The Caldwell Objects

much sky as does the full Moon, though it's the
dense inner portion you'll be spying visually.
It's possible that keen-eyed skywatchers in
the Southern Hemisphere had detected this
cluster's glow many years before Lacaille found it
with his ½-inch 8x telescope. But Lacaille was the
first to list it. In his 1755 catalog it's the 11th entry
under his Class III (stars accompanied by
nebulosity). His description reveals little else.
lames Dunlop was the first to describe the
object's true nature, writing, the "nebula is
resolvable into stars." NGC 6397 is the 366th
entry in his 1827 catalog.
Like 47 Tucanae, NGC 6397 resides close to
the plane of our galaxy and is about 20,000 lightyears
from the galactic center. It probably formed
in the galactic halo some 16 billion years ago.
Evidence of its birthplace is its paucity of
"metals" (chemical elements heavier than
helium); on average, each of the cluster's stars
contains only 1/90 as much iron (per hydrogen
atom) as does our Sun. The cluster's spectral class
is a yellowish F4.
NGC 6397 is so close, so large, and so loose
that the Hubble Space Telescope had no trouble
seeing right through it to far more distant
The Caldwell Objects
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galaxies beyond. This was not what astronomers
expected. Star-formation theory had predicted
that red dwarfs with only ⅕ the mass of the Sun
(or less) should abound, and that's what
astronomers expected to see when HST trained
its powerful eye toward NGC 6397. But while
HST did discover many previously unseen stars,
their brightnesses and colors implied masses of
0.2 Sun and above. This showed that low-mass
red dwarfs are much rarer than expected, and in
doing so, ruled out a leading candidate for dark
matter. HST has also discovered 100 white-dwarf
stars, four cataclysmic variables, and six blue
stragglers in this dense core-collapse globular.
Two of the latter are excessively blue and hot
even by blue-straggler standards. This is
probably the result of either yet-undetected
companions or mass accretion within these
binary-star systems. HST also identified a
nonvariable blue star that may represent a new
class with properties reminiscent of white
dwarfs. Such stars could result from the stripping
of red-giant envelopes in binary systems. Overall,
the HST results spotlight the exotic outcomes of
stellar interactions and binary evolution in the
cores of densely populated globular clusters.
But how serene is the view on dark starlit
nights, when through 7x35 binoculars NGC 6397
stands out not as a stellar dynamo but as a soft
and gentle glow. Even in binoculars you can feel
its closeness, the cluster is so large and
conspicuous. With averted vision look for a
gradual brightening toward the center. Many
sources say the telescopic view is similar to that
of M4, but this is not the case. Yes, the cluster is
at essentially the same distance as M4, and both
shine at about 5th magnitude, but no other
globular has M4's brilliant spine of suns, which
seems to sizzle with unbridled energy. In the 4inch
at 23x NGC 6397 appears as a clean wash of
loosely packed starlight.
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