The Caldwell Objects

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3 9
Eskimo Nebula; Clown-Face Nebula; Lion Nebula
NGC 2392
Type: Planetary Nebula
Con: Gemini
RA: 07 h 29 m 10.8 s
Dec: +20° 54' 43"
Mag: 9.2
Dim: 47" x 43"
Dist: ~4,000 light-years
Disc: William Herschel, 1787
W. H ERSCHEL: [Observed 17 January 1787] A star of 9th
magnitude with a pretty bright milky nebulosity all around. A
very remarkable phenomenon. (H IV-45)
GC/NGC: Bright, small in angular size, round, a star of 9th
magnitude, a star of 8th magnitude north following [to the
northeast] 100".
T HROUGH SMALL TELESCOPES MOST planetary
nebulae look like pale shells or symmetrical
patches of light. Their enchanting central stars —
whose ionizing ultraviolet rays energize the
nebulae — usually lie just beyond the grasp of
normal vision. But NGC 2392 in Gemini is an
exception. Not only is it bright, being a mere 0.4
magnitude fainter than the famous Ring Nebula
(M57) in Lyra, but its central star all but
dominates the view. Actually it is somewhat
misleading to say that NGC 2392, or any other
planetary nebula for that matter, is "bright";
unlike every other type of deep-sky object, no
planetary nebula is readily accessible to the
unaided eye. NGC 2392 is unquestionably a
telescopic wonder, however. Its singular
appearance compelled the great William
Herschel to call it "a very remarkable phenomenon."
His son John examined the nebula in
an 18¼-inch reflector and agreed; he saw it as an
8th-magnitude star "exactly in the center of an
exactly round bright atmosphere." His
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use of the word "atmosphere" was appropriate
for the times. In 1755 the Prussian philosopher
and physicist Immanuel Kant hypothesized that
the Sun and planets were born from a rotating
cloud of gas and dust. By the 19th century, when
John Herschel was turning his attention to his
father's discoveries, geologists were using Kant's
nebular hypothesis to explain the formation of
the Earth. In his 1914 work The Problem of
Volcanism, which devotes a chapter to nebular
hypotheses, Joseph Iddings explains that
geologists of the 19th century conceived a nebula
as a "homogeneous gas revolving about a central
nucleus and separating into annular zones that
were supposed to condense into the planets and
satellites of the solar system." Any views they
might have had of NGC 2392, with its multiple
shells and bright central core, certainly would
have excited them. Of course, Kant was right in
his thinking, as we know today, but the evidence
that would ultimately verify his theory would
Deep-Sky Companions: The Caldwell Objects