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The Caldwell Objects

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85<br />

varies between magnitude 3.56 and 3.67 every 3<br />

hours and 7 minutes. <strong>The</strong> cluster has long been<br />

estimated to be about 35 million years young, but<br />

in 1999 a team led by David Barrado y Navascues<br />

bestowed a new age of 53 million years (plus or<br />

minus 10 percent) on it. "While this is<br />

considerably older than the age most commonly<br />

attributed to this cluster," they wrote in<br />

Astrophysical Journal Letters, "this [age increase]<br />

for IC 2391 is comparable to those recently<br />

derived for the Pleiades and Alpha Persei<br />

clusters." <strong>The</strong> scientists had looked for lithium in<br />

the spectra of each cluster's faintest stars. <strong>The</strong>y<br />

then assessed just how bright a star (or brown<br />

dwarf) could be without consuming this fragile<br />

chemical element in its central nuclear furnace. In<br />

each case, this lithium-depletion measurement<br />

yielded a much higher cluster age than had been<br />

deduced from more classical analyses.<br />

Like the Pleiades, IC 2391 is a fertile hunting<br />

ground for faint, red brown dwarfs —<br />

340<br />

not-quite-stars whose masses (80 lupiters or less)<br />

can't sustain nuclear fusion reactions. As of 1999,<br />

extensive photometry had revealed 24 possible<br />

brown dwarfs in IC 2391, but more data<br />

(particularly near-infrared photometry and<br />

spectroscopy) were needed to determine whether<br />

or not these candidates truly are substellar brown<br />

dwarfs.<br />

Under a dark sky IC 2391 is visible to the<br />

naked eye about 1¾° north-northwest of<br />

magnitude-1.95 Delta (δ) Velorum, the star<br />

marking the northwestern arm of the False Cross.<br />

<strong>The</strong> object is quite bright, and my naked-eye<br />

estimate of magnitude 2.8 is reasonably<br />

comparable to Brent Archinal's value of 2.5.<br />

Persian astronomer Abd al-Rahman al-Sufi (who<br />

is also credited with discovering M31, the<br />

Andromeda Galaxy) first noted IC 2391 on or<br />

around 964 A.D. Of course, the object is bright<br />

enough that anyone, from poet to pig farmer,<br />

would have seen its light if they happened to<br />

gaze toward that region of sky. Abbé<br />

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

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