This
image was created via color synthesis of Red & Blue Digital
Sky Survey images. North is down and east is to the right, as
typically seen in a telescope.
For many years the
Crescent nebula was on my list of objects that I wanted to see,
but it took some time to finally detect it. Catalogs list it as 10th magnitude, but I found it to be much more
difficult than this would imply. I tried and tried one year to see
the Crescent in my 6-inch to no avail. When I finished my
18-inch Dob, this was my first serious target. But I was
rather disappointed when I failed to see the nebula with my big new
telescope on several different nights.
One night I was very careful with my dark
adaptation and the conditions were
quite good. Only then was I finally able to discern the nebula,
which makes an arc
among several bright stars. I was able to detect the
nebulosity on the northeast side first. Eventually I found
the curving haze to the southwest. With time and averted
vision I was able to make out the entire arc of the
nebula. But why was it so difficult?
It turned out that I was
not using the optimum eyepiece. Later on I obtained an
eyepiece with a wider
field and it made a big difference. The elongated white area near
the center-bottom of the image at the top of the page was clearly visible
and much
as it appears in the image.
Filters made an even bigger
difference. With a UHC filter the rest of the nebula
appeared, looking very much like the image (without the
colors). The best view was with an OIII filter which
greatly increased the contrast. With the OIII in place the
faint wisps in the center appeared, filling the nebula into a
complete "pear."
The
circle represents the simulated eyepiece view in a 12-inch (30 cm)
LX200 at 76x.
The
Crescent nebula (aka The Euro Sign Nebula and NGC 6888) is a
faint, arcing diffuse
nebula that appears in one of the most beautiful star fields of the summer
Milky Way, about 2/5th of the way from Gamma Cygni to Eta Cygni.
It was discovered by William
Herschel in 1792 using his 18.7-inch telescope, when he noted it
as H IV-72. It was designated NGC 6888 when the NGC catalog was
compiled. The Dreyer summary reads, "Faint, very large, very much extended, double star
attached."
The double star referred to is HD 192182,
which is the bright star at the bottom-middle of the picture
above. This is the double star pair STT 401, which was also
discovered by Herschel. It consists of a 7th magnitude star with a
10th magnitude companion 12.9 arc seconds away.
Observing
the Crescent Nebula for Yourself
In
smaller instruments this is a challenging object to
detect. A wide-field eyepiece is a must, and a UHC or OIII filter
may be necessary to view this nebula in scopes smaller
than 8-10 inches. AJ Crayon writes, "In my 8 inch f6
at 60X the nebula was about 15'X5' in a northeasterly PA
with a larger northeastern side. On this particular
night the nebula was of uniform brightness and the famous
crescent shape appeared pear like with several stars
involved. Yes, pear like!"
In 16-inch (41 cm) or
larger telescopes the Crescent becomes a
showpiece, particularly when an OIII filter is
employed.
Look for the Crescent in
Cygnus, which is well placed in the evening in September.
The diagram in the right shows Telrad circles centered at
the location of the nebula. There is a link to a printable
finder chart below.
If you are not going to
use a filter, a dark site and a dark night are essential.
You also need to be fully dark adapted. I often use an eye patch
over my observing eye so that the light from my
red-covered tablet or red flashlight don't affect my night
vision. Red filters help, but they don't fully protect
your eyes. Remember, it takes about 30 minutes for your
eye to become fully adapted to the dark.
If not immediately
obvious, the trick to finding the nebula is to identify
the four brighter stars that form a parallelogram near the
northern end (bottom in the pictures here) of the nebula.
Having good charts that match the orientation, scale, and
magnitude limit of your telescope can really help. The
southern-most star of the parallelogram is the Wolf-Rayet
star that is responsible for the nebula, and the northern-most
is the double discovered by Herschel. When you find the
double, look to the northeast for the brightest portion of
the nebula. Once you can spot the bright portion, try
tracing it to the southwest of the pair, and finally, look
for the fainter parts of the C-shaped nebula.
Finder
Chart (pdf)
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The
Science of The Crescent Nebula
At the heart of this
nebula is the Wolf-Rayet star WR 136 (HD 192163). It
was once a swollen Red Giant star, gently puffing its outer layers
into space. Now having evolved into a Wolf-Rayet star, it has a
fast and strong stellar wind, which is overcoming the older
layers. As the gasses collide they are heated by shock waves,
ultimately causing the gas to glow.
T.A.
Rector (NRAO/AUI/NSF and NOAO/AURA/NSF)
The above image is a combination of three
filters, isolating the emission lines of Hydrogen H-alpha (red), Oxygen
OIII (blue) and Sulfur SII
(yellow). So the red traces hydrogen gas, blue traces oxygen, and
yellow traces the (mostly) background sulfur.
When we use an OIII filter visually, we are
isolating the oxygen emission line. The emission line is a single
color, so we can make a filter that passes only the OIII color of
light. The background skyglow covers a wide range of colors, so we
only pass a small portion of it through the filter. As a result,
the contrast between the nebula and the sky is greatly improved.
Sadly, the H-alpha line is too far into the infrared for our eyes
to easily see, or it would be another good choice for improving
the visibility of emission nebulae in the eyepiece.
SkyTools 3 was used in the preparation of
this article.
Greg Crinklaw — Developer of
SkyTools
SkyTools
3, because the astronomy matters.
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