The Caldwell Objects

76
76
False Comet Cluster
NGC 6231 Type:
Open Cluster
Con: Scorpius
RA: 16 h 54.2 m
Dec:-41° 50'
Mag: 2.6
Diam: 14.0'
Dist: 6,000 light-years
Disc: Giovanni Batista
Hodierna, before 1654; but
known since antiquity
J. HERSCHEL: A fine bright large cluster pretty rich,
class VII. 10', stars [from] 10[th to] 13th
magnitude]. Place of a double star 5th
magnitude], the preceding [western] but one of 7
H AD THE 18TH- CENTURY F RENCH COMET
hunter Charles Messier lived farther south than
Paris he most certainly would have noticed the
bright 2°-long "comet" that perpetually shines in
the Scorpion's Tail. I'm confident Messier would
have noticed it, because my wife, Donna, never
fails to discover the same "comet" each time she
looks upon the stunning Scorpius Milky Way
with her unaided eyes from Hawaii. Of course,
the object is not a comet, but it is the most
cometlike naked-eye spectacle in the heavens that
is not a comet. Open cluster NGC 6231 (Caldwell
76) forms the "head" of our pseudocomet and lies
only 30' north of the 4th-magnitude double star
Zeta 1,2 (ζ 1,2 ) Scorpii; the comet's "tail" is the large,
elongated open cluster Trumpler 24 to the north.
Although these objects have separate catalog
identities, they belong to the same group of
luminous stars, the Scorpius OB1 Association.
This widely scattered family of young stars
marks
300
bright stars in the middle, (h 3652)
GC / NGC: Cluster, bright, considerably large, pretty rich,
consisting of stars of 10th to 13th magnitude.
the onetime location of a giant Η II region. It
resides in the Sagittarius-Carina spiral arm,
which lies closer to the galactic center than the
one containing our Sun. Both of these clusters
would have been appropriate for Messier's catalog
had he known of their existence.
Which brings us to the next point. Most
sources credit Nicolas Louis de Lacaille for discovering
NGC 6231 during his exploration of the
southern skies from the Cape of Good Hope in
the 1750s. But true credit belongs to long-forgotten
Giovanni Batista Hodierna, an astronomer
at the court of the Duke of Montechiaro, who
compiled a catalog of nebulous objects that he
observed with a simple 20x Galilean refractor.
Printed in Palermo in 1654, his work listed 40
nebulous objects, 19 of which (NGC 6231
included) were of his own discovery Since
Galileo had resolved the misty Milky Way into
stars, Hodierna believed that all the heavens'
nebulous objects were star clusters, and
Deep-Sky Companions: The Caldwell Objects