Look heavenward on a moonless summer night far from city light. The first thing to strike you is the shear volume of stars, but as your eyes adjust, your gaze is drawn to a luminous band spanning the sky. Ranging from magnificently brilliant to barely visible, this is the Milky Way, home to our Sun and nearly a half trillion other stars of varying size, age, and temperature.
Though every star you’ve ever seen is part of our Milky Way galaxy, stargazers use the Milky Way label more specifically to identify this river of starlight, gas, and dust spanning the night sky. As you feast your eyes, appreciate that some of the Milky Way’s starlight has traveled 25,000 years to reach your eyes, and light from a star on one edge of the Milky Way would take 100,000 years to reach the other side.
The rest of the sky appears to be filled with far more discrete stars than the region containing the Milky Way, but don’t let this deceive you. Imagine that you’re out in the countryside where the lights of a distant city blend into a homogeneous glow—similarly, the stars in the Milky Way’s luminous band are simply too numerous and distant to resolve individually. On the other hand, the individual pinpoints of starlight that we name and mentally assemble into constellations are just closer, much like the lights of nearby farmhouses. And the dark patches in the Milky Way aren’t empty space—like the trees and mountains that block our view of the city, they’re starlight-blocking interstellar dust and gas, remnants of exploded stars and the stuff of future stars.
Just as it’s impossible to know what your house looks like by peering out a window, it’s impossible to know what the Milky Way looks like by simply looking up on a dark night. Fortunate for us, really smart people have been able to infer from painstaking observation, reconstruction, and comparison with other galaxies that our Milky Way is flat (much wider than it is tall) and spiral shaped, like a glowing pinwheel, with two major arms and several minor arms spiraling out from the center. Our solar system is in one of the minor arms, a little past midway between the Milky Way’s center and outer edge.
Sadly, artificial light and atmospheric pollution have erased the view of the Milky Way for nearly a third of the world’s population, and eighty percent of Americans. Worse still, even though some part of the Milky Way is overhead on every clear night, many people have never seen it.
Advances in digital technology have spurred a night photography renaissance that has enabled the Milky Way challenged to enjoy images of its splendor from the comfort of their recliner, but there’s nothing quite like viewing it in person. With just a little knowledge and effort, you can enjoy the Milky Way firsthand; add the right equipment and a little more knowledge, and you’ll be able to photograph it as well.
Understanding that our Solar System is inside the Milky Way’s disk makes it easier to understand why we can see some portion of the Milky Way on any night (assuming the sky is dark enough). In fact, from our perspective, the plane of the Milky Way forms a complete ring around Earth (but of course we can only see half the sky at any given time), with its brightness varying depending on whether we’re looking toward our galaxy’s dense center or sparse outer region.
Though the plane of the Milky Way stretches all the way across our sky, when photographers talk about photographing the Milky Way, they usually mean the galactic core—the Milky Way’s center, its most densely packed, brightest region. Unfortunately, our night sky doesn’t always face the galactic core, and there are many months when this bright region is not visible at all.
To understand the Milky Way’s visibility in our night sky, it helps to remember that Earth both rotates on its axis (a day), and revolves around the sun (a year). When the side of the planet we’re on rotates away from the sun each day, the night sky we see is determined by our position on our annual trip around the sun—when Earth is between the sun and the galactic core, we’re in position to see the most brilliant part of the Milky Way.
Put in terrestrial terms, imagine you’re at the neighborhood playground, and that the merry-go-round is our solar system—the Sun is the stationary post at the center, and Earth on the outer edge (we’re ignoring our solar system’s other planets because they’re not a factor here). Our celestial merry-go-round takes a year to complete a single revolution. The Milky Way would be like a tree growing beside the merry-go-round; the tree’s trunk is the galactic core, and and a branch arcing above the merry-go-round is a less dense (fewer stars) outer extension of a spiral arm. No matter where the merry-go-round is in its revolution, the branch remains visible overhead, but the tree’s trunk is only visible when the merry-go-round rotates to face it.
Just like every other celestial object outside our solar system, the Milky Way’s position in our sky changes with the season and time of night you view it, but it remains constant relative to the other stars and constellations. This means you can find the Milky Way by simply locating any of the constellations in the galactic plane. Here’s an alphabetical list of the constellations* through which the Milky Way passes (with brief notes by a few of the more notable constellations):
If you can find any of these constellations, you’re looking in the direction of some part of the Milky Way (if you can’t see it, your sky isn’t dark enough). But most of us want to see the center of the Milky Way, where it’s brightest, most expansive, and most photogenic. The two most important things to understand about finding the Milky Way’s brilliant center are:
Armed with this knowledge, locating the Milky Way’s core is as simple as opening one of my (too many) star apps to find out where Sagittarius is. Problem solved. Of course it helps to know that the months when the galactic core rises highest and is visible longest are June, July, and August, and to not even consider looking before mid-March, or after mid-October. If you can’t wait until summer and don’t mind missing a little sleep, starting in April, with a dark enough sky you can catch Sagittarius and the galactic core rising in the southeast shortly before sunrise. After its appearance in April, the Milky Way’s core rises slightly earlier each night and is eventually well above the horizon by nightfall.
People who enjoy sleep prefer doing their Milky Way hunting in late summer and early autumn, when the galactic core has been above the horizon for most of the daylight hours, and is high in the southwest sky as soon as the post-sunset sky darkens enough for the stars to appear. The farther into summer and autumn you get, the closer to setting beneath the western horizon the Milky Way will be at sunset, and the less time you’ll have before it disappears.
The Milky Way is dim enough to be easily washed out by light pollution and moonlight, so the darker your sky, the more visible the Milky Way will be. To ensure sufficient darkness, I target moonless hours from an hour or so after sunset to an hour before sunrise. New moon nights are easiest because the new moon rises and sets with the sun and there’s no moon all night. But on any night, if you pick a time before the moon rises, or after it sets, you should be fine. Be aware that the closer the moon is to full, the greater the potential for its glow to leak into the scene from below the horizon.
Getting away from city lights can be surprisingly difficult (and frustrating). Taking a drive out into the countryside near home is better than nothing, and while it may seem dark enough to your eyes, a night exposure in an area that you expect to be dark enough reveals just how insidious light pollution is when you realize all of your images are washed out by an unnatural glow on the horizon. Since the galactic core is in the southern sky in the Northern Hemisphere, you can mitigate urban glow in your Milky Way images by heading south of any nearby population area, putting the glow behind you as you face the Milky Way.
Better than a night drive out to the country, plan a trip to a location with a truly dark sky. Here those in the western US have an advantage. The best resource for finding world class dark skies anywhere on Earth is the International Dark-Sky Association. More than just a resource, the IDA actively advocates for dark skies. If the quality of our night skies matters to you, spend some time on their site, get involved, and share their website with others.
Once you’ve feasted your eyes on the Milky Way, the next step is to photograph it. In my next blog post I pick up from here, sharing my tips for capturing the Milky Way with your camera.
Click an image for a closer look and slide show. Refresh the window to reorder the display.