50 ASTRONOMY • JANUARY 2022
72 M78
Wonderfully mysterious, M78 is the brightest swath of neb-
ulosity in a group of specterlike glows some 2½° northeast
of Alnitak (Zeta [ζ] Orionis), the eastern star in Orion’s Belt.
Pierre Méchain discovered M78 in early 1780 and wrote
to Charles Messier that the nebula surrounded two “fairly
bright nuclei.” These nuclei are two 10th-magnitude stars
(HD 38563A and HD 38563B), and the chief illuminators
of the surrounding nebulae. While the ultraviolet radiation
streaming from these stars is not hot enough to cause
the nearby gas to glow, their light does scatter off dust
particles, making them shine by reflected light.
Messier resolved M78 into a “[c]luster of stars, with a lot
of nebulosity,” while William Parsons, Earl of Rosse, added
it to his list of spiral nebulae. However, in wasn’t until 1919
that Vesto Slipher at Lowell Observatory took its spectrum
and found its true nature was a reflection nebula. Lord
Rosse was likely seeing arcing filaments of dust that curl
around the central stars, giving it a pseudo-spiral form.
These dark bands also divide the shining clouds into
sections, which we see as the nebulosities NGC 2064,
NGC 2067, NGC 2071, and McNeil’s Nebula (which flares
into view only when its illuminating variable star V1647
Orionis experiences an outburst). All are associated with
the Orion Molecular Cloud Complex some 1,350 light-years
distant, which includes the Orion Nebula (see #19).
M78 shines at 8th magnitude and measures 8' by 6' —
about as large as M77 (see #89) in Cetus and a magnitude
brighter! Under a dark sky, it is visible through binoculars.
Small telescopes will show its two 10th-magnitude stars
burning through the nebulosity like bloodless eyes peering
through a frosty window. Larger telescopes at moderate to
high magnifications will shatter the nebulosity into bright
fans, wisps, and streamers tickled by dark skeletal fingers
of dust. Unseen to our eyes are the clusters with hundreds
of newborn stars embedded deep within the nebula’s
dusty folds, as well as a family of 45 infant T Tauri stars
less than 10 million years young, whose cool cores have
yet to ignite. — S.J.O.73 Copeland's Septet
When it comes to obscure objects, Copeland’s Septet (also called Hickson
Compact Group 57) ranks with the best of them. Ralph Copeland discovered
this group in the spring of 1874 while employed by William Parsons, Earl of
Rosse, in Ireland. He was using the Leviathan, a 72-inch speculum mirror
telescope at Birr Castle. At that time, it was the largest telescope in the world,
enabling him to pick up faint galaxies with relative ease.
Located 3° northwest of 93 Leonis, this group is unusual in that it is dominated
by barred spirals. A 12th-magnitude foreground star sits in the middle. NGC 3753
is the brightest and largest member. Also called Hickson 57a, it is magnitude 13.6
and appears edge-on, covering 1.7' by 0.4'. In images, it sports a dark lane; blue,
knotty arms; and faint extensions that may be associated with a collision and
appear to stretch hundreds of thousands of light-years to neighboring NGC 3746.
NGC 3746 (Hickson 57b) is the group’s second-largest member. This magni-
tude 14.2 galaxy has produced two recorded supernovae: 2002ar and 2005ba. A
barred spiral, this galaxy has a more distinct inner ring and S-shaped spiral halo
than fellow group member NGC 3745. The latter is a tiny (0.4' by 0.2') magnitude
15.2 barred spiral also cataloged as Hickson 57g. Photos show it as compact
with an extended faint spiral halo.
Although comparable to NGC 3745 in size, NGC 3754 (Hickson 57d) is
magnitude 14.3. NGC 3748 (Hickson 57e) is a magnitude 14.8 edge-on lenticular
or Sa-type spiral with a narrow dust lane. It spans 0.7' by 0.4'. NGC 3750
(Hickson 57c) is a magnitude 13.9 lenticular galaxy 0.8' by 0.7' in size. It shows
little structure in photos. NGC 3751 (Hickson 57f ) is as bright as NGC 3750 and
an elongated 0.8' by 0.5'. An eighth member, the barred spiral PGC 36010 with
a magnitude of 17.4, was too faint for Copeland to observe. — A.G.
ADAM BLOCKDAN CROWSON