The Caldwell Objects

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NGC 6397
Type: Globular Cluster
Con: Ara
RA: 17 h 40 m 41.3 s
Dec: -53° 40' 25"
Mag: 5.7; 5.3 (O'Meara)
Diam: 31’
Dist: 7,500 light-years
Disc: Abbe Nicolas Louis de
Lacaille, listed in his 1755
catalog
J. H ERSCHEL: Globular cluster,
fine, large, bright, round,
gradually brighter to the
middle, not very
compressed, 5' diameter, but stragglers extend a great way. In
the middle is a more compact group of much smaller stars. The
stars at circumference are larger than in the middle, at [the
northeastern] border is a double star, (h 3692)
G LOBULAR CLUSTER NGC 6397 IN A RA HAS
an impressive résumé. At magnitude 5.3 it's the
fourth-brightest globular cluster in the heavens;
only Omega Centauri (Caldwell 80), 47 Tucanae
(Caldwell 106), and M22 in Sagittarius are
brighter. It is also the second-closest globular to
our Sun; only M4 in Scorpius is closer, and by a
statistically insignificant 700 light-years at that.
What's more, NGC 6397 ranks first among
globulars for brightest member stars (10th
magnitude) and resolvability (its horizontalbranch
magnitude is 12.9, so even a good 2½-inch
refractor can penetrate its inner sanctum).
Unfortunately for Northern Hemisphere
observers, NGC 6397's declination places it below
the horizon from midnorthern latitudes. Even
from more southerly locales in the continental
United States it hardly clears the
342
86
GC: Globular cluster of stars, bright, very large, rich, stars of
13th magnitude and fainter.
NGC: Globular cluster of stars, bright, very large, rich, stars of
13th magnitude.
horizon. Yet Texas amateur Barbara Wilson calls
it a "gorgeous cluster even at this low altitude,
only 3° above [the] mountaintops." If you are
among the lucky ones who can reach a
declination of -54° from home, or if you travel,
say, to the Texas Star Party, you'll find the cluster
easy to locate. Look 11° south and slightly east of
2nd-magnitude Theta (θ) Scorpii in the
Scorpion's Tail. There sits the cluster on the
eastern fringe of the Milky Way. The apex of a
bright acute triangle comprising Theta and Eta
(η) Scorpii and Alpha (α) Arae points right to it.
From Hawaii the cluster is easily visible to the
naked eye as a 5th-magnitude glow 2° east and a
tad south of the midpoint between Alpha and
Beta (β) Arae. It lies just 1° north-northeast of 5thmagnitude
Pi (π) Arae. In deep photographs the
cluster is quite large, covering as
Deep-Sky Companions: The Caldwell Objects