Sky and Telescope - July 2016 - 45

OBSERVING

Northern Hemisphere's Sky

Fred Schaaf welcomes your
comments at fschaaf@aol.com.

Fred Schaaf

Sailing a Starry River
Take a journey down the summer Milky Way.
. . . It's dark and wide and deep
Towards the sea it creeps
I'm so glad I brought along my mandolin
To play the river hymn

Cygnus Star Cloud

....If you hear a lonesome drone
That's as common as a stone
And gets louder as the day is growing dim
That's the river hymn
The whole congregation was standing
On the banks of the river
We are gathered here
To give a little thanks, thanks
- Robbie Robertson, The River Hymn
We give thanks for the rivers of Earth - all the rivers
on this water planet we call home. But we also should
remember to give thanks for the river of the sky.
I'm not talking about the constellation Eridanus, the
River, which meanders through the evening sky of late
autumn and winter. For as wonderful as that constellation is, its marvels are enormously exceeded by those of
what many cultures have seen as the Heavenly River. I'm
referring, of course, to the luminous band of the Milky
Way - and especially the Milky Way of summer nights.
Starting the voyage at Cygnus, the Swan. Where
do we start our journey on the heavenly river? The lower
parts of the Milky Way, especially the grand Sagittarius
region, may be hidden from you by trees or buildings, or
may often be veiled by summer haze, which is thicker
lower in the sky. So the starting point for our ship voyage
on the summer Milky Way will usually be the bright
part that passes overhead as seen from mid-northern
latitudes: the Cygnus Star Cloud.
By the year 2000, light pollution had worsened to
the point where it prevented at least 20% of the world's
population and more than 2/3 of the U.S. population from
seeing the Milky Way. The Cygnus Star Cloud, however,
can be seen overhead - just barely - even when the

EDDIE YIP / CC BY-SA 2.0

You can ride on it or drink it
Poison it or damn it, ﬁsh in it and wash in it
Swim in it and you can die in it
Run, you river, run....

limiting magnitude is as poor as +5.0 or +4.8. So the
Cygnus Star Cloud is a last trace of the Milky Way for
many of us to hold onto and - hopefully for all of us -
from which to begin the restoration of dark skies.
As deep-sky pioneer Walter Scott Houston once
noted, under the very best conditions of sky darkness
and transparent atmosphere, the glow of the Cygnus Star
Cloud can break up into individual 7th-magnitude and
even 8th-magnitude stars to the naked eye. But under
common dark skies we actually see the bright stars of
the Swan's pattern reduced in prominence by the Milky
Way glow in which they are immersed. This may be a
key reason why there are fewer myths connected with
Cygnus itself than with many other constellations.
Sailing around even just Cygnus, even just with the
naked eye, brings us to many delightful places. Can
you ferret out the form of the North America Nebula
from the surrounding Milky Way with your naked eye?
Binoculars or ﬁnderscopes can sometimes show the
brightest parts of the Veil Nebula.
The Great Rift of dark clouds of interstellar dust that
splits a long section of the Milky Way begins in Cygnus,
with the Northern Coalsack, southeast of Deneb. But the
patch of darkness in Cygnus that ﬁrst attracted me back
in the 1970s was a deep sideways cut - a mighty indentation in the Milky Way located not far north-northeast
of Deneb. It was only later I learned this narrowing of
the Milky Way's ﬂow is primarily caused by a huge dark
cloud called Le Gentil 3.
Our voyage continues next month. In the next column, we'll continue our journey south through Scutum
and Serpens and through comparisons of the Milky Way
to two rivers of fantasy, the Anduin and the Yann. ✦
Sk yandTelescope.com July 2016

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Sky and Telescope - July 2016

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