By Jim Kaler

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.