The Caldwell Objects

9th magnitude. Of the
latter, 23 have spectral
types of Β and A while
the others range from F0
to K 5 Theta . Carinae, of
course, is the brightest,
shining at magnitude 2.8
with a spectral class of
B0V. The nearly 5thmagni-tude
star 6' to the
southwest of Theta
Carinae is V518 Carinae.
Its variability was
discovered by the Hipparcos
satellite, which
recorded apparent visual
magnitudes ranging from 4.60 to 4.76.
IC 2602 is very young. Its stars burned
through their dusty cocoons some 30 million
years ago, which makes this cluster less than half
the age of the Pleiades. Further evidence of the
cluster's relative youth can be found in the spin
rates of its stars. Younger stars generally spin
faster than older ones. However, there can be a
wide range of rotation periods even among a
young cluster's stars. Recent studies have found
that the rotation periods for 29 of IC 2602's stars
range from 0.2 days (one of the shortest known
rotation periods of any single open-cluster star)
to about 10 days (which is almost twice the
longest period previously known for a cluster of
this age). Images of IC 2602 from the Rosat X-ray
satellite revealed a total of 110 objects within an
area of 11 square degrees. Of these, 68 have been
identified with at least one visible counterpart; 44
of these are new candidates for cluster
membership. In IC 2602 Rosat detected X-rays
from stars of all spectral types. The F-, G-, and
early K-type stars appear to be more X-ray
luminous than their counterparts in the
The Caldwell Objects
102
Pleiades, while no significant difference was seen
among the two cluster's late-K- and M-t y p
dwarfs. This may be related to the different rates
at which stars of different masses shed rotational
energy.
If we accept its distance as 492 light-years, IC
2602 is 85 light-years more distant than the
Pleiades and 23 light-years closer than M44, the
Beehive in Cancer. It spans 14 light-years, as does
the Pleiades. Furthermore, IC 2602 has some 10
stars shining at 6th magnitude or brighter, like
the Pleiades, and it is only 0.4 magnitude (39
percent) fainter than the Pleiades. So the two
clusters really are twins. Visually, however, IC
2602 resembles M6, the Butterfly Cluster in
Scorpius, more than it does the Pleiades. IC 2602
is a member of the Scorpius-Centaurus O B
association, a widespread group of hot, young
stars that appear to have a common origin.
Like the Pleiades, IC 2602 is best observed
through binoculars or a rich-field telescope. In
the 4-inch at 23x the cluster is a loose aggregation
of suns with two distinct groupings. The first
grouping includes Theta Carinae and a
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