The Caldwell Objects

this wreathlike network of dust and gas is about
as large as M42, the Orion Nebula, on the plane
of the sky. But it is more than three times as far
as M42, and therefore that much larger
intrinsically; at its greatest extent the Rosette
spans nearly 115 light-years, while the Orion
Nebula spans only 40 light-years. In photographs
the Rosette appears to ripple outward from a
central hollow, as if a stone had been dropped
into a pool of celestial vapors. A more elegant
metaphor might be a snow-dusted rose seen by
the frosty light of the Moon.
The Rosette Nebula surrounds the rectangular
open cluster NGC 2244 (Caldwell 50),
whose 100 members populate the nebula's ½°wide
central cavity. (Note that the position given
in the table on page 194 is for 12 Monocerotis.
That star appears to be on the southeastern end
of the cluster's densest portion. But that is an
illusion. Cluster members do extend farther to
the southeast, though they are swaddled in
nebulosity. Taking these "hidden" suns into
consideration, Brian Skiff estimates that 12
Monocerotis marks the cluster's center as projected
onto the plane of the sky.) Collectively
these stars shine at magnitude 4.8, say sources, so
they make a reasonable naked-eye target (and
my own estimate makes them 0.4 magnitude
brighter still). Ultraviolet radiation streaming
from the cluster's young, hot stars excites the
surrounding nebula, whose spectrum shows
strong emission lines of hydrogen and singly
ionized oxygen as well as the so-called "forbidden"
blue-green lines of doubly ionized oxygen.
But, unlike the cluster, the Rosette Nebula burns
brightly only in photographs. Visually the nebula
is dim and challenging, and while some have
seen it with the unaided eye (myself included),
others have argued that seeing it without optical
aid is as likely as seeing a unicorn in New York
City. But more on that later.
Glance at almost any photograph of the
49 & 50
Rosette and you'll see dark veins of obscuring
matter peppering the nebula's northwestern
quadrant; it's as if someone had spray-painted
the nebula with celestial graffiti. Magnified views
of these features reveal the presence of tiny dark
blobs. These blobs are known as Bok globules
after the late Harvard College Observatory
astronomer Bart J. Bok, who turned astronomers'
attention to them in the late 1940s. (Bok called
them "globules" because they reminded him of
the small globules of fat floating atop the cream
that the H.P. Hood Company delivered to his
Belmont, Massachusetts, home.) Bok proposed
that these diminutive dark masses, which span
less than 10,000 a.u., were once attached to
umbilical filaments of neutral hydrogen gas.
Radiation from nearby stars separated, then
compressed, them, and the orphaned dark clouds
continued to contract under their own gravity
until they formed the tiny globules we see today.
These small black orbs may be crucibles of
creation, for they are the primeval dust clouds
within which new suns and new worlds may be
forming. By studying Bok globules, we stand at
the threshold of understanding the origins of new
solar systems. Indeed, life itself may have been
conceived within the confines of dusty cocoons
like these.
The visual gem of the nebulous Rosette
complex, however, is NGC 2244, the dazzling
cluster at its heart. The cluster's physical diameter
is 43 light-years. It is very young on the
cosmic time scale, having formed within the
Rosette Nebula less than 1 million years ago
(about when the Trapezium stars formed in the
Orion Nebula). More than 3 million years before
these stars burned through their cocoons, the first
hominids already had appeared on Earth. And
by the time these stars bathed the Earth with
their light, Homo erectus had migrated out of his
native Africa and ventured
The Caldwell Objects 195