Sky & Telescope - May 2021 - 23

ou don't have to be an amateur astronomer for long
before you learn this: When springtime rolls around, the
most sought-after deep-sky objects lie outside the Milky
Way. I'm talking about galaxies, of course.
Often we're content to casually inspect these vast wonders, but I'm going to challenge you to go even deeper and
look for super-giant H II/star-forming regions (H II/SFR).
These are immense stellar nurseries where new stars are
born and the hydrogen cloud surrounding them glows from
the ionizing ultraviolet radiation of the hot, massive O- and
B-type stars within. The nearest one to us is the Tarantula
Nebula, visible naked-eye in the neighboring Large Magellanic Cloud (S&T: Nov. 2017, p. 24).
Seeing such features might sound daunting for galaxies
beyond our Local Group, but it's really not - I've observed
many in my 10-inch Schmidt-Cassegrain telescope. I'll even
start with one H II/SFR that's so bright you'll be kicking
yourself if you've never looked for it! So let's begin in the far
north and work our way south.
For most people, the dwarf irregular galaxy NGC 2366
can be hard to ﬁnd simply because it lies in the dim constellation Camelopardalis. For me, it's even harder because the
fork arms of my Schmidt-Cass get in the way! So, I ﬁnd it's
easiest to ﬁrst locate the binocular-bright galaxy NGC 2403.
From there it's only a 3° hop north-northwest to the magnitude-5.6 star HD 58425 and then another 0.8° to our quarry.
Sweeping around with an eyepiece yielding close to a
1° ﬁeld at 73×, my eye quickly catches sight of a dim glow
3.5′ × 1′ in size and elongated north-northeast to southsouthwest. At its southern end is a soft-looking, 12.5-magnitude star that forms a narrow diamond pattern with two

stars about 4′ north and a 10th-magnitude star a farther 3′
north. Bumping the magniﬁcation all the way to 322× reveals
that the soft-looking " star " is actually a high surface brightness knot 7″ across and elongated east to west.
Known as Markarian 71 (Mrk 71), this knot is actually
a massive H II/SFR that spans roughly 30 times the width
of the entire Orion Nebula and contains a pair of super
star clusters, massive clusters of very young stars that may
eventually evolve into globulars. I've been able to see it in
my 10-inch with the aperture stopped down to just 50 mm
with an off-axis mask - and that's before adding an O III or
UHC ﬁ lter!
Not surprisingly, it was Mrk 71 and not the rest of the
galaxy that caught William Herschel's eye when he discovered NGC 2366 in 1788. However, it wasn't until 1935
that Mrk 71 was ﬁ rst studied. Today, NGC 2366 and others
like it are classiﬁed as tadpole (formerly cometary) galaxies
because they exhibit dense, luminous " heads " and dimmer
trailing " tails. "

Going to the Dogs
Heading farther south into galaxy-rich Canes Venatici you'll
ﬁnd three more of the galaxies on our tour. The ﬁrst and
easiest to locate is the asymmetrical barred spiral galaxy
NGC 4618, which is visible in 8×56 binoculars almost midway between Beta (β) Canum Venaticorum and M94. Low
power in my 5.1-inch (130-mm) reﬂector reveals an 11thmagnitude star 4′ to its south and the smaller, 12.4-magnitude galaxy NGC 4625 only 8′ to its north-northeast.
At 153× in my 10-inch, NGC 4618 appears as a softlooking elliptical glow 3′ across with a bright, broad core

X NGC 2366 Look for this dwarf galaxy in the
sparse and dim constellation Camelopardalis,
the Giraffe. Nestled in the galaxy is the bright
H II/star-forming region, Markarian 71, which
in turn harbors two super star clusters that go
by many names, including Knot A and Knot B.
One reason Mrk 71 is the second brightest of
its kind in the Northern Celestial Hemisphere
is because its host galaxy only lies 10 million
light-years away.

+75°

HD 49878

Star magnitudes

MICHAEL SINISCALCHI

CAMELOPARDALIS

+70°

3
4
5
6
7

2366

HD 42818

Mrk 71

HD 58425

8h 00m

2403

7h 30m

+65°
7h 00m

6h 30m

s k y a n d t e l e s c o p e . o r g * M AY 2 0 2 1

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Sky & Telescope - May 2021

Table of Contents for the Digital Edition of Sky & Telescope - May 2021