70 ASTRONOMY • SEPTEMBER 2019
SECRET SKY
Crisp September nights
often bring high-contrast
skies, which allows us to
look up and see darkness well.
That may sound paradoxical, but
we need bold swaths of starlight to
see the “deep wells” of darkness that
mar the Milky Way like celestial
graffiti. Mariners of the 16th cen-
t u r y c a l led t hem “coa lsack s”; tod ay,
we c ategor i z e t hem a s d a rk nebu lae.
Let’s explore some prominent exam-
ples in Cygnus.
The Northern Coalsack is an
el l ipt ic a l spla sh of d a rk ne s s bet we en
Deneb (Alpha [α] Cygni), Sadr
(Ga mma [γ] Cyg), and Epsilon (ε)
Cyg. Despite its popularity, the Northern Coalsack is
not particularly obvious to unaided eyes for two rea-
sons. First, it is large (7° by 5°), and second, it is not fully
su r rou nded by br ig ht patche s of Mi l k y Way. I’ l l ret u r n
to this later.
To target a smaller object, slip over the North America
Nebula (NGC 7000) and look for Barnard 352,
which sits slight ly less t han 3° east-nor t heast
of Deneb. On a globe, this 20' by 10' dark
cloud would lie at about the position of the
Northwest Passages above Hudson Bay. Try
to spy it first through binoculars. If that’s too
difficult, use a telescope at low power.
Arguably the best Cygnus coalsack is
Le Gentil 3, sometimes called the Northern
Inkspot. You’ll find it 8° north-northeast of
Deneb. French astronomer Guillaume Le
Gentil recorded this 5°-wide naked-eye
cloud in 1749, noting that it “seems opaque
and very dark.” And while Le Gentil 3 is as large as the
Northern Coalsack, the Milky Way surrounding it is
more uniform in intensity, boosting its contrast and
enhancing the darkness.
When the 19th-century French astronomer Étienne
Léopold Trouvelot drew the naked-eye Milky Way, he
wrote, “It enters Cygnus, where it becomes very com-
plicated and bright, and where several large cloudy
masses are seen terminating its left branch, which
passes to the right, near the bright star Deneb, the leaderof this constellation.” In his drawing, this terminating
branch is separated by a dark gap at the position of Le
G ent i l 3. Most tel l i ng is t hat he doe s not d r aw (or fa i led
to notice) the dark Northern Coalsack.
Now use your scope and low power to look roughly
midway between the North America Nebula and open
cluster M39 for B361, which I call the Little Cygnus
I n k spot. It l ie s about ¼ ̊ we st of t he 9 t h-ma g n it ude open
cluster IC 1369. Through a 4-inch scope under a dark
sky, the 20' well of darkness stands out prominently
against the surrounding Milky Way. The view is
enhanced by the star cluster’s presence, which is only
half the dark nebula’s size.
We’ll end this survey with B168, one of
the northern sky’s most visually stunning
dark nebulae through binoculars and tele-
scopes. It’s also a great naked-eye challenge.
The Cocoon Nebula (IC 5146) lies in a pool
of darkness at the southeast end of this
nearly 2°-long stream of darkness. Look just
south of Pi^2 (π^2 ) Cygni, where the Milky Way
appears tangled in a cobweb of naked-eye
da rk nebu lae. It is out of t his web t hat a sur-
geon’s cut of darkness trickles like blood into
the pool surrounding the Cocoon. It’s dif-
ficult to see this dark stream without optical aid because
it is so narrow. But try sweeping your gaze up and down
and left to right for a few minutes and see if you catch
glimpses of it. If not, binoculars will show it clearly
slicing across the Milky Way.
As always, share what you see and don’t see at
[email protected].Turn your gaze to some of the darker regions of our universe.
The coalsacks
of Cygnus
ABOVE: Barnard 361
is relatively easy to
spot because of the
dense Milky Way
background in its
region. DIGITIZED SKY SURVEYTOP RIGHT: This
illustration from The
Trouvelot Astronomical
Drawings (Charles
Scribner’s Sons, 1881)
shows how the author
of that work saw the
naked-eye appearance
of the Northern
Coalsack and
Le Gentil 3.Arguably the
best Cygnus
coalsack is
Le Gentil 3,
sometimes
called the
Northern
Inkspot.
BY STEPHEN
JAMES O’MEARA
Stephen is a globe-
trotting observer who
is always looking
for the next great
celestial event.BROWSE THE “SECRET SKY” ARCHIVE AT
http://www.Astronomy.com/OMearaST
EPHEN^ JAMESO
'M
EARA