OF THE ROGER N. CLARK

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VISUAL ASTRONOMY OF THE DEEP SKY
A VISUAL ATLAS OF DEEP-SKY OBJECTS
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M 17 (NGC 6618), THE OMEGA NEBULA
AND OPEN CLUSTER IN SAGITTARIUS
RA. IS h 20.9m, Dec. _160 11' (2000.0)
T chnical. M 17 is a beautifully detailed
n:bula in the (Northern Hemisphere) summer
Milky Way between MI6 and M20. The
st side resembles a capital Greek letter
ega. t he brightest portion looks like the
number 2, or perhaps the letter V, or perhaps
a swan, from whence comes the other common
name, Swan Nebula. Ml 7 contains
glowing gas and dark dust, some of which is
cold some warm. The warm dust is heated
by sars embedded in the cloud to the west of
the bright "2". The cold dust is to the east of
the "2". There are at least 35 stars in and
around the nebula, which is roughly 5 000
light-years away. Its estimated mass is about
4 000 suns, and its size, about 27 light-years
acrosS.
Visual. Ml7 shows a wealth of detail in
telescopes large and small. Its total magnitude
is about 6, and its faintest portions extend
45 by 35 arc-minutes. The mean surface
brightness is thus 22.6 magnitudes per square
arc-second, but the "2" is much brighter. Its
base is about 13 arc-minutes long and its
height 8 arc-minutes.
The "2" is detectable in telescopes as small
as 2 inches, while larger telescopes begin to
show additional detail. Through the 8-inch
under moderate to good skies at medium
powers, dark areas were seen crossing the
base of the "2", and at the joint between the
base and hook of the "2" a bright wedge was
easily seen. Inside the hook the sky appears
darker than elsewhere around the nebula.
The detail in the brighter portions was best
seen at lOOX to 200 x. This nebula provides a
truly magnificent view under excellent skies
through medium to large amateur telescopes.
200
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