The Caldwell Objects

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fog. I could see it in the 4-inch, but with difficulty,
because the region's clustered starlight
forms its own pseudohaze. I find a combination
of keen averted vision, tube tapping, and backand-forth
sweeping helps to pull the nebula out
from the background noise. IC 4628 is actually
the brightest part of a 4°-wide circular loop,
discovered by Colin Gum in 1952 and
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centered on NGC 6231. Astronomers once believed
that this loop was a supernova remnant,
but in 1971 David Crampton and Andrew David
Thackeray revealed that the nebula is a normal Η
II region. As Burnham adds, it is "one of the great
Ή ΙΓ regions which we often see in the external
galaxies, outlining the course of the spiral arms
like so many 'pearls on a string.'"
Deep-Sky Companions: The Caldwell Objects