The Caldwell Objects

13
accompanying photograph and color-magnitude
diagram," Archinal writes, "it looks fairly obvious
that the two brightest stars in NGC 457 [Phi1 and
Phi2 Cassiopeiae] are non-members. They are
much brighter than the other (cluster) stars, and
much higher on the color-magnitude diagram
plot. There's, of course, no way to tell for sure
from just this information whether they are
members, but it seems unlikely." Sky & Telescope's
Roger W Sinnott . adds that parallax data from the
Hipparcos satellite suggest a probable distance
for Phi Cassiopeiae of 2,000 light-years, though
uncertainties in the data could position the
double star anywhere from 1,000 to 4,000 lightyears
out. Sinnott remarks that this distance is
better than anything we've had before, despite
the uncertainty.
Ambiguities also exist in determinations of
NGC 457's distance. Gosta Lynga's value of 9,965
light-years, published in his 1983 Catalogue of
Open Cluster Data, is probably the most reliable
pre-Hipparcos estimate. Hipparcos based a
provisional distance of 2,100 light-years on four
bright
stars that presumably belong to the
cluster. Other estimates recently
published in professional journals
are 9,400,10,400, and 8,150 lightyears.
Archinal says that "either the
ground-based estimates have some
common large error, the brightest
stars are not members, or there is
some large error common to the
four Hipparcos stars used in this
most recent study." On the one
hand, if Phi Cassiopeiae is not
related to the cluster, then NGC
457's brightest member shines at
magnitude 8.6. On the other hand,
if new data one day reveal
unequivocally
60
that Phi Cassiopeiae is a true member, the cluster's
total apparent magnitude would certainly
have to jump to a higher value.
Phi Cassiopeiae is troublesome in another
way. NGC 457 should be near the limit of nakedeye
visibility for a diffuse object, but glare from
Phi Cassiopeiae renders it invisible (at least I
couldn't convince myself of its visibility). But the
jury is still out. So here is a decent visual
challenge for observers (especially young ones)
who live under dark skies: see if you can pick up
the cluster unambiguously without optical aid.
Some guidance may come from descriptions of
NGC 457's appearance in binoculars. In my 7x35s
I see the tapered tail of a fragmenting comet.
More poetically, MacRobert likens his binocular
view to a "wisp of a candle flame blown from the
two stars by the wind." One should wonder why
Messier failed to notice this beautiful comet-like
object.
Through the 4-inch at 23x the cluster has a
more sinister appearance than it does in
Deep-Sky Companions: The Caldwell Objects