OF THE ROGER N. CLARK

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VISUAL ASTRONOMY OF THE DEEP SKY
NGC 6960 AND NGC 6992-5 THE VEIL
NEBULA IN CYGNUS
NGC 6960: R.A. 20h 45.7m, Dec. 30° 43'
NGC 6992-5: R.A. 20 h 56.4m, Dec. 3]0 42'
(2000.0)
Technical. The Veil Nebula is a spectacular
supernova remnant showing beautiful, intricate
detail. Observers have given it many
names in attempts to convey its delicate
appearance: the Cirrus, Bridal Veil,
Filamentary, Network, and Loop Nebula.
Each is appropriate.
The nebula, discovered by William Herschel
in 1784, is fully 2.7° in diameter. Modern
investigations have shown it to be expanding
by 0.06 arc-second per year. Spectroscopic
analysis shows its present physical expansion
rate to be 45 kilometers per second. These
two values allow us to compute a distance of
I 300 light-years.
The original expansion rate after the
supernova explosion must have been well
over 1000 kilometers per second. The expansion
has slowed because the gas has been
plowing into interstellar matter. Using a
reasonable estimate of the deceleration rate,
the time since the supernova can be calculated
as about 30 000 years. No remnant star
has been identified, and none may exist. The
energy to light the Veil probably comes from
its ongoing collision with interstellar matter,
a process known as shock excitation.
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Visual. The Veil Nebula is a p : ized object
ng amateur astronomers and IS sure to be
arnO .
a ta .
A VISUAL ATLAS OF DEEP-SKY OBJECTS
rget for medlUm to large telescopes at any
(N rthern Hemisphere) summer star party.
NC 6960 and 6992-5 are arcs each with a
total magnitude of about . NGC 6960 has a
length of about O arc-mlllutes, an average
'dth of .6 arc-mlllutes, and a mean surface
WI .
b ightness of23.2 magmtu d es per square arcs;cond.
NGC 6992-5 has a size of 78 by 8
arc-minutes and a mean surface brightness of
23.6 magnitudes per square arc-second. Bo h
appear brighter than the Merope Nebula III
.
the Pleiades or the outer loop of the Onon
Nebula, M42. They are fainter than the Trifid
Nebula, M20.
The Veil is divided into three parts. The
brightest is NGC 6992-5, which is centered
2.7° northeast of the 4th-magnitude star 52
Cygni. The nex brightest part is NG 6960,
which passes nght next to 52 Cygm. The
th ird and faintest is an unnamed triangular
patch between the brighter two. There are
other, smaller and fainter fragments of the
nebula that have not been reported visually
by amateurs in recent years.
How hard it is to detect the Veil is somewhat
controversial. Some observing manuals
say that NGC 6960 and NGC 6992-5 require
a 6- or 8-inch telescope at low powers under
an excellent sky, and that the triangular
patch cannot be seen visually in any amateur
telescope. Others say the brighter parts are
visible in 7 X 50 binoculars. The reason for
this disparity is unclear. I suspect it lies in
particular observers' experiences trying to
find the objects. Under only moderate skies,
the nebula may indeed be difficult to detect in
an 8-inch. But slight improvements in sky
quality show the Veil's beauty in smaller telescopes.
The drawings of August 12, 1983,
illustrate the view through the 8-inch tele­
Scope under good skies. Only one magnification,
117 x, was tried because clouds interrupted
the observing session. Even so considerable
detail could be seen.
Of NGC 6960, the part north of 52 Cygni
was easy. The edges were sharp, and the
width slowly decreased to a fine point at the
northernmost extreme. The bend in the
nebula 12 arc-minutes north of 52 Cygni was
easily detected, as was a small fork that extended
an arc-minute or so to the northeast.
The part south of the star (actually curving to
the southeast) could not be detected. That
portion becomes very wide and dim. The
glare from 52 Cygni makes the observation
difficult by adding stray light to the field.
NGC 6992-5, the brighter arc, showed
considerable detail through the 8-inch under
good skies. The brightest portion, near the
northern extreme, is NGC 6992. The southern
end becomes broad and looked forked in
the 8-inch. That end is NGC 6995. The surface
brightness varied along the length of the
arc, an indication that if the sky were slightly
better, the individual filaments that make up
the nebula could be resolved.
Under good skies, none of the Veil was
visible in the 2.4-inch, 7.9x finder. But under
skies showing stars only about 0.3 magnitude
fainter, the Veil showed much better. The
change resulting from this slight increase in
sky quality was amazing. Now NGC 6992-5
could be seen through the 2.4-inch finder,
and in the 3-inch finder at 31 X, all three
portions of the Veil were visible! This
observation, on August 26, 1984, was the first
time I had seen the triangular patch.
Under these improved conditions, the 8-
inch showed the faint part of NGC 6960
south of 52 Cygni easily at 117x. At 188X,
the narrow tip at the northern end had sharper
edges than when seen the year before.
NGC 6992-5 contained a wealth of detail. At
the northern end of this arc, in NGC 6992,
individual filaments appeared. Throughout
the Veil, the filaments range from only I to 5
arc-seconds in width. Resolving them demands
excellent skies and at least an 8-inch
telescope.
For the drawing made that night, the best
view was at 188x. At that power only portions
of NGC 6992-5 could be seen at one
time. I had the impression that the filaments
coincided with star chains. In fact, about
two-thirds of the way toward the southern
end I drew a nearly circular loop. This is not
nebulosity but many faint stars that gave that
impression. Farther south, where the nebula's
width increases, several additional arcs
could be seen.
Some amateurs using large telescopes have
reported seeing colors of individual filaments.
The Veil is truly a beautiful structure that
will reveal its intricate splendor to medium
size instruments under good to excellent
skies.
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