The Caldwell Objects

stars"). Lacaille also likened NGC 4833's appearance
to that of a small, faint comet. John
Herschel placed NGC 4833 in his Class VII,
deeming it a very loose and sparse cluster.
NGC 4833 is easy to find. It lies ¾° northwest
of 3.6-magnitude Delta (δ) Muscae, and there is
no mistaking its glow. But how big and bright is
that glow? Well, that's a matter of some debate,
and I wish I'd known as much before I went to
New Zealand! Here's the present — and still
unresolved (no pun intended!) — situation:
The Deep Sky Field Guide to Uranometria
2000.0 lists NGC 4833's magnitude as 7.0 and its
diameter as 13.5'. Ernst Hartung fails to record
the cluster's magnitude but says it's a "fairly
compact globular cluster with small outliers to a
diameter of about 4'." The original Caldwell
Catalog lists a magnitude of 7.3 and a diameter of
14.0'. Burnham's Celestial Handbook offers a
magnitude of 8.5 and a diameter of 6'. In his
globular-cluster database Brian A. Skiff gives a
magnitude of 8.5 but no diameter. The
professional literature is just as confusing. "The
[cluster's] total magnitude," Brent Archinal
explains, "apparently is poorly determined, given
the range of values in the professional papers of
6.80 to 8.35. However, this is mostly dependent
on what diameter was chosen for this total
magnitude. The cluster has a fairly obvious 'halflight'
core that extends to a diameter of about 5'
with outer halo stars out to at least about 10' on
the Digital Sky Survey. Thus the 13.5' Deep Sky
Field Guide value and 14.0' Caldwell Catalog
values seem a little large, especially for visual
observers." For better or worse, the values I've
tabulated on page 417 come from the Catalog of
Parameters for Milky Way Globular Clusters that
McMaster University astronomer William E.
Harris compiled in 1999, and from the Deep Sky
Field Guide to Uranometria 2000.0.
The Caldwell Objects
105 & 108
Adopting an angular diameter of 13.5' and a
distance of 19,600 light-years implies that the
cluster's physical diameter is 77 light-years,
making it 1.7 times smaller than M13 and 1.3
times smaller than M92. NGC 4833's colormagnitude
diagram places it in a class of
extremely metal-deficient globular clusters, like
M92. The cluster's stars are metal-poor because
they formed about 15 billion years ago from gas
that contained only a small sprinkling of heavy
chemical elements. The typical star in NGC 4833
contains only about 1/60 as much iron (per unit
of hydrogen) as there is in our Sun.
NGC 4833's brightest star shines at magnitude
12.4, and its horizontal-branch magnitude is
15.5, meaning the cluster can be resolved in
moderate-sized amateur telescopes but
challenges smaller instruments like my Genesis. I
spotted NGC 4833 in 10x50 binoculars from citybound
Auckland Observatory, under the light of
a waxing gibbous Moon. It lies just 15' south of
an 8th-magni-tude star. In the binoculars the
cluster's core appeared to possess a stellar
radiance. But through the 4½-inch finderscope on
the observatory's 20-inch reflector I discovered
that this was an illusion created by a 9thmagnitude
star superposed on the cluster's
envelope, only about 2.5' north of the compressed
center. In the 4½-inch the cluster's core looks
layered. The brightest inner part measures about
2' and appears distinctly square. The square is
lined by moderately bright stars, making the core
look rather hollow (save for a small gathering of
about four tightly packed "stars" — really
bunches of suns — in the very center). A dark
lane running east-west separates the box from a
thin, similarly sized bar of stars to the north. The
rest of the inner core sweeps out from this boxy
body in four magnificent loops, to the northeast,
northwest, southwest, and
419