Evening passes of the International Space Station (ISS) / shuttle combo are winding down this week for our region. There are just three nights left before a several-week-long hiatus followed by a re-appearance in the morning sky. The ISS will first appear in the western sky, cross the southwest and disappear in the south-southeast.
Tonight at 10:20 p.m.
Fri. July 24 at 9:09 p.m. (bright pass!)
Sat. July 25 at 9:34 p.m.
A huge cumulus congestus cloud billows upward on a recent evening here in Duluth. The white portion is lit by the sun while the darker cloud is in shadow and silhouetted against the taller. Credit: Bob King
The clouds have been spectacular here the last couple days. The eye-grabbers are the big cumulus clouds that develop into towering thunderheads with multiple turrets topped by anvils of delicate cirrus. They dump rain in fine purple strands that you see from miles away. In the clouds pictured above, the contrast between the illuminated cloud and the one in shadow is very dramatic. This photo will help us understand why the Milky Way appears to be split in two by what astronomers call the Great Rift.
The Great Rift, composed of giant clouds of interstellar dust, is the chunky dark lane cutting through the center of the picture. It begins at the top of the Summer Triangle and continues all the way to the southern horizon. The photo is tipped so north is left and south is at right. If you need a map to find the Summer Triangle, click here. Credit: Ingo Berg
Round about late July the Milky Way is high in the east at nightfall. If you live in the suburban fringe or the country, you’ll have no problem seeing the soft band of hazy light that first-time observers often mistake for approaching clouds. Even a casual inspection will show that the band is not evenly-textured. In particular, there’s a large gap or split extending from the Northern Cross all the way down to Sagittarius in the south. That’s the Great Rift.
While it may appear to be starless, the rift is packed with suns, millions of them, but they’re obscured and hidden by enormous clouds of fine dust many light years deep. Tiny dust particles as well as tasty molecules of water, ammonia, salt and others are spewed by older, evolved stars and exploding stars called supernovas. The fine molecular mix settles in the plane of the galaxy where over the eons, it gets re-compressed into new stars. While the dust is really sparse, it adds up over the light years to form a thick, dark band that appears to slice the Milky Way right in half.
Interstellar dust, arranged in huge patches and tentacles called molecular clouds, is silhoutted against the stars in the midplane of the galaxy NGC 891 in Andromeda. Credit: NASA/Hubble Space Telescope
That brings us back to the cumulus cloud photo. We see the dark dust because it’s silhouetted against a rich backdrop of stars just like the unlit cloud is silhouetted against the lit one. If you look around at other galaxies, you see these dark bands too. The inhabitants of planet Gloobo 3 orbiting the center of NGC 891 (above) would have their own version of the Great Rift. Should we meet someday, we’ll have something in common to talk about.
About one outstretched fist to the left of Deneb at the top of the Summer Triangle, you’ll spot a dark slash in the Milky Way sometimes called the Funnel Cloud Nebula. This string of dark, dusty nebulas form a funnel shape that’s easy to see from a dark sky. It’s pretty big — more than a fist from top to bottowm. Details: 20mm lens at f/2.8, 30-second time exposure at ISO 3200. Credit: Bob King
Another name for all these dark dusty patches is dark nebulas. Now that you’ve found your way into the Great Rift, it’s time to visit one of my favorite dark nebulas, sometimes called the Funnel Cloud Nebula. It’s an inky slash perpendicular to the Great Rift and visible just to the left or north of Deneb. The shape is dramatic, starting out wide at top and narrowing to a tip a fist below or east of Deneb. To see it even better, use the astronomer’s technique of averted vision, where you don’t start directly at it but instead sweep your eye around the nebula.
Once upon a time, the materials that eventually formed the sun and solar system were part of a larger dusty nebula. Now all is light and life. As you gaze into the dusty deep, consider its potential.