The Caldwell Objects

On four plates taken with the Bruce 16-inch
telescope in September I find an extended
nebula, which attracts attention both by its
situation and its appearance.
Practically all extended nebulosities are
found in or near the Milky-way, but this
object is about 70° distant from the plane of
the Milky-way. Its densest part is situated at
[RA] = 0 h 57 m .4 [Dec] = + 1° 20' (1855.0)
Round this is spread faint nebulous matter
of varying intensity. At several points small
clouds of greater density shine forth.
Further out, the intensity is so feeble that it
is impossible to exactly define its limits. It
seems that these will be materially altered
by lengthening the exposure. My exposures
extend to four hours. With these the nebulous
cloud extends about 40' in declination
and about 30' in R.A. [Three] B.D. stars
[whose positions are given here] are all
involved in the cloud. Southwards it reaches
nearly to the B.D. star
+ 0°.207 0h56m.9 +1° 0'
but it cannot be certainly detected up to the
star. Especially towards the west the nebulous
matter promises to go much further.
Under the microscope the brighter parts of
the mass are filled with numerous very
small spots and short trails, so that the
appearance is very different from that presented
by the Milky-way nebulae. It seems
possible that this object is a multitude of
very small planetary nebulae collected in a
cluster, which a more powerful instrument
than mine may perhaps resolve.
Astrophysical Observatory, Heidelberg: 1906
October.
Archinal updated to equinox 2000.0 the
positional information supplied by Wolf and
The Caldwell Objects
51
discovered that Wolf's position for the galaxy
"was off." He also noticed that the size listed in
table on page 202 (18.8' x 17.3') is noticeably
smaller than Wolf's estimate of 40' x 20'. "Did
Wolf's exposure go so deep that it showed outer
regions not indicated by the size in modern catalogues?"
Archinal asks. "Is that why his position
is off? It is also interesting that [Wolf] describes
what are (now) obviously lots of Η II regions
within the galaxy."
That Messier and the Herschels failed to spot
this loose assemblage of dim suns is not
surprising. Although IC 1613 shines at 9th
magnitude, its integrated brightness is deceiving.
The galaxy's light is spread across an area of sky
about the size of a quarter Moon, so each square
arcminute of its surface shines with an apparent
magnitude of 15.5 — more than six times fainter
than Pluto's maximum brightness. This breath of
extragalactic light was far beyond the visual
grasp of Messier and his contemporaries, and it
was too dim and large for the powerful but
narrow-field Herschel telescopes. Many amateur
astronomy guides pass over IC 1613, claiming it
is too dim and difficult for amateur telescopes,
but this is not necessarily the case. Before delving
into that controversy, let's first look at the
wonder of this tiny system, which astronomers
often refer to as the dimmest member of our
Local Group of galaxies.
IC 1613 lies 2.3 million light-years distant, so
it is as far as M31, the Great Andromeda galaxy.
It measures 12,400 light-years across, which is
comparable to the sizes of NGC 147 and 185
(Caldwell 17 and 18), two of the dwarfs
attending M31. But the Andromeda dwarfs are
much easier than IC 1613 to see, because they
have higher surface brightnesses. IC 1613 has the
intrinsic luminosity of a mere 80 million Suns,
making it one of the least luminous galaxies
known. Like the Andromeda dwarfs, IC
203