The Caldwell Objects

85
varies between magnitude 3.56 and 3.67 every 3
hours and 7 minutes. The cluster has long been
estimated to be about 35 million years young, but
in 1999 a team led by David Barrado y Navascues
bestowed a new age of 53 million years (plus or
minus 10 percent) on it. "While this is
considerably older than the age most commonly
attributed to this cluster," they wrote in
Astrophysical Journal Letters, "this [age increase]
for IC 2391 is comparable to those recently
derived for the Pleiades and Alpha Persei
clusters." The scientists had looked for lithium in
the spectra of each cluster's faintest stars. They
then assessed just how bright a star (or brown
dwarf) could be without consuming this fragile
chemical element in its central nuclear furnace. In
each case, this lithium-depletion measurement
yielded a much higher cluster age than had been
deduced from more classical analyses.
Like the Pleiades, IC 2391 is a fertile hunting
ground for faint, red brown dwarfs —
340
not-quite-stars whose masses (80 lupiters or less)
can't sustain nuclear fusion reactions. As of 1999,
extensive photometry had revealed 24 possible
brown dwarfs in IC 2391, but more data
(particularly near-infrared photometry and
spectroscopy) were needed to determine whether
or not these candidates truly are substellar brown
dwarfs.
Under a dark sky IC 2391 is visible to the
naked eye about 1¾° north-northwest of
magnitude-1.95 Delta (δ) Velorum, the star
marking the northwestern arm of the False Cross.
The object is quite bright, and my naked-eye
estimate of magnitude 2.8 is reasonably
comparable to Brent Archinal's value of 2.5.
Persian astronomer Abd al-Rahman al-Sufi (who
is also credited with discovering M31, the
Andromeda Galaxy) first noted IC 2391 on or
around 964 A.D. Of course, the object is bright
enough that anyone, from poet to pig farmer,
would have seen its light if they happened to
gaze toward that region of sky. Abbé
Deep-Sky Companions: The Caldwell Objects