The Caldwell Objects

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About half of the stars on NGC 2506's main
sequence are binaries, as are more than 20
percent of the cluster's 800 or so total members.
Some blue stragglers — stars that apparently
linger on the main sequence — also have been
confirmed as members of this cluster. Most
astronomers believe that a blue straggler forms
when two closely orbiting stars merge, producing
an unusually massive single star that has yet to
depart the hydrogen-fusing main-sequence stage
of stellar evolution.
NGC 2506's apparent size of 12' is deceivingly
small. The cluster lies 10,000 light-years
from Earth; that's twice as far as M46 and more
than 5 times the distance of M48. NGC 2506
spans some 37 light-years, making it 16 percent
larger intrinsically than M46 and nearly 50
percent larger than M48. If we could bring NGC
2506 as close as M48, it would shine at 4th
magnitude and span a full degree of sky, making
it a near rival to M44, the famous Beehive Cluster
in Cancer. And if we could bring NGC 2506 as
close as the Beehive Cluster (515 light-years), we
would see a spectacular 1st-magnitude object 4°
across. Poets would have been inspired to glorify
this scintillating fury of grizzled suns, and we
amateurs would have
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had the chance to ponder another cluster as
magnificent as the Pleiades.
We can at least imagine such glory as we
peer at NGC 2506 through our telescopes. To find
it I follow a river of suns that meanders eastward
from Sirius, past 4th-magnitude Gamma (γ)
Canis Majoris and beyond the clusters M46 and
M47, until I get to 5th-mag-nitude 19 Puppis — a
journey of more than 20°. Confirming 19 Puppis's
identity is simple, because the star is
superimposed on the southeastern side of
another dramatic open cluster, 6.5-magnitude
NGC 2539. The star is also the southernmost in a
4°-long chain of three stars pointing to the northnorthwest.
That chain's other two stars shine at
magnitude 6.3 and 6.2, respectively; the
magnitude-6.2 star, at the chain's northern end, is
also a visual double star. You will need to
confirm that you've correctly located these stars
in your finderscope or in binoculars before
carrying on. NGC 2506 makes a near-equilateral
triangle with chain's northernmost two stars, to
the west.
In 7x35 binoculars NGC 2506 looks unimpressive,
almost inconsequential — a celestial
afterthought. But stick with it for a moment,
because with averted vision a bright core should
Deep-Sky Companions: The Caldwell Objects