Sky and Telescope - September 2017 - 58

SEPTEMBER 2017 OBSERVING
Going Deep
FURTHER READING: To learn more

u John Herschel,
who sketched this
representation of
the nebula in 1833,
alternated between
the names Omega
and Horseshoe.
What do you see?
North is down,
west is to the left.

Although visually smaller and fainter
than the body, what we see as the gracefully curved head and neck of the swan
is actually the most massive part of the
nebula. The brightest portions look like
a vaporous number 2 to me, and yet this
is the area that inspired the Omega and
Horseshoe names.
The most dramatic part is actually
the black area inside the curve of the
2 - on the best nights it looks almost
impossibly black. This dense portion of
M17's molecular cloud creates a memorably high contrast region with the
bright head and neck curving around it.
On close inspection I was able to
see two faint stars in the foreground of
the darkness - close to its south and
northwest edges. I also noticed a rather
sharp corner on the inside edge near the
top of the 2.
Farther down the neck, and almost
on the border with the black molecular
cloud, shine three rather prominent
stars, which are opposite a star of
comparable brightness right near the
southern end, or top, of the 2. These
are some of the visible stars of the NGC
6618 cluster. Just east of the center of
the three stars is a small, bright knot of
nebulosity that I see only on the most
transparent nights.
Continuing the curve past the top of
the 2 are two subtle, not quite parallel
streaks that ﬂow into a faint haze that
circles back to the base of the 2. This
faint nebulosity completely encloses the
58

dark area inside the curve of the 2 and
reinforces its utter blackness. Also note
the two, equally faint, feathery streaks
pointing south from the head.
Above the 2 is a small puff of nebulosity with a fairly prominent star that
crowns the head of the swan. There are
other puffs of fainter nebulosity surrounding the head, but this is the most
obvious and picturesque.

The Swan's Heart,
Wing & Tail
Extending mostly east-west, the swan's
body is the brightest and most detailed

portion of M17. It's also the most fun
area to observe at various magniﬁcations, especially around the two brightest areas along the southern edge of
the body. The one closest to the swan's
neck and head is shaped much like an
elongated heart.
The next brightest area lies closer to
the tail end of the body and, with
pareidolia now in full force, I can even
conjure up a wing. Boosting magniﬁcation brings out its undulations and
ragged outline, and a thin extension -
the wing tip - curving toward the
southeast. Its shape is reinforced by an
arc of stars along its southern border.
There's a longer, beautifully curved
extension off the eastern end that
gracefully arcs toward the southeast
that makes an excellent tail. It shows up
best with averted vision.
Although we can't see much of the
NGC 6618 cluster, its main concentration of stars is located in the crook
between the swan's neck and body. If
human vision was sensitive to nearinfrared wavelengths, this might be the
most spectacular part of M17.

"2"
Tail
Very dark
molecular cloud

The Wake
Heart

Wing

p In this sketch by the author, the components of M17 have been labeled to match the descriptions in the text. Note how dark the molecular cloud inside the curve of the head and neck
appears, as well as the sharp corner it makes. Can you spot the two arcs of relatively faint stars
highlighted by the dashed lines? These stars reinforce the appearance of the faint haze in this
area, looking as if they've collected the last bit of nebulosity that drained from the darkness inside
the 2. North is down, west is to the left.

S E P T E M B E R 2 0 1 7 * SK Y & TELESCOPE

SKE TCH: HOWA RD BA NICH; HERSCHEL SKE TCH: POPUL AR SCIENCE MONTHLY / WIKIMEDIA COMMONS / PUBLIC DOMAIN

about Messier 17 and view the author's
earliest sketch of the region, see
https://is.gd/BanichM17.

https://www.is.gd/BanichM17

Sky and Telescope - September 2017

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