OF THE ROGER N. CLARK

M42 (NGC 1976), M43 (NGC 1982), THE
GREAT NEBULA IN ORlON
M42: RA. 05h 35.4m Dec. -05° 23 ' (2000.0)
M43 : RA. 05h 35.6m Dec. -05° 16'
Technical. Messier 42 and 43 are probably
the brightest and most spectacular nebulae in
the sky, rivaled only by the Eta Carinae
Nebula. Often referred to as the Great Nebula
in Orion, M42 and M43 are a beautiful
example of an H II region: an emission nebula
containing mostly hydrogen, fluorescing in
the ultraviolet light of very hot, newborn
stars in its midst. The light of the Orion
Nebula consists primarily of green emission
lines of oxygen, with other colors from blue to
red coming from emission of hydrogen,
helium, nitrogen, and neon.
The distance of the Orion Nebula is usually
given as around 1300 light-years and its
size as about 30 light-years. The bright region
is surrounded by vastly larger, dark
clouds of gas and dust, which in fact fill much
of Orion itself. The dust is thought to be
primarily silicate (rock) particles only a micron
in diameter. The composition of the
glowing gas has been given as: hydrogen
90.8%, helium 9.08%, carbon 0.05%, oxygen
0.02%, nitrogen 0.02%, sulfur 0.003%, neon
0.0009%, chlorine 0.0002%, argon 0.0001 %,
and fluorine 0.00001 %.
Deep within the cloud many stars are
forming. One sign that the dark cloud is
much bigger than the portion we see is that
the region is full of bright infrared sources -
stars whose visible light is blocked by dust.
The bright nebula appears to be a hole blown
in the dark cloud's wall, allowing us to see
part way in.
In the brightest part of the hole are the four
bright Trapezium stars, which are responsible
for illuminating the gas. These very
young, hot stars are estimated to be only
100 000 years old. They are among the
youngest stars that amateurs can see.
VISUAL ASTRONOMY OF THE DEEP SKY
Visual. The Orion Nebula is easily seen
the middle of the "Sword of Or ion" as a
star. Curiously, however, its haziness is
mentioned in ancient records. The
known discovery of the nebula was by Nicholas
Pieresc in 1611.
It is very pretty in any instrument
binoculars to the largest telescopes. M42 is
arc-minutes in diameter (twice the size of
full Moon) with a total visual magnitude
M43 is about 7 by 5 arc-minutes and has
total magnitude of 8. The average
brightness of the Orion Nebula is 21. 7
tudes per square arc-second - but the fai
parts are much dimmer and the T
region very much brighter, in the range of }
magnitudes per square arc-second. This
range in brightness is difficult to
and most pictures overexpose the Tra
region. The eye has a larger dynamic
than film and, given good skies, can see
faint and bright portions of the nebula at
same time.
The detail visible in the Orion Nebula
truly spectacular. Here is one of the
sights in the sky that, when seen
modest amateur telescopes, impresses
those not excited by astronomy. The in
cy is beyond description, and even after
dreds of hours of viewing this nebula, it
remains a beautiful and wondrous sight.
Its very complexity makes the nebula
ficult to draw. The drawing made with
8-inch on page 101 took two nights U
17 and 18, 1983) and nearly six hours of
(2 hours of observations and 4 hours for
final drawing). This short a time was
possible because many hours had been
viewing and sketching the nebula on
nights. In fact a drawing done a year
shown on page 103, shows considerably
detail. At that time I had less
making detailed observations, used
magnifications, and spent less time
telescope.
The Trapezium region contains the
est patch of the nebula, about 4 by 3
minutes in size. The surface brightness
is so high that the amount of detail is
limited by atmospheric turbulence (
rather than the limitations of the eye
situation almost unknown in visual
work. With so much light, very high
can be used.
--
Many stars dot this region. The four
b . htest are the famous multiple star system
ei l brionis. They form a trapezoidal figure
t h a t inspired the group's name. The four
s are easily resolved . in small telescopes,
as their separatIOns range from 8.7 to 19.2
c _seconds and their magnitudes from 5 to 8.
wo additional components, shiing at 1 th
magnitude, can be :nade out With a 6-mch
telescope in steady alf. The rest of the nebula
's full of fainter stars: over 300 brighter than
agnitude 17 are within just 5 arc-minutes of
star
the Trapezium. The nebula contains more
than 50 variable stars with maxima greater
than magnitude 14.
The fainter regions of the nebula contain
photographs. They are visible in moderate
sized amateur telescopes. The brightest arc,
extending south on the east side, is visible in
telescopes as small as about 2 inches. Since
many loops of nebulosity familiar from
M42 is over one degree in diameter, low powers
are needed to view it all at once. However,
only at moderate to high powers do most of
the elegant arcs become visible. In the 8-inch,
they were best seen at powers near 200x.
The faintest portions of the nebula form a
loop at the southern end, opposite the
Trapezium, giving the nebula a circular,
almost bubble-like shape. At low power this
outer loop is seen only on very dark nights.
Surprisingly, though, at medium to high
powers it can be seen on moderate nights
such as those on which the drawing of J anuary
1983 was made. The outer loop appears
continuous at low powers, but at 117 X in the
8-inch, the individual sections seen on photographs
could be resolved.
.
Powers onoox to 200x show a dark patch
Just south of the Trapezium. This patch is
next to a star 2 arc-minutes south and slightly
west of the southern tip of the bright zone
containing the Trapezium stars. This bright
ange, and its edges appear remarkably
corner of the nebula is a nearly perfect right
straight. It looks almost unnatural.
To most observers the Orion Nebula
appears pale green. The visibility of other
colors is somewhat controversial. A few
oservers report a faint reddish color, especlal
. ly on the southern edge of the Trapezium
region - which is indeed quite red in photographs.
On extremely dark nights, I have
seen the T
_
.
d r .
A VISUAL ATLAS OF DEEP-SKY OBJECTS
rapezlUm an lamt outer regIOns to
.
appear green while most of the nebula
appeared a vivid pastel pink, and the bright
arc extending south on the east side appeared
pastel blue. Blocking the bright area around
the Trapezium makes the detection of red in
the outer regions easier.
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