The Caldwell Objects

89
S Normae Cluster
NGC 6087
Type: Open Cluster
Con: Norma
RA: 16 h 18.9 m
Dec:-57° 54'
Mag: 5.4; 5.2 (O'Meara)
Diam: 15.0'
Dist: 3,300 light-years
Disc: James Dunlop, included in his 1827 catalog
J. HERSCHEL: ClusterVIII class, large, loose, brilliant, irregular
figure, fills field, chief star about 7th magnitude] taken. (h 3619)
GC/NGC: Cluster, bright, large, little compressed, stars of
magnitude 7 to 10.
AS I LAMENTED IN THIS BOOK'S INTRODUCTION,
the Caldwell Catalog, as originally published in
Sky & Telescope magazine, included a serious
typographical error. There Caldwell 89 was
identified as the open cluster NGC 6067 in
Norma. In fact it is NGC 6087 — another open
cluster in Norma. As fate would have it, NGC
6067 lies just 3¾° north and slightly west of our
true mark, the delightful S Normae Cluster, a
dynamic assortment of 349 suns centered on the
bright Cepheid variable star S Normae.
James Dunlop first discovered this "group of
very small stars of an irregular branched figure,"
and he listed it as the 326th object in his Catalogue
of Nebulae and Clusters of Stars in the Southern
Hemisphere. The 5th-magnitude cluster can be
found in southern Norma, just beyond a
fabulously rich and dense part of the Milky Way.
To find it with the naked eye, I start with Alpha
(α) and Beta (β) Centauri. Just 5° east-northeast of
Alpha Centauri (which is also known as Rigil
Kentaurus) is a naked-eye double comprising
two 4th-magnitude stars,
89
Beta and Gamma (γ) Circini. And a little more
than 5° farther east-northeast of Gamma Circini is
Iota 1 (ι 1 ) Circini, the westernmost of four 5th-and
6th-magnitude stars that form a 2°-long, eastwest-trending
chain. That chain's easternmost
"star" is the open cluster NGC 6087.
Under dark skies the 15'-wide cluster is just
visible to the naked eye, though it's difficult to
separate it from the magnitude-5.6 star some 25'
to its west. Averted vision is required to see the
combined glow of the cluster's brightest suns.
Most of NGC 6087's light comes from S Normae,
a true cluster member. A Cepheid variable, S
Normae fades from magnitude 6.12 to 6.77, then
brightens again, every 9.75 days. The naked-eye
view may take a lot of squinting; try brief,
relaxed bouts of averted vision.
In 10x50 binoculars I estimated NGC 6087's
magnitude to be 5.2, which is 0.2 magnitude (20
percent) brighter than the published value.
Actually, I was quite taken with the binocular
view of this cluster. Train your binoculars on it,
and you should see the cluster as an east-west
The Caldwell Objects 351