8 ASTRONOMY • JANUARY 2022
4 Barnard’s E
Barnard’s E has to be one of amateur astronomers’
all-time favorite binocular objects. This combination of
two dark nebulae (Barnard 142 and Barnard 143) comes
from American astronomer Edward Emerson Barnard’s
catalog of such objects.
The E lies against the rich Milky Way in Aquila. Start at
yellow-hued magnitude 2.7 Tarazed (Gamma [γ] Aquilae).
If you center Tarazed, you shouldn’t have to move your
binoculars at all. Barnard’s E lies 1.6° west-northwest of
the star.
Barnard 143 (often designated B143) is the easiest
of the two to spot. Its most visible part is a narrow bar
about 15' long, which stretches east to west. A second
dark bar of the same length connects at its east end
and heads northward. The end of that bar is the start
of a third one, which parallels B143. The combination
of these three bars forms a U-shape whose open end
points to the west.
Just to the south of B143 lies Barnard 142 (B142),
which also parallels its northern counterpart. This dark
bar isn’t quite as distinct as B143, making it more dif-
ficult to see.
Astronomers classify the clouds that form Barnard’s E
as dark nebulae. These objects, which will become
future star-forming regions, are composed of dust and
cold gas. Really cold gas: The temperature of the hydro-
gen in dark nebulae is always within about 10 degrees
of absolute zero. If it were warmer than that, the atoms
within the cloud would be moving too fast to merge
together when they collide, and stars would never form.
Dark nebulae give off no light of their own and they
don’t reflect light from nearby stars. The only way we
see them is if they lie in front of bright backgrounds.
Barnard’s E lies closer to us than the Milky Way behind
it. If you view it through binoculars from a dark site,
you’ll see it silhouetted before many tens of thousands
of unresolveable stars. — M.B.5 The Cone Nebula
The Cone Nebula (NGC 2264) and its associated open cluster,
the Christmas Tree Cluster, is in the faint constellation Monoceros
the Unicorn. The easiest way to find it is to point your telescope
3.2° south-southwest of magnitude 3.4 Xi (ξ) Geminorum. At
magnitude 3.9, the cluster glows brightly enough for you to spot
with your naked eyes, albeit as an indistinct fuzz ball. It lies some
2,700 light-years away and measures about 7 light-years across.
With an eyepiece that yields a magnification of 50x, you’ll see
10 to 15 stars to the east and west of 15 Monocerotis, which glows
at magnitude 4.7. This line forms the ½°-long base of the tree.
Note that while its top points to the south, the tree may seem
upside down to you, depending on your scope’s optical configu-
ration. The stars south of the line may appear to make the tree’s
top, but they’re not actually moving through space with those in
the main cluster. They only happen to lie in the same direction.
At the southern end of the Christmas Tree Cluster is the Cone
Nebula. This dark nebula is a cloud of dust and cold gas with a
conical shape. And while the cluster is easy to spot with your eye,
the nebula is not for observers. Instead, it’s astroimagers who
capture it quite easily, superimposed on the brighter emission
nebulosity behind it. The emission nebula’s crimson glow results
from a two-part process: Its hydrogen first absorbs ultraviolet
radiation from the intensely hot star 15 Monocerotis (the brightest
star in the Christmas Tree Cluster), then reemits it as red light.
To see the Cone Nebula, you’ll need a large amateur tele-
scope at a dark site. A 14-inch or larger scope might show a
strip of nebulosity about 5' long that originates at the brightest
star and proceeds to the west. This gas belongs to the emission
nebula Sharpless 2–273, which stretches an additional 2° to the
west. — M.B.MICHAEL STECKER