Birds of a Feather

© 2001 by Robert C. Moler

Well, not exactly. Cygnus the swan and Aquila the eagle are not birds of a feather, but they do share the summer sky and the Milky Way, but they're flying in opposite directions. These two constellations have bright stars that are two thirds of the Summer Triangle, the asterism that's the sign of summer. Also we'll look at other constellations, and asterisms nearby. Asterism's are informal constellations that aren't listed with the official 88 constellations.

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Cygnus the swan is seen flying, neck outstretched, like the familiar mute swans we have in our area. The constellation outline above is that of the asterism the Northern Cross. The wingspan of Cygnus extends to the next star out on either side. The trailing edges of the wings can also be seen in the stars. Cygnus' beak and tail stars are special. The tail is the bright star Deneb, the word actually means tail. It's barely a first magnitude star, but since it is 1,600 light years away, Deneb turns out to be one of the brightest stars known.

Alberio, in the beak of Cygnus is a beautiful double star, whose components are blue and gold. I like to call it the U of M star. (Sorry Spartan fans, there's no green stars around to make a double star for you.) Alberio resists splitting in binoculars, but is an easy sight in any small telescope.

One of Cygnus' great deep sky wonders is the North American Nebula. This is a huge cloud that has the shape of the North American Continent. It is detectable with the unaided eye and binoculars. It's a bit large for telescopes. The actual shape is hard to pick out. Cygnus also has two Messier open star clusters, M29 and M39.


Aquila is easy to spot with its brightest star Altair flanked by two shoulder stars. Its wings and tail are also easy to spot. There are no Messier objects in it, though off its tail, officially in Scutum the shield is M11, a beautiful compact open cluster with an intriguingly triangular shape. For that reason it's also known as the Wild Duck Cluster.

In Aquila the dark rift that splits the Milky Way, which starts near Cygnus reaches its widest. The milky band is really the light of thousands of stars, too faint to be seen individually with the unaided eye. But their light combines to create the milky glow. The rift has fewer stars visible, so it's dark. That doesn't mean there are really fewer stars.

The Great Rift is caused by a dark cloud of gas and dust that lies in front of vast clouds of stars behind it, blocking their light. It's neat to wander the edges of the rift with a pair of binoculars on a dark night.


Sagitta the arrow is a small, but distinct constellation. It has nothing to do with Sagittarius, the archer to the south. It's supposed to represent Cupid's dart, but Cupid doesn't even appear in the sky. Unfortunately its only visible in the early morning sky on St. Valentine day. Sagitta has the small globular star cluster M 71 within it, which is hard to resolve.

Behind and a bit north of the tail feathers of Sagitta, and not visible on my chart is a telescopic asterism called the Coat Hanger. It appears upright in finder telescopes, which invert the image.


Vulpecula the fox is very difficult to spot, and if you can see a fox there, you're a better astronomer than I. It's a relatively modern constellation invented by Hevelius an astronomer of the 17th century. The neatest thing about Vulpecula is the Dumbbell nebula, M27. I find it by using the star at the tip of Sagitta and head north about 3 degrees, which is half a binocular of finder field of view. M27 is much larger than the Ring Nebula, M57 I discussed last month, but it has a lower surface brightness. The dumbbell shape is easily seen.


Delphinus is a tiny constellation, but its stars easily portray a playful dolphin leaping out of the water. The box of stars that is the top part of its body is the asterism Job's Coffin.

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Uploaded: 08/16/2001