiar expectation of summertime skywatching.
We live in a barred spiral galaxy about 100,000 light-years wide shaped like a pancake with a bulge in the middle. It's home to at least 200 billion stars and an estimated 100 billion planets. The Sun and planets orbit within the 1,000-light-year-thick disk near the mid-point of the galactic plane, 26,000 light-years from the central bulge. Because we're stuck inside the galaxy, we can only ever see it edge-on, never face-on, a view reserved for stars orbiting above or below the galactic plane. For a musical description of the Milky Way Galaxy, listen to Monte Python's Galaxy Song.
So, what do we see exactly when we look at the band of the Milky Way?
Although our edge-on perspective prevents a view of the galactic core due to intervening dust, it's responsible for the magnificent band of the Milky Way, a perspective effect from looking through the disk and seeing billions of stars stack up over vast distances to create a ribbon of glowing haze across the sky. When we look above or below the band, which defines the galactic plane, we're looking out of the galaxy. That's the reason the stars thin out rapidly and also why the band has a well-defined edge. Stars are concentrated in the disk and core; they dwindle quickly once our gaze soars above the plane.
Locations of objects within the Milky Way are described by their galactic latitude and longitude with the Sun as the center point. The line connecting the Sun through the center of the galaxy defines the galactic equator (0° latitude), while the direction from the Sun to the galactic center serves as 0° longitude point. The other longitudes are arranged around the galactic circle in the diagram above similar to a compass. The galactic north and south poles at +90° and –90° lie far above and below the galaxy's plane.
On winter nights we look outward toward the Perseus and Outer Arms between about 120° and 240° longitude centered on the galactic anticenter (180°) directly opposite the core. But in summer we face inward toward the galactic core, toward 0° longitude in the direction of the constellation Sagittarius. As our gaze sweeps from Cassiopeia and Cepheus in the northeastern sky to Sagittarius in the southeast, we first look "over our shoulder" into the Perseus Arm toward Cassiopeia (130° – 110°). Then our eyes glide effortlessly across an inter-arm gap until we arrive at our local arm called the Orion–Cygnus Arm or the Orion Spur. Minor though it may be, it still extends an impressive 10,000 light-years.
Looking down the local arm, the Milky Way thickens in Cygnus (90°– 60°) as stars bunch up along our line of sight to make such notable spectacles as the Cygnus Star Cloud. You can't miss this bulbous glow tucked between Gamma (γ) Cygni and Albireo — it's one of the brightest sections of the summertime Milky Way and a great place for star-sweeping with a small telescope or binoculars.
From Cygnus we slide southward to Aquila and Scutum, crossing into the Sagittarius Arm, the outermost of the galaxy's inner arms. Lots of highlights here! One of the best is the Scutum Star Cloud, a strawberry-shaped glitter-puff about 5° across located 6,000 light-years away. It's home to the Wild Duck Cluster (M11) and numerous dark nebulae. The Lagoon Nebula (M8), the Eagle (M16), the Omega Nebula (M17), and Carina Nebula and many more of our favorite deep sky objects reside in the Sagittarius Arm.
Next in is a massive Scutum-Centaurus Arm. While there may be faint fuzzies and stars visible in amateur telescopes within this distant galactic furl, I'm not aware of any. Clouds of interstellar dust mucking about the plane of the Milky Way hide distant stars and nebulae except in rare cases. One of those scarce portholes is Baade's Window, a 1°-wide clearing just above the tip of the Sagittarius Teapot, that will shoot you across 25,000 light-years straight into the core of the galaxy!
Truth to tell, cosmic dust and the natural dimming of stars across great distances means we only see a bubble's worth of space from our perch midway in the Milky Way. Maybe out to 5,000 light-years or so with the naked eye. Much of the galaxy is obscured or lies beyond visibility. And yet it's enough to set your soul sailing off to great adventures on a dark summer night.