The Caldwell Objects

according to theory, the gas density is high
enough to trigger star formation.
Locating NGC 2403 is really not much of a
problem at all. It lies about 7½° northwest of
Omicron (o) Ursae Majoris (Muscida), the Great
Bear's well-known nose, and 1° west of 6thmagnitude
51 Camelopardalis. Under dark skies
the galaxy is immediately obvious in 10x50
binoculars, looking much like a tailless comet
trying to squeeze through a close pair of llthmagnitude
suns. In his Celestial Handbook Robert
Burnham Jr. reminds us that the galaxy is
frequently picked up by comet
hunters; reading between the lines, one infers
that the galaxy is often mistaken for a comet.
Understandably so. Certainly the galaxy's
obscure location and cometlike appearance have
caused more than a few hearts to flutter.
Actually, NGC 2403 may have sent many comet
hunters' hearts aflutter more than once, because
once they learn that they have stumbled upon a
galaxy, how could they not suddenly wonder if
one of the many stars across its face might be a
supernova?
Burnham also points out an obvious discrepancy
in the galaxy's apparent magnitude.
The Caldwell Objects
7
The Shapley-Ames catalog lists NGC 2403's magnitude
as 10.2, which, Burnham says, "seems
definitely too faint, and should be corrected to
magnitude 8.8." Indeed, several sources now list
the galaxy's magnitude as 8.8; others list it as
bright as magnitude 8.4. But I find even these
"new" magnitudes demonstrably too faint. On
the evening of January 26, 1998 (Hawaiian
Standard Time), I estimated the brightness of
Comet Tempel-Tuttle at magnitude 7.3.1 then set
out to find NGC 2403. The galaxy's brightness, it
turned out, looked remarkably similar to the
comet's that night. Interestingly, my entirely
independent estimate for the galaxy's brightness
turned out to be magnitude 7.3; I even used
different stars to gauge the brightnesses of the
comet and the galaxy.
The wide range of values for NGC 2403's
brightness may be a result of the method used to
estimate the galaxy's magnitude. Noted comet
observer Charles Morris and I have
independently developed an unconventional
method for estimating the brightnesses of
comets, or of diffuse objects like galaxies. Now
known as the "Modified-Out Method," the technique
is described in Deep-Sky Companions: The
Messier Objects. (An excellent, comprehensive
review of all methods used in making magnitude
estimates of comets appears in the Guide to
Observing Comets, a special issue of the
International Comet Quarterly that was published
in January 1997). The Modified-Out Method is
most effective when a diffuse object displays a
bright inner region surrounded by an equally
large but fainter outer halo — as NGC 2403 does.
Standard magnitude-estimation methods tend to
diminish the contribution of light from the outer
halos of such objects, making them seem fainter
than they really are.
At 23x in the 4-inch, the galaxy is an extremely
well-defined elliptical glow punctuated
by two 11 th-magnitude stars on either side of
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