The Caldwell Objects

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NGC 188 may have formed when our solar
system did, but it will not last as long. Both
internal and external forces tend to rob open
clusters of their stars. Massive binary stars within
a cluster can cast out solitary members passing
near them. Stars grazing a cluster's outskirts can
free members from the cluster's weak
gravitational grip. And open clusters are
subjected to the tidal shear of our galaxy, which
is constantly working against the gravitational
forces that hold the clusters together. Over the
next few billion years we can expect NGC 188's
stars to slowly drift apart, each following a
random path into space, until the cluster fully
dissipates and its constituents become part of the
scintillating backdrop of the night sky.
Northern Hemisphere observers can find the
loose and aged NGC 188 year-round. The cluster
is conveniently placed 1° south-southwest of a
4th-magnitude star in Cepheus. The first edition
of Sky Atlas 2000.0 identifies this star as 2 Ursae
Minoris, but you will not find this designation on
other star charts. Walter Scott Houston notes that
the star was engulfed
by Cepheus when the constellation boundaries
were redefined by Belgian astronomer
Eugene Delporte, then adopted by
international agreement in 1930; the
Flamsteed designation has since been
dropped to avoid confusion. Brent Archinal
of the U.S. Naval Observatory adds that
NGC 188 lies so close to the pole that the
cardinal directions here will change
noticeably in several decades because of
precession.
The cluster's cataloged magnitude of 8.1
may be deceiving. Using 7x35 binoculars I
estimated the cluster to be a full magnitude
brighter. I could not detect the object with
the unaided eye, however; its dim glow is
spread across nearly ¼° of sky,
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and it does not stand out from the bland background
sky. A low-power glimpse through the 4inch
Genesis reveals what appears to be the spirit
of a deceased open cluster, one seemingly fading
from view. Houston himself called NGC 188 a
"ghostly glow" as seen through his 4-inch Clark
refractor. (He also noted that "since the cluster is
always at nearly the same altitude, amateurs will
find it a good object for checking observing
conditions" — a classic "Scotty" remark.)
Deep-Sky Companions: The Caldwell Objects