Flanked by Orion and Canis Minor, with Gemini above and Canis Major below,
the faint constellation Monoceros ("the Unicorn")
is often overlooked.
The stars of Monoceros are as dim as the
constellation's history: only a few fourth-magnitude stars that are
difficult to notice except on very clear nights. Alpha Monocerotis
is only a 3.93 visual magnitude, slightly brighter than gamma
While the constellation may have been in existence prior to the seventeenth
century, its first historical reference appears in Jakob Bartsch's star
chart of 1624, under the name "Unicornu". It is believed that Bartsch
(who incidentally was Johannes Kepler's son-in-law) relied on earlier works,
but such works have never been identified.
It takes a lot of imagination to fashion a unicorn out of this group of
stars. In fact, there are several variations. While our figure puts the
horn in front, from gamma Monocerotis through epsilon and up to S
Monocerotis, another popular form has the horn instead coming from delta
Monocerotis through 18 Mon and over to epsilon.
But it isn't the stars which hold most of our interest here. Instead,
Monoceros has several celebrated deep sky objects as well as the most
massive binary system yet discovered.
The only Bayer star of interest is beta Monocerotis, which is a
splendid triple. See below for a detailed description of this system.
Double stars in Monoceros:
Beta Monocerotis is a wonderful triple star system, especially
for smaller telescopes. William Herschel, who discovered it in 1781,
thought it was one of the best he'd ever seen. You may well agree.
The three stars form an elegant triangle that doesn't change much, if at
all, over time. Thus the system may be considered "fixed". The visual
magnitudes and separations are as follows: AB (4.7, 5.2; 132º,
7.3"), AC: (6.1, 124º, 10").
Epsilon Monocerotis is a fixed binary: 4.4, 6.6; 30º, 12".
15 Monocerotis, also known as S Monocerotis, is another
multiple system consisting of six stars. However most of them are
AB: 4.7, 7.8; 213º, 2.9".
C: 9.9, 14º, 16.4"
D: 9.7, 308º, 40.7"
E: 8.9, 140º, 73.8"
F: 9.0, 222º, 155.5".
Variable stars in Monoceros:
R Monocerotis is an irregular variable, and the nucleus of
Hubble's Variable Nebula (see below). It is an RW Aurigae type
variable, changing from visual magnitude 10 to 12.
S Monocerotis is also irregular, the central star in NGC 2264.
This star is a bright 4.5 visual magnitude, dipping down at times to
Deep Sky Objects in Monoceros:
NGC 2237, a large diffuse nebula ("Rosette Nebula") which engulfs
the open star cluster NGC 2244 (see below). This nebula actually carries
four separate NGC numbers (2237, 2238, 2239, and 2246) although it
usually goes under the name of NGC 2237.
It takes a large telescope to distinguish the ring shape. Usually all
one sees is a ghostly bit of fluff around the star cluster.
This nebula has been extensively studied, for it seems to be
extraordinarily massive (over 10,000 Suns). Dark matter is woven in and
out of the surrounding gases. It is surmised that eventually the gases
will coalesce, producing either a new star or perhaps even a whole new
system of sun and planets, similar to our own.
NGC 2244, the open cluster at the centre of the Rosette Nebula,
may actually be stars formed out of the Rosette Nebula. However, the central star, 12 Mon (magnitude 6), probably does not belong to the group.
NGC 2264 is a large and bright cluster with associated nebula (The Cone Nebula, so called because of its shape). The brightest star here is the variable S Monocerotis, which is found near the top of the cluster.
Like other clusters in this constellation, NGC 2264 is surrounded by gaseous matter not revealed in small scopes. The spectacular dark Cone Nebula is found at the southern edge of this cluster. However, much like the Horsehead Nebula, it appears best in long-exposure photographs.
M50 (NGC 2323) is surprisingly the only Messier object in this
This is a cluster of about a hundred bright stars, rather tightly
grouped, ideal for small telescopes. It can even be seen by the naked eye
on a good night. There is a red star near its centre. The cluster is considered
to be about 2500 light years away.
To find M50 draw a line between Sirius and Procyon; you'll find the
cluster about a third of the way up from Sirius.
Another way to find M50 is to locate the roughly-shaped square formed by
alpha, delta, and beta Monocerotis, along with Sirius. Right in the
middle of that square lies M50.
Then there is Plaskett's Star:
This giant double star system is recognised as the most massive pair yet discovered.
John Stanley Plaskett began his career at the Dominion Observatory in
Ottawa. As he became aware of its limitations, he lobbied the Canadian government to support the development of a new astronomy facility.
In 1913 the federal government provided funding for the construction of a 183 cm (72 inch) reflecting telescope, to be built near Victoria, BC. The Dominion Astrophysical Observatory officially opened in 1918 and was, for a time, the largest telescope in the world. Plaskett served as DAO's first director from 1917 to 1935.
It was here that Plaskett set about studying binary stars and in 1922 this work resulted in his discovering the very massive binary star which now bears his name.
The system is comprised of two giant O-type stars, each of which orbits
a common centre of gravity every 14.4 days. While Plaskett arrived at a mass of 90 Suns for each star, it is now probable that the total mass of the two does not exceed 100 Suns. Even so, this pair still stands as the most massive double star sytem yet discovered.
Plaskett's Star is probably a member of the NGC 2244 cluster (see above).
Hubble's Variable Nebula (NGC 2261) and the mystery star R
The nebula has a curious shape, somewhat like a comet's tail. At the
"head" of the comet is where the variable R Monocerotis will be found
This reflection nebula has a usual visual magnitude of about 10, but
this fluctuates sporadically. It was originally thought that as R
Monocerotis's visual magnitude changed, so did the visual magnitude of
the nebula. But this proved to be false; the nebula's variations do not
seem to be associated with the star's variablility.
However, Valerie Illingworth (Facts On File Dictionary of Astronomy)
states that the variability of the nebula comes from what is called a bipolar
flow of emissions originating from R Monocerotis. This ejection of gas in two
opposite directions is typical in very young stars.
Other observers question the existence of R Monocerotis, considering
the area nothing more than an extremely dense gaseous area. Tirion's
SkyAtlas 2000.0 doesn't show the star, and major star catalogues
don't list the star. Burnham calls it "a bright nebulous condensation
with perceptible apparent size."
Observations at Kitt Peak and Mauna Kea have concluded that R Monocerotis
is a protoplanetary system. That is, that planets may presently be forming
in a highly condensed region: another "solar system" being born.
While the nebula is easily seen in small scopes, it is a little tricky
to find. Burnham's Celestial Handbook has a finder's guide (p.
1201). Or you may try this: once you locate epsilon Monocerotis star-hop
up to 13 Monocerotis. Farther up, to the northeast, is S Monocerotis.
Between these two, just about half way, the great nebulosity surrounding
S Monocerotis begins. At the extreme southern edge of this nebulosity is
the distinguishing form of Hubble's Variable Nebula: the comet-like
shape is unmistakeable.
For a more detailed appreciation of Monoceros, visit the Binocular Section.