The Caldwell Objects

porarily) even the most practiced of observers.
NGC 2261 is easy and fun to find. First, note the
feet of Gemini, which are marked by a roughly
10°-long line of four reasonably bright stars
oriented northwest to southeast — 3rdmagnitude
Mu (μ) Geminorum, 4th-magni-tude
Nu (ν) Geminorum, 2nd-magnitude Gamma (γ)
Geminorum, and 3rd-magnitude Xi (ξ)
Geminorum. Xi Geminorum also marks the
northeastern end of a fainter 10°-long line of four
stars that flows to the southwest; the other three
stars are 5th-magnitude 15 Monocerotis, 4.5magnitude
13 Monocerotis, and 4th-magnitude 8
Monocerotis. Raise your binoculars to 15
Monocerotis. Here you'll see one of the winter
Milky Way's most celebrated clusters — NGC
2264, Leland Copeland's famous Christmas Tree,
whose base, 15 Mon-
46
ocerotis, is a famous
semiregular variable star
that also goes by the
name S Monocerotis. In
his book Star-Hopping for
Backyard Astronomers,
Alan M. MacRobert says
this luminous young star,
whose spectral type is 07,
is "about as blue a star as
you'll ever see; they don't
get bluer than this pale
shade no matter how
high the temperature
goes." Of course, the
color one sees depends
on the sensitivity of one's
eyes. For instance, Adm.
William Henry Smyth
saw the star as
"greenish." (Bear in mind,
though, that this man's
vision
would often ignite behind the eyepiece, allowing
him to see the most vivid colors in the most
mundane stars). In the 4-inch at 23x I could detect
a thin stream of nebulosity flowing a few
arcminutes southwest from S Monocerotis
toward a small grouping of three 8th- to 9thmagnitude
suns. In photographs this entire
region is adorned with a symphony of nebulosity
whose bright and dark folds seem to ripple across
space like visible sound waves. The scene
includes the majestic, though dark, Cone Nebula,
a dense stalagmite of obscuring matter. (Had the
ancients known of such a wonder, wrote Robert
Burnham Jr. of the Cone, they would not have
called it by "any lesser name than 'The Throne of
God.'")
Hubble's Variable Nebula is about 1¼° to
the south-southwest of the Christmas Tree
The Caldwell Objects 185