Sky & Telescope - September 2021 - 30

Observer's Sketchbook
Fleming's Triangle,
Simeis 3-188
Western Arc
Southern Blowout Region
Eastern Arc,
Simeis 3-210
VEIL #3 The Southern Blowout Region presents a challenge for observers,
but if you manage to see these delicate features, especially the
Western Arc, you'll be richly rewarded.
[GS55] 222
southwest from the main bubble of the Loop and - another
surprise - is mostly over the border in neighboring Vulpecula.
Fairly straight at its northern end, it divides into two segments
- a bright knot in each branch makes this clear. The main
branch curves almost due west at its southern end, gradually
disappearing before it could merge with the Western Arc.
By far the most visually challenging part of the entire Cygnus
Loop, the Western Arc of the SBR flows mostly northsouth.
I could barely see a couple of wisps with determined
averted vision, so to confirm my observation I slowly scanned
back and forth and noted where the sky background looked a
little brighter. Although quite subtle, those wisps may not be
the only parts of the Western Arc of the SBR that are visible.
At the southern end of the First Veil are two relatively
VEIL #2 The tail of Fleming's Triangle closes in on the First Veil the
farther south one looks. Keeping track of what's what in these far southern
reaches requires concentration.
The Third Veil - Southern Blowout Region
As you reach the southern limit of the tail of Fleming's Triangle,
you come to the Southern Blowout Region (SBR).
This area is faintly involved with Fleming's Triangle and the
Fourth Veil at their wispy southern ends. The brightest part is
the Eastern Arc of the SBR, also designated Simeis 3-210.
To me, this is the most unexpected Veil, and I'm flabbergasted
I never thought to look for it until I started this project - it's
far from being the faintest area of the Loop.
Research first suggested the SBR might be the wisps of a separate
supernova explosion overlapping the Cygnus Loop. But
we now know it to be an area of low interstellar dust density
where the supernova shock wave has simply traveled farther
due to less resistance. The brighter Eastern Arc is a fascinating
series of streamers blended together that extend a degree
30 SEPTEMBER 2021 * SKY & TELESCOPE
bright streaks of nebulosity that point directly at the Western
Arc. I don't know if that means they're all part of the same
arc, but it sure looks like they could be. If so, they would be
the easiest bits of the entire Third Veil to see.
The Fourth Veil - NGC 6974, NGC 6979,
and Assorted Filaments
It didn't strike me until I began sketching NGC 6974 and
NGC 6979 that they're just the brightest areas of nebulosity
that parallel Fleming's Triangle to their west. The more I
sketched, the more I saw, until I realized this extensive feature
is its own Veil. The softly glowing wisps of the Fourth Veil have
enough streamers and patches to connect them into a visually
coherent, separate section of the Loop. It meanders southward
until it nearly connects with the SBR. This Veil is expanding
toward us on the near side of the supernova shock wave bubble.
Make no mistake: Most of the Fourth Veil is challenging to
see and is only accessible to those observing under a truly dark
and transparent sky. Its overall character is of soft, smoothly
curving, and difficult-to-see nebulous streamers. If we could
HOWARD BANICH (2)
Sky & Telescope - September 2021

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