Astro Bob blog: Clear with a chance of star cloudsA short tour of the Milky Way's finest southern star clouds
By: Bob King, Duluth News Tribune
Clear with a chance of star clouds
The southern half of the Milky Way from Aquila to the tail of Scorpius slices across the sky earlier this week. The distinctly brighter patches are several of our galaxy's finest star clouds. The long, irregular dark gap splitting the Milky Way in two is called the Great Rift. Details: 16mm lens at f/2.8, ISO 800 and 4-minute exposure on a tracking mount. Photo: Bob King
If you live where the sky is dark and look south on an otherwise clear summer night, you might think clouds are gathering there in advance of a storm. It would be fair to say that's a common first impression of people who've never seen the Milky Way before. Most of us live where the sky is light polluted and don't have the opportunity to see our galaxy in its full glory.
When I drove north in search of real darkness several nights ago, I came to a stop on a narrow dirt road in a field of weeds and wildflowers. It was twilight and the sky looked the same as it does at home -- Venus burning low in the west, the Summer Triangle ascendant in the east. But when darkness finally settled in, I wasn't prepared for the startling brightness of the Milky Way. Cascades of stars seems to tumble from high in Cygnus all the way to the southern horizon. The region around Sagittarius, normally diluted by manmade light, was chunky with star clouds and spattered with stars.
When we look south on summer nights we face the constellation Sagittarius, home to the center of the Milky Way galaxy 30,000 light years away. Thousands of clouds of interstellar dust and gas -- dark nebulas -- lie between us and the center, obscuring it from view. Radio waves and infrared light can penetrate that dust and see the star-rich core and its supermassive black hole, but our eyes cannot go there. Astronomers estimate the dust dims stars in that direction by 25 to 30 magnitudes or 100 billion times. Someone get a vacuum cleaner, please!
Here's a cropped and annotated version of the photo above. You can use the Teapot of Sagittarius and Altair, the bottom star of the Summer Triangle, to get oriented. The red dot marks the location of the galactic center. Each star cloud will appear as a noticeable enhancement in the brightness of the Milky Way. Photo: Bob King
You can see the dust very clearly in the photographs. It's that large dark gap, called the Great Rift, as well as numerous other smaller patches running the length of the Milky Way. Here and there however the dust thins and we get these little windows straight into the heart of the galaxy. Think of wiping fog off a bathroom mirror. With little intervening dust, the distant stars stand out as distinct bright star clouds. A star cloud is not exactly a star cluster. It's more of a pseudo-cluster or concentration of associated stars and star clusters that form a clump in one of the arms of our spiral-shaped galaxy. Three of the finest -- the Scutum (SKEW-tum) Star Cloud and the Small and Large Sagittarius Star Clouds are arranged at equal intervals along the Milky Way from the Teapot to Scutum the Shield. All three are about 10,000 light years away and lie in the Sagittarius-Carina spiral arm interior to the one in which the sun and planets are located. When you gaze at the these misty patches, you look across the vast interarm gap separating the two spiral arms.
Our galaxy the Milky Way is a spiral assemblage of stars, clusters, dust and gas clouds and of course, planets some 100,000 light years wide. The sun is located in the Local Arm (also called the Orion Spur) about halfway between center and edge. When we look at the Milky Way in Sagittarius, our gaze reaches inward toward the Sagittarius Arm. Credit: NASA/CXC/M.Weiss
Once you've feasted on these radiant hunks with your naked eye, examine them in binoculars. You'll be in for a treat as you meet snaky dark nebulas, dense spots that are true star clusters and a host of individual stars. These really are magnificent regions to begin your exploration of the galaxy. You won't believe how rich the view is until you look. A telescope will help you probe the patches even more deeply and show so many stars you're guaranteed to be wowed. Just make sure you bring mosquito repellant.
Look low in the northwest about an hour after sunset tonight to spy a beauty pair of celestial jewels: Venus and Regulus. Created with Stellarium
One other item of interest while you're out waiting for the sky to darken is Venus. Tonight it will be in conjunction with Leo's brightest star Regulus. The two will be just a degree apart. Watch over the next night or two as Venus soon leaves the star behind.