Sky & Telescope - April 2021 - 19

L

eo was the first constellation my dad pointed out to me
when I was very young.
As he held me in his arms with my older sister
standing at his side, he tried to show us which stars belong
to Leo, the Lion. Almost instantly, my three-year-old brain
began panicking at the idea of a wild animal in the sky. My
father tried to calm me by saying they're not real animals -
and then proceeded to point out that Ursa Major and Ursa
Minor are big and little bears. Now I was really scared.
I was carrying on so much that my braver sister was starting to wonder if she should be afraid, too. Dad promptly took
us inside, to the safety of the house, all the while reassuring
us that the animals in the sky were just make-believe. Once
we were back indoors, he managed to gently calm us down.
This early memory, possibly my first, left its mark. Even
now, that long-ago night comes to mind while star-hopping
through Leo to one of my favorite telescopic sights, the three
galaxies of the Leo Triplet.

The Trio in Leo
Finding the Triplet, which is located about 2° south-southeast
of the star Theta (θ) Leonis and almost halfway along a nearly
straight line from Theta to Iota (ι) Leonis, is pretty straightforward. But even though all three galaxies are obvious in
backyard telescopes, the Triplet wasn't discovered all at once.
Charles Messier discovered M65 and M66 in 1780, and
William Herschel completed the triplet in 1784 with his
discovery of NGC 3628. All three galaxies are gravitationally
interacting with one another, although only the distorted
shapes of M66 and NGC 3628 make this apparent. Studies of
NGC 3628's tidal tail have even revealed the formation of a
rare tidal dwarf galaxy within its plume.
M66 is the closest to us at 33 million light-years (Ml-y),
with M65 next at 36 Ml-y and NGC 3628 the farthest at 40

COSMIC TRIO: STEFA N BINNE WIES / JOSEF PÖPSEL / CA PELL A
OBSERVATORY; SK E TCH: HOWA RD BANICH

W COSMIC TRIO The Leo Triplet consists of NGC 3628 (top right), M66
(bottom left), and M65 (bottom right). Note the tidal tail of NGC 3628
stretching eastward toward the top left of the image, as well as the
subtle X pattern of its core. Also note the distant galaxies in the far background. The three Triplet galaxies are less than a half degree apart and
together fit into the low-power eyepiece field of view of most amateur
telescopes. All images have north up.

S TIGHT FIT This sketch of the Leo Triplet hints at the low-power (155×)
view on a dark, transparent night through my 28-inch scope, and shows
how they barely fit into the field of view. A short focus instrument is better suited to observe all three galaxies at once.

Ml-y. They're a tight clustering, tighter than the Local Group
according to current distance estimates. Astronomically
inclined inhabitants on M66, say, would likely see M65 the
way we view Andromeda - or even larger.
Halton Arp's Atlas of Peculiar Galaxies (published in 1966)
lists the Triplet as Arp 317, due to the interaction between
M66 and NGC 3628. M66 gets its own designation - Arp
16 - because of a " large concentration at end of the S. arm. "
Cryptic, but concise.
There are other bright galaxies in the area that are gravitationally bound to the Triplet. For instance, NGC 3593
is located about a degree to the west-southwest of the trio,

Triplet in the Lion
Other
Designation

Distance
(million l-y)

Surface
Brightness

Mag(v)

Size/Sep

RA

Dec.

M65

NGC 3623

36

12.8

9.3

9.8′ × 2.9′

11h 18.9m

+13° 06′

M66

NGC 3627

33

12.7

8.9

9.1′ × 4.2′

11h 20.3m

+12° 59′

Triplet Member

NGC 3628

-

40

13.4

9.5

14.8′ × 3.0′

11h

NGC 3593

-

30

13.3

10.9

5.2′ × 1.9′

11h 14.6m

20.3m

+13° 35′
+12° 49′

Angular sizes are from recent catalogs. Visually, an object's size is often smaller than the cataloged value and varies according to the aperture and magnification of
the viewing instrument. Right ascension and declination are for equinox 2000.0.

sk yandtelescope.org * APRIL 2 021

19


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Sky & Telescope - April 2021

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

Contents
Sky & Telescope - April 2021 - Cover1
Sky & Telescope - April 2021 - Cover2
Sky & Telescope - April 2021 - 1
Sky & Telescope - April 2021 - Contents
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