The Caldwell Objects

33 & 34
to see such color in dim nebulosity, but aperture
certainly helps, as does a narrowband filter
favoring the nebula's fine-tuned emissions.
Overall, the nebula's structure reminded me of
coils of DNA.
Of course, to the peering eye of the Hubble
Space Telescope even a tiny section of the Veil
Nebula is inconceivably intricate, looking like a
close-up photograph of colored smoke rising
from an unseen cigarette. HST imaged an area
where the supernova blast wave recently hit a
cloud of denser-than-average interstellar gas.
After comparing the HST image to a 1953
Palomar Observatory photograph, two Johns
Hopkins University scientists recently concluded
that the Veil is a good deal younger (about 5,000
years old) and closer to Earth (about 1,400 lightyears)
than widely believed. However, these
revisions depend on a still-incomplete
understanding of shock-wave propagation in the
interstellar medium, and they have yet to gain
universal acceptance.
Once you have contemplated the Veil Nebula's
details to your eye's delight, return to low
power and try to glimpse two fainter wisps
inside the main loop. The westernmost wisp is
the more prominent of the two. Look for a triangular
"head" on the north end of this tadpolelike
feature. Can you see the tadpole's "tail,"
at the southern end, curve northward? From a
dark site, the entire inner arc is prominent
enough to fool an unwary observer into believing
that he or she has seen the Veil's easternmost
segment. The perceived dimensions of these
fainter, enclosed features are tentative because
they tend to follow star streams, and it is hard to
judge where the nebulosity ends and the star
streams begin.
Like you, no doubt, I have wondered how
the various names associated with this nebula
came about. None of the names appear to be
associated with pre-photographic (i.e., purely
134
visual) descriptions. William Herschel wrote of
"branching nebulosity" that "divides into several
streams," while the Rev. T W Webb . simply refers
to NGC 6992 as a "large nebulosity in a curve"
and to NGC 6960 as a "nebulous ray." The terms
"Network Nebula" and "Filamentary Nebula,"
however, were in use at the beginning of the 20th
century. They ostensibly describe the
appearances of the eastern and western
segments, respectively, on high-resolution photographic
plates. Writing about the "Nebular
Hypothesis" in his 1914 work The Problem of
Volcanism, Joseph R Iddings discusses several
"[c]louds of luminous matter. . . separated by
rifts of darkness whose character is unknown."
Among them is "another great cloud-like nebula
of this kind . . . the 'network' nebula in Cygnus."
His use of quotation marks reveals that "Network
Nebula" was already in use by the 1910s. Iddings
goes on to describe the nebula as "a cirrus cloud
of inconceivable extent." Iddings, then, might
have been the first person to associate the nebula
with Cirrus, another occasionally invoked
moniker. In his popular 1938 work Astronomy,
Forest Ray Moulton describes the nebula's
appearance on a wide-field plate by Lowell
Observatory astronomer Wilbur A. Cogshall as
"Veil-like"; his may have been the earliest use of
that term. Indeed, by 1957 the "Veil" was the
common name used by Bart Bok for NGC 6960.
"Cygnus Loop" was bestowed upon the nebula
after Rudolph Minkowski photographed the
entire supernova remnant with the 48-inch
Schmidt camera on Palomar Mountain. The
names, then, appear to follow progress in
astrophotography.
By the way, supernovae occur in our galaxy
roughly once every hundred years on average,
which suggests that we are well overdue for
another. So keep your eyes peeled to the night
sky, or, as Jack Horkheimer, TV's Star Gazer,
would say, "Keep looking up!"
Deep-Sky Companions: The Caldwell Objects