The Caldwell Objects

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ciety-Palomar Observatory Sky Survey prints. This
rarefied bubble is centered on the Tau Canis
Majoris Cluster and is excited both by it and by
29 Canis Majoris. Lada believes the cloud is the
remains of the cluster's natal cocoon, and that it
has been blown away by radiation pressure from
the cluster's hot, young stars. But that does not
mean, he continues, that the nebula should be
given an NGC designation; nor should
Trumpler's original work be altered to note the
nebula's presence. Such revisions only pervert
the histories of interesting objects like these.
In a 1978 Astrophysical Journal paper, Lada
described another interesting feature of the
bubble. Along its edge are a number of isolated
dark clouds and one bright one at the tip of
which lies the peculiar variable star VY Canis
Majoris 1¼° southeast of Tau. VY Canis Majoris
lies at the same distance from Earth as the Tau
Canis Majoris cluster and appears to be associated
with a vast complex of two colliding
molecular clouds. A 10' x 5' reflection nebula,
vdB 96, lies about 1° north-northeast of Tau Canis
Majoris; it may also be illuminated by Tau Canis
Majoris and 29 Canis Majoris as well as by the
chain of three 9th-magnitude stars embedded
within it.
Brent Archinal's database has 60 members in
NGC 2362 spread across 6' (9 light-years). But
that number increases substantially if we take
into account the several hundred pre-mainsequence
stars that the Rosat X-ray satellite
revealed in the cluster. The cluster's crown
"jewel," of course, is Tau Canis Majoris itself.
With a spectral type of 091b, the magnitude-4.4
star is now known to be a multiple-star system
with five components, none of which can be
resolved by amateur instruments. The system's
core is a close binary, with two stars of nearly
equal brightness separated by 0.16". NGC 2362
also contains three short-period var-
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iables; one of them may be the hottest Delta (δ)
Scuti star known. Delta Scuti stars are relatively
young and hot pulsating variable stars with
about 1.5 to 3 solar masses. Though numerous,
these stars are difficult to detect because of their
very small brightness variations (typically about
0.01 to 0.4 magnitude, or 1 to 4 percent).
If the star Tau Canis Majoris looks fuzzy to
the naked eye, it's because of 5th-magnitude 29
Canis Majoris, which lies a mere 25' north of it.
Our eyes tend to make close pairs of stars look
fuzzy, but the fuzziness is just an optical illusion.
For those with really young eyes, here is a
fantastic test: can you resolve the magni-tude-7.3
star about 7' east of Tau Canis Majoris? The rest
of the cluster's members are so tightly packed,
and Tau Canis Majoris's brilliance so
overpowering, that small-binocular users may be
challenged to see any of the cluster's other
members. If you do find yourself under a dark
sky with binoculars, take a moment to sweep
them around this area. The part of the Milky Way
that runs through the region is laced with ribbons
and arcs of dark nebulosity.
I've given this cluster another nickname: the
Liquid Pyramid. Here's why: at 23x in the 4-
Deep-Sky Companions: The Caldwell Objects