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The Caldwell Objects

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78<br />

trated glow. Christian Luginbuhl and Brian Skiff<br />

found NGC 6541 visible even in a 12-inch<br />

telescope "as a well-concentrated spot with a<br />

substellar center." Sharp-eyed observers under<br />

decent skies should have no problem detecting<br />

this cluster with the unaided eye; the trick is to<br />

separate it from its slightly brighter stellar<br />

companion. Try keen averted vision on a crisp<br />

night just as astronomical twilight is drawing to<br />

an end.<br />

James Dunlop discovered NGC 6541 while<br />

exploring the southern heavens from his home in<br />

Parramatta, New South Wales. It is the 473rd<br />

object listed in his 1827 A Catalogue of Nebulae and<br />

Clusters of Stars. Of its appearance through a 9inch<br />

reflector he wrote: "A very bright round<br />

highly condensed nebula, about 3' diameter. I<br />

can resolve a considerable portion round the<br />

margin, but the compression is so great near the<br />

centre, that it would require a very high power,<br />

as well as light, to separate the stars; the stars are<br />

rather dusky." John Herschel found it "easily<br />

resolvable" with an 18¼-inch reflector, adding:<br />

"<strong>The</strong> left eye resolves it." Here we get a taste of<br />

an early experiment in astronomical perception.<br />

Which eye works best for you?<br />

It is tempting to think that NGC 6541 looks<br />

so compressed because it is one of the 20-odd<br />

known globular clusters, like M30 in Capricornus<br />

and Μ15 in Pegasus, that has experienced<br />

a core collapse. In dense gravitating systems like<br />

globular clusters, stars can swarm toward the<br />

cluster's center like bees to a hive. This swarming<br />

episode can last a few million years, until the<br />

stars get too close. At that point two things can<br />

happen. <strong>The</strong> stars either continue collapsing until<br />

they form a black hole (which is unlikely, given<br />

the observed properties of globular clusters), or<br />

more likely they will rebound back out until they<br />

reach a point where they remain in globularcluster<br />

limbo, being both attracted and repelled<br />

by one anoth-<br />

312<br />

er. In a sense, when we peer into the heart of<br />

NGC 6541, we are looking at a snapshot of this<br />

drama at some critical point in the globular's<br />

evolution. However, our imaginations must be<br />

tempered by the fact that the effects of core<br />

collapse on a globular cluster almost certainly<br />

cannot be resolved visually.<br />

NGC 6541 is a poorly studied cluster. Its age<br />

is estimated at 16 to 17 billion years. <strong>The</strong> cluster's<br />

metallicity places it among the metal-poor<br />

globulars; each of NGC 654l's stars, on average,<br />

contains less than l /70 as much iron as does our<br />

Sun. This is yet another clue that the cluster is<br />

one of our galaxy's senior citizens. <strong>The</strong> cluster's<br />

overall spectral type is F6, and it is moving away<br />

from us at 155 km per second.<br />

At 23x in the 4-inch Genesis, which provides<br />

a stunning nearly 3°-wide view of this rich Milky<br />

Way region, the globular glows to the southeast<br />

of SAO 228708, which is actually an extremely<br />

challenging double star whose equally bright<br />

components were separated by a mere 1.7" in<br />

1999.1 see these type-A5V stars as a single sun<br />

burning with a pale yellow light. <strong>The</strong> tiny (6.9')<br />

magnitude-8.5 globular star cluster NGC 6496<br />

can also be seen in the same field, about 1¾°<br />

west-southwest of NGC 6541. <strong>The</strong>re is no<br />

mistaking the two globulars for one another;<br />

NGC 6496 is but a mere fleck of dandruff in the<br />

Milky Way compared to the magnificently crisp<br />

form of NGC 6541. When I use averted vision on<br />

NGC 6541 at low power, the cluster's outer halo<br />

Deep-Sky Companions: <strong>The</strong> <strong>Caldwell</strong> <strong>Objects</strong>

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