OF THE ROGER N. CLARK
OF THE ROGER N. CLARK
OF THE ROGER N. CLARK
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VISUAL ASTRONOMY <strong>OF</strong> <strong>THE</strong> DEEP SKY<br />
--<br />
Mi6 (NGC 6611), <strong>THE</strong> EAGLE NEBULA IN<br />
SERPENS<br />
B. A. IS h IS.Sm, Dec. _13° 47' (2000.0)<br />
A VISUAL ATLAS <strong>OF</strong> DEEP-SKY OBJECTS<br />
Technical. MI6 is another beautiful mixre<br />
of bright and dark nebulosity, not very<br />
:r from M8 and M20. All three lie in the<br />
eneral direction of the center of our Galaxy<br />
g<br />
nd proviae marvelous views to (Northern<br />
a .<br />
Hemisphere) summertIme observers. 16,<br />
however, is fainter and probably farther away<br />
than M8 or M20. Its distance has been estimated<br />
as about 8000 light-years, and the<br />
mass of its gas as about 12 000 times that of<br />
the Sun.<br />
The nebulosity has an embedded star cluster.<br />
Curiously, early observers noticed only<br />
the cluster and not the nebulosity. The cluster<br />
was discovered by P. L. de Cheseaux in<br />
1746. Charles Messier in 1764 mentioned a<br />
weak light surrounding the stars, but he may<br />
have thought it came from unresolved stars.<br />
Later observers such as John Herschel and T.<br />
W. Webb mention only the cluster.<br />
M<br />
Like M8 and M20, MI6 consists of hydrogen<br />
gas that is illuminated and blown about<br />
by newly formed stars near its center.<br />
The expanding gas is encountering dark,<br />
denser clouds, resulting in bright rims that<br />
show intricate' and unusual detail on modern<br />
photographs. The cluster's age is estimated<br />
to be slightly less than 1 million years, with<br />
some stars only 50 000 years old.<br />
Visual. MI6 appears as a loose star cluster<br />
surrounded by a faint nebula. The nebulosity<br />
can be seen in small amateur telescopes,<br />
given dark skies, so it is a real mystery why<br />
early observers failed to report it. Did their<br />
optics scatter so much light that the glow of<br />
the stars hid it<br />
The magnitude of MI6 is given as about<br />
6.5, but this probably includes the integrated<br />
light of the cluster as well as the nebulosity,<br />
so the mean surface brightness of the nebulosity<br />
can't be stated. MI6 is about 25 arcminutes<br />
in apparent diameter. The nebulosity<br />
is more difficult to detect than that of<br />
M20, but much easier than that surrounding<br />
Merope in the Pleiades. Professional photometry<br />
puts the surface brightness of the<br />
Merope Nebula at 21.6 magnitudes per<br />
square arc-second, but the calculated surface<br />
brightness for M20 is 24.4. However M20<br />
appears brighter than the Merope Nebula.<br />
Photolectric photometry needs to be done to<br />
resolve the discrepancy.<br />
Through the 8-inch telescope under good<br />
skies, the general extent of the nebula was<br />
apparent but no small details such as dark<br />
patches could be seen. Even though the nebula<br />
was quite large, a magnification of 117 X<br />
was best for detecting the nebulosity; it filled<br />
the field of view of the Erfle eyepiece, showing<br />
an apparent diameter of about 60°. A power<br />
of 188x was best for detecting faint.stars in<br />
the cluster. In similar skies a 17-inch telescope<br />
showed some dark patches. Under excellent<br />
conditions some dark patches can be<br />
seen in 8- to lO-inch telescopes.<br />
196<br />
197
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