The Caldwell Objects

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NGC 2506
Type: Open Cluster
Con: Monoceros
RA: 08 h 00.0 m
Dec:-10° 46'
Mag: 7.6
Diam: 12.0'
Dist: 10,000 light-years
Disc: William Herschel, 1791
W. H ERSCHEL: [Observed 23
February 1791] A very com-
pressed and very rich cluster
of stars. The stars are of two
sizes, some considerably large
and the rest next to invisible. The compressed
part 5’ or 6' in diameter. (H VI-37)
GO OUT ON A CLEAR WINTER' S EVENING,
when Sirius is sparkling like fire-fused crystal
high in the south, and sweep your gaze upward
along the winter Milky Way Among its frosty
ribbons of starlight are some of the heavens' most
magnificent open star clusters, including M46
and M47 in Puppis and M48 in Hydra. And
halfway between M46 and M48 lies one of the
region's most understated galactic clusters, an old
salt of the river Milky Way whose core of aged
suns seems like a collection of faded memories.
NGC 2506 is among the oldest open clusters
known. In 1981, Robert McClure (Dominion
Astrophysical Observatory, Canada) and his
colleagues derived an age of 3.4 billion years for
the cluster, making it a contemporary of M67 in
Cancer. More recent studies, however, have
dropped that value to about 2 billion years,
making NGC 2506 closer in age to NGC 752
(Caldwell 28).
Old open clusters are noticeably deficient in
"metals," the term astronomers use to de-
The Caldwell Objects
GC/NGC: Cluster, pretty large, very rich, compressed, stars
of 11th to 20th magnitude.
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scribe all chemical elements heavier than
hydrogen and helium. For instance, on average
each star in M67 has only 82 percent as much iron
as our Sun. Given their roughly comparable ages,
one would expect that NGC 2506 might have a
similar metal abundance. But it doesn't. NGC
2506 is much poorer in metals than M67, with its
average star containing only one-third as much
iron as our Sun. Current models of open-cluster
formation show that the farther a cluster is from
the galactic center, the poorer its metal content. It
is the cluster's position in the galaxy, not its age,
which determines its metal content. NGC 2506
currently lies near the direction of the galactic
anticenter at a relatively high galactic latitude,
and it is nearly 10,000 light-years from the
galactic center as well as 1,600 light-years above
the galactic plane. Together with its position, its
low metal abundance places this cluster into a
class of objects that includes NGC 2420 (the
prototype), NGC 2158, NGC 2204, NGC 2243, and
Melotte 66.
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