The Caldwell Objects

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trated glow. Christian Luginbuhl and Brian Skiff
found NGC 6541 visible even in a 12-inch
telescope "as a well-concentrated spot with a
substellar center." Sharp-eyed observers under
decent skies should have no problem detecting
this cluster with the unaided eye; the trick is to
separate it from its slightly brighter stellar
companion. Try keen averted vision on a crisp
night just as astronomical twilight is drawing to
an end.
James Dunlop discovered NGC 6541 while
exploring the southern heavens from his home in
Parramatta, New South Wales. It is the 473rd
object listed in his 1827 A Catalogue of Nebulae and
Clusters of Stars. Of its appearance through a 9inch
reflector he wrote: "A very bright round
highly condensed nebula, about 3' diameter. I
can resolve a considerable portion round the
margin, but the compression is so great near the
centre, that it would require a very high power,
as well as light, to separate the stars; the stars are
rather dusky." John Herschel found it "easily
resolvable" with an 18¼-inch reflector, adding:
"The left eye resolves it." Here we get a taste of
an early experiment in astronomical perception.
Which eye works best for you?
It is tempting to think that NGC 6541 looks
so compressed because it is one of the 20-odd
known globular clusters, like M30 in Capricornus
and Μ15 in Pegasus, that has experienced
a core collapse. In dense gravitating systems like
globular clusters, stars can swarm toward the
cluster's center like bees to a hive. This swarming
episode can last a few million years, until the
stars get too close. At that point two things can
happen. The stars either continue collapsing until
they form a black hole (which is unlikely, given
the observed properties of globular clusters), or
more likely they will rebound back out until they
reach a point where they remain in globularcluster
limbo, being both attracted and repelled
by one anoth-
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er. In a sense, when we peer into the heart of
NGC 6541, we are looking at a snapshot of this
drama at some critical point in the globular's
evolution. However, our imaginations must be
tempered by the fact that the effects of core
collapse on a globular cluster almost certainly
cannot be resolved visually.
NGC 6541 is a poorly studied cluster. Its age
is estimated at 16 to 17 billion years. The cluster's
metallicity places it among the metal-poor
globulars; each of NGC 654l's stars, on average,
contains less than l /70 as much iron as does our
Sun. This is yet another clue that the cluster is
one of our galaxy's senior citizens. The cluster's
overall spectral type is F6, and it is moving away
from us at 155 km per second.
At 23x in the 4-inch Genesis, which provides
a stunning nearly 3°-wide view of this rich Milky
Way region, the globular glows to the southeast
of SAO 228708, which is actually an extremely
challenging double star whose equally bright
components were separated by a mere 1.7" in
1999.1 see these type-A5V stars as a single sun
burning with a pale yellow light. The tiny (6.9')
magnitude-8.5 globular star cluster NGC 6496
can also be seen in the same field, about 1¾°
west-southwest of NGC 6541. There is no
mistaking the two globulars for one another;
NGC 6496 is but a mere fleck of dandruff in the
Milky Way compared to the magnificently crisp
form of NGC 6541. When I use averted vision on
NGC 6541 at low power, the cluster's outer halo
Deep-Sky Companions: The Caldwell Objects