The Caldwell Objects

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(γ) Velorum marks the southern one. Of course,
NGC 2451 also is obvious to the naked eye in the
northern wing. The challenge, however, is to see
NGC 2477, our magnitude-5.8 target, with the
unaided eye. From Hawaii I find it just visible.
The difficulty is that the cluster lies less than ½°
north-northwest of magnitude-4.5 b Puppis.
(Note that b Puppis is marked as such on the
original Sky Atlas 2000.0 and in Urano-metria
2000.0. However, the star is not labeled in the
second edition of Sky Atlas 2000.0, and the
Millennium Star Atlas tags it as QZ Puppis, an
eclipsing binary with a period somewhere
between 1 and 9 days.) The cluster also is large
(nearly 70 percent as wide as the full Moon) and
has a moderate surface brightness.
I find the best way to see NGC 2477 with the
naked eye is first to take several deep breaths,
then to stare at b Puppis with direct vision while
using averted vision to spy the cluster. This is
harder than it sounds. It takes time. After several
tries, though, NGC 2477 should pop into view I
have no success when gazing directly at the
cluster.
NGC 2477 has long been one of my all-time
favorite binocular objects because its compact
form appears strikingly like that of a tailless
comet. The view at 23x in the 4-inch, however, is
absolutely gorgeous. I immediately fell in love
with the telescopic view of NGC 2477 because
even at low power the open cluster looks like a
finely resolved globular. In his 1930 book Star
Clusters, Harvard College Observatory
astronomer Harlow Shapley wrote that NGC
2477 is "the richest of galactic clusters; or perhaps
it is the loosest of globular clusters," a description
that recalls M71 in Sagitta. But unlike M71, which
is a true globular, NGC 2477 is an open or
galactic cluster with a paltry radial velocity of
about six km per second. It's a shame we cannot
simultaneously compare and contrast NGC 2477
and M71
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under a dark sky from northern sites, but from
temperate southern latitudes the two clusters do
appear very briefly just above opposing horizons
after sunset in November. As NGC 2477 rises in
the east, M71 starts to set in the west — just like
the Great Hunter, Orion, and his nemesis,
Scorpius.
Brent Archinal says that NGC 2477 has 1,911
known members spread across 22 light-years of
space. Since the cluster is seen against a rich
Milky Way background, it is difficult to separate
its stars from others in the field. The cluster's
brightest member shines at magnitude 12, and
many others are not much dimmer. NGC 2477
has an estimated 4,400 solar masses and a metal
abundance like our Sun's. In 1995 the Hubble
Space Telescope was used to image stars as faint
as 26th magnitude in the cluster. A study of these
stars suggests that the cluster is 1.3 billion years
old, about twice as old as the Hyades (Caldwell
41). The Hubble images also revealed four whitedwarf
stars, and it's believed that perhaps 120
remain to be discovered. The fraction of a
cluster's total mass that is stored in faint white
dwarfs is important for determining the
evolution of star clusters as well as the masses of
our
Deep-Sky Companions: The Caldwell Objects