The Caldwell Objects

star in a large naked-eye triangle of similarly
bright stars; the other two members are Lambda
(λ) and Zeta (ζ) Geminorum. Delta Geminorum is
also the northernmost star in a small kite-shaped
asterism that also contains the 5th-magnitude
stars 56, 61, and 63 Geminorum (itself a wide
double). NGC 2392 is just 40' southeast of 63
Geminorum, the easternmost star in the "kite." I
generally draw a line between 61 and 63
Geminorum and point my telescope's sighting
device just east of the halfway point (but a little
closer to 63 Geminorum). At low power NGC
2392 will be in the field, looking like the southern
member of a tight pair of 8th-magnitude stars
separated by 1.6'. It's that simple. To be sure
you're on target, use averted vision, and the
planetary should appear to swell.
How did Charles Messier miss this spectacular
object during his comet hunts? The
answer is simple. NGC 2392 appears so nearly
stellar at low power that it would not be difficult
to sweep past it while looking for a diffuse
object. In fact, I tried sweeping up the nebula at
23x with the 4-inch, as if I were Messier, and
never once suspected its presence. And when I
pointed the telescope directly at the Eskimo I had
to use 72x to verify that I was seeing a nebula.
Once I returned to 23x with that knowledge in
mind, the nebula stood out as the "bloated star"
noted by Smyth. Remember,
39
too, that the planetary lies close to a star of
similar magnitude. Any fuzziness Messier might
have spotted here could also have been
dismissed as an illusion. (Just such an illusion is
posed by the double star M40 in Ursa Major;
Johannes Hevelius mistook it for a nebula in the
17th century.)
The Rev. T. W Webb . penned something a
little more entertaining about NGC 2392. "I was
so much surprised at the result in my inferior
telescope," he wrote, "that I cannot help
supposing some temporary impediment to
distinct vision at Bedford, for on coming accidentally
across it in 1850 I found such a conspicuous
nebulosity that I thought it was either
damp on the eye lens or a telescopic comet."
Webb's suspicions were understandable but
unwarranted. With some effort it's not difficult
for the users of even the smallest telescopes to see
in NGC 2392 two obvious concentric shells of
debris surrounding a tack-sharp central star — a
type-O7 white dwarf, which may have a hot
unseen companion. The central star's brightness
seems a tad uncertain, since apparent
magnitudes have ranged from 9 to 10.5 in the
modern literature.
If you plan to seek NGC 2392 with binoculars
or a very low-power telescope, beware: you
may think you've seen the nebula because it
appears slightly swollen compared to other field
stars. But this may be a "false positive"
identification. The nebula lies 1.6' south of a
magnitude-8.5 star, and in binoculars you almost
certainly will perceive the combined light of both
objects. (I could just split the two with 10x50
binoculars, but their apparent separation was too
small for me to reliably estimate the nebula's
brightness with such a small aperture.) Note that
our photo on page 154 shows the magnitude-8.5
star. In January 2001 1 estimated NGC 2392's
brightness with the 4-inch refractor at 23x and
got magnitude 9.2, which
The Caldwell Objects 157