The Caldwell Objects

went out and looked at the cluster with the naked
eye, I saw a l°-wide, wishbone-shaped grouping.
Three 6th-magnitude stars in a gentle northeastto-southwest
curve formed the thin northwestern
branch of the wishbone, while a fuzzy wedge of
knitted starlight formed the thicker southeastern
branch. The two segments are separated by about
½°.
Imagine my surprise, then, when I noticed
that Uranometria 2000.0, the second edition of Sky
Atlas 2000.0, and the Millennium Star Atlas all
show the cluster as if it were only ½ across. ° All
three of these atlases center NGC 2516 on the
dense core of stars in the southeastern part of the
wishbone. They also place the region's four
brightest naked-eye stars — the three stars in the
northwestern branch of the wishbone and the
northernmost star in the southeastern branch —
outside the cluster.
More recently, Steven Hynes and Brent
Archinal have moved the cluster's center 7' to the
north (from -60° 52' to -60° 45') and shrunk
96
the cluster's width by another 8' (from 30' to 22').
So in the last quarter century, NGC 2516's
diameter has diminished by 63 percent. Hynes
and Archinal present another problem. They
report that the cluster's brightest star shines at
magnitude 7.0. But when I center a 22'-wide circle
at their position for the cluster I find the variable
star V374 Carinae (which fluctuates between
magnitude 5.72 and 5.84) within the circle. Is this
star a member of the cluster?
Puzzled by this apparent dilemma, I referred
to astrophysical journals and checked the
Hipparcos data as well as the second edition of
Sky Catalogue 2000.0. The Hipparcos satellite
revealed that the middle star of the trio in the
wishbone's northwestern branch is not a cluster
member; it is only 53 light-years distant, while
the cluster lies 1,300 light-years away. But the
other two bright stars in that branch are at the
cluster's distance, so they probably are members.
Therefore, we've come full circle (to use a pun),
because NGC 2516's
The Caldwell Objects 385