I spent much of the summer kayaking or canoeing the salt marshes of the Cape, the Nashua River, and of course our beloved Bare Hill Pond. Those excursions were often shared with water birds. There were snowy egrets and great blue herons. There was the thrill of first seeing a bald eagle from Blueberry Island. And as fall settled in, I came upon a single swan. Though swans are known to pair for life, this one swam alone. Fellow sky walkers, this week we seek that swan’s mate among the stars, in its namesake constellation Cygnus, the swan.
On this adventure, you will want to bring a telescope such as the library’s StarBlast lend-a-scope. While Cygnus itself is a naked-eye sight, its finest treasures require a telescope’s observational oomph.
Flight of the swan
Our stellar swan soars through the summer skies, accompanied on its journey by Aquila (the eagle) and Lyra. Each of these three constellations contributes an especially bright star to the Summer Triangle, one of the most easily recognized features of the warm-weather skies. With summer having turned to fall, this starry trio now spends the evening hours in the western sky.
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Cygnus and neighboring constellations as they appear at 9 p.m. in early November and at 8 p.m. towards the end of the month. Stars marked as ★ make up the summer triangle. Credits: Photo of Albireo,Wikimedia Commons. Chart and eyepiece drawings, Marc Vilain)
Summer constellation notwithstanding, I always find it easier to observe Cygnus in autumn. Most amateur telescopes, including my own scope and the StarBlast, are supported by a boxy Dobsonian mount; these mounts are more effective when aiming part-way up the sky than when aiming straight up to the zenith.
The star chart shows where to find Cygnus in the November sky. Look about 45 degrees up from the horizon: Cygnus is a large constellation that occupies much of the sky. If you can’t immediately picture a swan in flight, look instead for Charlie Brown’s kite, whose cross-like shape is formed by the brightest stars in Cygnus.
A bounty of stars
Cygnus lies along the Milky Way. In areas with less light pollution than Harvard, Cygnus appears to bathe in a foam of stars. If you are lucky enough to find yourself under dark skies, you may have a surprisingly hard time locating Cygnus’ bright shape among all the starry foam.
Point your telescope or binoculars anywhere around Cygnus, and your eyes will fill with stars: single stars, pairs of stars, and star clusters coming in shapes limited only by your visual imagination. To me, it always feels like I’m falling through a hole in the sky into a starry otherworld; spend any time at the eyepiece and you will know what I mean.
Homework: Double stars
If you can bring yourself back to earth, Cygnus holds some particularly compelling double stars. The most famous is Albireo, at the end of the swan’s long neck, the name having been westernized from the medieval Arabic for the beak of the hen (as our swan was known). To the naked eye, Albireo is a modest orange star, but the scope reveals it as a star pair, one deep orange and the other cold blue, thus giving the lie to stars having only the color twinkle.
The two components of Albireo most likely represent a chance alignment, making it a so-called optical double star. Not so delta Cygni, or Rukh, a binary star on the front edge of the swan’s wing. The two components of delta Cygni are gravitationally bound, meaning that they follow an astronomical dance, traveling in circles around a common center.
Here’s how that dance starts: Stars form out of a spinning disk of gas, mostly hydrogen, which gradually collapses around a growing nucleus. When the nucleus becomes sufficiently massive, the hydrogen starts fusing and a star is born. With single stars, the story ends there, but the proto-stellar disk may sometimes fragment into multiple nuclei. These nuclei continue traveling along the plane of the disk, and once the disk collapses, the resulting stars just keep dancing along that same path.
You will need keen eyes and a night without turbulence to split the components of delta Cygni in the eyepiece. Earlier this week, at 63x magnification, breaks in the wind allowed fleeting glimpses of the gap between the primary component and its much smaller secondary.
Our final target for today is the pair of stars known as omicron-1 (o1) and omicron-2 (o2) Cygni. This naked-eye optical pair lies on the back edge of the swan’s wing; the easiest way to locate them is to center the scope on delta Cygni, and just lift the scope straight up to o1. You will find o1 as part of a colorful star trio shaped like a hockey stick, with orange o1 at the bend and two blue stars completing the figure. Then hop from o1 to o2, another orange star, which you will find in a star cluster that looks like a misshapen umbrella.
About that swan
Greek myths supply plenty of swans that might have inspired our constellation. Most likely it’s Zeus, who having also morphed into a bull to make off with Europa, played the same trick as a swan to seduce Queen Leda of Sparta. Among the progeny of this not especially consensual union, hatched out of giant eggs, no less, were Castor and Pollux, the twins who became the constellation Gemini.
It strikes me as massively chauvinistic that Leda got nothing for her #MeToo moment, while Zeus-swan and his twins have constellations to their name. I’d like to rewrite the myth, with Leda becoming a swan herself as a result of the encounter. Let’s have Leda immortalized in the stars as Cygnus, where she now watches over Castor and Pollux from her stellar perch. And maybe she at times returns to earth. And maybe she was herself the long-lost mate of the swan I saw on Bare Hill Pond. Fellow sky walkers, as our earth-bound swans head south for the winter, I wish you many a mythical night under the autumn skies.
Marc Vilain sees the stars come out at night from the roads, fields, and ponds of Harvard.
