The Caldwell Objects

he classified them accordingly. NGC 6231 was
the seventh listing under his Class I, Lum-inosae
("stars visible to the naked eye"). Of course, NGC
6231 is so bright and obvious that skywatchers
throughout time would have known of its
existence, though not necessarily its true nature.
Today we recognize NGC 6231 as one of the
youngest open clusters in the heavens. Its precise
age, however, appears to be a matter of some
debate. Most estimates place its age somewhere
between 3.8 and 6.5 million years, though as
recently as 1999 Argentine astronomer Gustavo
Baume (Astronomical Observatory of La Plata,
Argentina) and his colleagues have found stars as
old as 10 million years that may be associated
with the cluster. Either way, NGC 6231 's stars
are much younger than those of the Pleiades (70,
though some would argue 100, million years).
They form the nucleus of the Scorpius 051
Association, which is noted for its preponderance
of spectroscopi-cally peculiar stars. Indeed, of the
cluster's 93 stars many are luminous O- and Btype
super-giants, including two Wolf-Rayet
stars, 6 Beta Cephei stars, and several Ρ Cygnitype
eruptive
76
variables. The cluster also hosts several eclipsing
binaries. If we accept a distance of 6,000 lightyears
to NGC 6231, the cluster would span nearly
25 light-years of space while the size of the
association would be 10 times greater. Robert
Burnham Jr. notes that if NGC 6231 were as close
to us as the Pleiades are, the two groups would
appear equally large. But NGC 6231 's members
would outshine the individual Pleiads by a factor
of about 50, with the very brightest shining as
brightly as Sirius.
From Hawaii NGC 6231 becomes visible in
twilight, when 3rd-magnitude stars start to
appear. As the sky darkens, the cluster, seen
through 7x35 binoculars, seems to gradually
energize into a ball of shimmering starlight.
Shortly thereafter a nearby asterism of stars
resembling a bent Eiffel Tower appears. This is
part of the aforementioned Scorpius OB1
Association, with Trumpler 24 at the northern
end and NGC 6231 and Zeta 1,2 Scorpii at the
southern end. The two clusters are joined in
celestial longitude by parallel strands of stars,
which, with a little imagination, look like strands
of mozzarella cheese stretching from a slice of
The Caldwell Objects 301