The Caldwell Objects

93 & 101
sight. These impressions don't hold together at
high power, though. Rather, high magnification
makes the cluster look like an old windmill with
10 broken arms. Or so I see it. Marilyn Head of
Wellington, New Zealand, sees these bright,
curving arms differently. She calls NGC 6752
"The Starfish," which I rather enjoy, because it fits
well with my "atoll" metaphor.
Just 1° southeast of NGC 6752 is a cluster of
five NGC and IC galaxies: IC 4845, NGC 6769-70-
71, and IC 4842. Another galaxy, IC 4836, lies
about halfway between the globular and these
galaxies. The NGC objects are all about 12th
magnitude, while IC 4845 shines at magnitude
11.6 and IC 4842 at 13th magnitude. These six
galaxies are plotted in the Millennium Star Atlas,
along with others. Can you detect any of them?
Located 4° due south of NGC 6752, one of
the sky's most aesthetically stunning galaxies,
NGC 6744 (Caldwell 101), glows from behind a
rich thicket of Milky Way suns. This enormous
barred-spiral system extends 193,000 light-years
and has a total luminosity of 56 billion Suns. It
belongs to the Pavo-Ara Cloud of galaxies and is
receding from us at 842 km per second.
Photographs taken with large telescopes reveal a
maelstrom of arms clotted with Η II regions.
These arms spiral out from a lens-shaped bar
centered on a fractured inner ring seen 35° from
edge on. The area between the prominent bulge
and the inner ring is virtually devoid of gas and
star-formation activity. The void may be the
result of an orbital resonance, like the gaps in
Saturn's rings. At least a half dozen major spiral
arms project tangen-tially from the inner ring like
the barbs of a feather before winding almost
continuously out to the edge of the disk. Two
arms display especially intense star formation,
though Η II regions pepper all the arms. Maps
made with
374
the Australia Telescope Compact Array of radio
telescopes reveal that the galaxy's invisible
atomic-hydrogen gas resides in a ring underlying
the outer portion of the galaxy's disk. They also
show two spiral arms extending almost 50
percent farther than the visible disk. Astronomers
also have found evidence for tidal interactions
between NGC 6744 and its two companion
galaxies, NGC 6744A and ESO 104-G44.
Alas, NGC 6744 is not a retina burner. It is
relatively bright (magnitude 8.6) as galaxies go,
but its light is spread across 21' of sky. As a
result, its surface brightness is a relatively feeble
13.9 magnitudes per square arcminute. This
means that each square arcminute of the galaxy's
luminous surface shines with roughly the
brightness of Pluto. Dunlop deserves considerable
credit for finding this low-surfacebrightness
object. It is the 262nd object in his
catalog, and he observed it on three evenings. He
described it as a "pretty large very faint nebula,
about 5' or 6' diameter, slightly bright towards
the centre; a minute star is north of the nebula,
and two stars of the 7th mag preceding [to the
west]." John Herschel's descriptions of it —
"pretty bright, round... first very gradually, then
suddenly very much brighter in the middle" and
"resolvable" — make NGC 6744 sound more like
a globular cluster than a galaxy. But, as we have
seen, with his 18/4-inch reflector Herschel was
undoubtedly resolving the galaxy's innumerable
Η II regions, some of which are as large as 5",
and not its individual stars, which would
challenge even HST.
At 23x the 4-inch Genesis shows NGC 6744
as a ghostly pale oval with a bright central pip
burning through a tight, slightly elliptical haze
with an enhanced rim. By concentrating I can
make out a brightening on the southeastern edge
of this rim; it could be a bright row of Η II
regions in the inner ring. A dark region separates
the ring and what appears to be the
Deep-Sky Companions: The Caldwell Objects