The Caldwell Objects

with the naked eye is a challenge. See if you can
spot it. In photographs, a thin black tail flows
several degrees to the southwest from the kite's
dark bottom. Can you see this delicate feature?
Now return to 5th-magnitude BZ Crucis. See
anything unusual? Just 5' northwest is the tiny 6'long
open star cluster NGC 4609 (Caldwell 98),
and directly opposite the star, on the southeast, is
an even smaller open cluster, Hogg 15. James
Dunlop discovered NGC
98 & 99
4609 from Parramatta, New South Wales; it is the
272nd object listed in his catalog. He described it
as "a group of five stars of the 8th or 9th
magnitude, with a great number of extremely
[faint] stars resembling faint nebulae." Brent
Archinal says the 7th-magnitude cluster contains
52 stars; if we accept a distance of 4,200 lightyears
for the cluster, these stars are spread across
7 light-years of space. Of course, that distance
also implies that the cluster lies behind the
Coalsack, and photometry confirms that its
starlight has been considerably reddened by
dust. Also, despite their apparent proximity to
NGC 4609, the stars of Hogg 15 — four of which
can be seen in the 4-inch, forming a small
triangular asterism about 1' across — are not
associated with it. NGC 4609 is a young cluster,
its age being about 60 million years, which makes
it a bit younger than the Pleiades.
A few of NGC 4609's stars can be made out
with averted vision through binoculars, and the
dimmer ones seem to flare off of BZ Crucis like
particles in a dim comet tail. At 23x in the 4-inch
the cluster appears as a tiny splash of about a
half-dozen suns oriented north-northwest to
south-southeast. Sweep your telescope around
this area and the cluster may look like a boat
sailing across long, rolling waves of darkness. At
72x NGC 4609 is a loose gathering of about 20
stars in a rectangle, many in pairs; under averted
vision a dim wedge of faint starlight can be seen
to the northwest. Increasing the magnification
widens the view, making the cluster appear like a
skeleton seen through a dusting of dirt. How
utterly different this cluster is from the lewel Box,
which resides just outside the black fog of the
Coalsack. The contrast between the two is
amazing to behold.
By the way, if you've ever wondered where
the Coalsack got its name, we can thank Sir
The Caldwell Objects 395