RIDING THE UNICORN
As summer and fall turn to winter, turn to an obscure and
often neglected part of the sky, to a modern constellation that
looks nothing whatever like what it is supposed to be, one that
looks nothing like anything. Appropriately, the constellation
honors a beast that does not exist, Monoceros, the
elusive Unicorn, which lies just to the east of brilliant Orion. Much of it is set within the Winter Triangle of Betelgeuse, Procyon, and Sirius. Even though home to the Milky Way,
its brightest star, Beta Mon, is only
fourth magnitude (at magnitude 3.92 just beating Alpha Mon by a
mere 0.01).
As treasured as our ancient memory of the mythical beast are
the objects found within it, from marvelous stars to vast
interstellar clouds, chief among which is the flower of the skies,
the Rosette Nebula, so great and disjointed that it carries not
one, but FOUR, NGC numbers (the all time record): 2237, 2238, 2239,
and 2246. Buried within its hollowed-out interior is an entire
open cluster, NGC 2244, which is dominated by enough hot blue class
O stars (the hottest a 50 solar mass class O5 monster that radiates
at a rate nearly half a million times solar) to ionize the nebula,
and so make it shine, out to an astonishing radius of 30 light
years, seven times the distance between the Sun and Alpha
Centauri. Even at its great distance of 5400 light years (as
determined from the colors and apparent brightnesses of the
cluster's stars), the Rosette is nearly a degree across, and shows
up on short-exposure amateur photographs. More absorbing, the
nebula is the backdrop for an extraordinary set of related dark
clouds, including long thin "elephant trunks" that thread in front
of it and are probably being evaporated by the hot stars within the
cluster, which was born only 1.5 million years ago. There is so
much dust in the line of sight, that were it not there, the stars
would appear 1.5 magnitudes brighter, making the tip of the cluster
visible to the naked eye. Curiously, the apparently brightest star
in the group is a class K giant that is an interloper only 500
light years distant that (like Aldebaran to the Hyades) is merely an interloper. Like many young
clusters, the Rosette and NGC 2244 are related to a much larger
structure, an "association" of O and B stars that is not
gravitationally bound together, "Mon OB2" also containing the class
A0 fifth magnitude supergiant 13 Mon.
Crossing the Galactic equator from south to north, we
encounter a vast nebulosity with no common name that fronts a dark
background molecular cloud that is rife with star-forming regions.
The southeastern corner of the nebula is home to another
spectacular cluster, NGC 2264. At a distance of some 2500 light
years, the cluster includes the magnificent fifth magnitude class
O star S (or 15) Monocerotis.
Though still not very well
understood, S Mon is a massive binary in which stars of around 30
and 20 solar masses orbit every 25 years at an average separation
of 26 Astronomical Units. Like the Rosette's luminary, they will
explode as grand supernovae. But we have a while to wait, as NGC
2264 is younger even than the Rosette's cluster, and possesses
great numbers of "T Tauri" stars (named after the prototype that
lies in the dark star-forming interstellar clouds of Taurus). T Tauri stars have just been
born and are still in the process of contracting and settling in to
become core hydrogen burners like the Sun, and possess surrounding
disks from which they accrete yet more mass. After 10 million or
so years, the disks dissipate, or consolidate to (so we believe)
form planets. The cluster is the core of yet another extended OB
association, Mon OB2. To the south of S Mon is one of the more
intriguing of celestial objects, the Cone Nebula, a long dark cloud
whose tip is being eroded away by another nearby hot star.
Related to the complex, but not part of the cluster, is a yet-
more famed body, fan-shaped Hubble's variable nebula (NGC 2261),
which is associated with the very young star R Monocerotis at its
southern tip. It was of such fascination and mystery that it was
the first object to be "officially" photographed with the Palomar
200-inch telescope. R Mon is a fine example of a "Herbig Ae/Be
star," a high-mass counterpart to the lower mass T Tauri stars that
flock within NGC 2264. The star is so young that it is surrounded
by the usual thick dark disk from which it is still accreting mass.
Flowing perpendicular to the disk is a 100 kilometer per second jet
that hammers the surrounding interstellar matter and produces
bright "Herbig-Haro objects." The fan-shaped nebula is formed by
reflection of light from the buried star, which, when it settles
down, will be a class B dwarf. As dark clouds orbit close to R
Mon, they project shadows onto the nebula, and cause it to vary in
brightness. Accompanying R Mon proper is a much fainter T Tauri
star, which will someday become another white star much like Vega
or Sirius.
Seeming also to be projected onto the great nebula that holds
NGC 2264 are two more open clusters, NGC 2259 and Trumpler 5.
Their apparent proximity to NGC 2259 and to R Mon is only a
coincidence, as they both lie much farther away, some 10,500 light
years. Although the two are 250 light years apart, the fact that
they are at nearly the same distance from us might imply that they
are somehow related. But no. Detailed examination shows that NGC
2259 is a middle-age 300 million years old, while Trumpler 5 is, at
an age of 5 billion years, one of the oldest clusters of the
Galaxy.
After all these wonders, some of which require sophisticated
instrumentation to appreciate, return to easily-visible Beta.
Point your telescope at it, and view a magnificent triple star that
consists of a close pair only 2.8 seconds of arc apart that is
accompanied 7.4 seconds of arc away by another, the orbital periods
estimated in the thousands of years. At magnitude 4.6, the
brighter "A" star of the trio dominates the other two, which shine
near magnitude 5.5. Beta Mon's slight domination of the
constellation is therefore a bit of a cheat, as it takes all three
stars to do it. If we take the three singly, then Alpha Mon
properly becomes the constellation's luminary. All are blue-white
hot class B3 stars jewels that range from 6 to 7 solar masses that
were born only 34 million years ago. The "A" component has only 9
million years left before it ceases hydrogen fusion and begins to
become a red giant that will contrast brilliantly with the two
remaining blue stars, this trio and other sights fitting
appropriately with the beauty of the Unicorn.
Copyright © James B. Kaler, all rights reserved.
These contents are the property of the author and may not be
reproduced in whole or in part without the author's consent
except in fair use for educational purposes. First published in
the January/April 2003 Newsletter of the Lowestoft and Great
Yarmouth Regional Astronomers, who are gratefully acknowledged.
Revised April 2005.