DON GOLDMANDOUGLAS J. STRUBLE26 ASTRONOMY • JANUARY 2022
41 M106
The challenges of observing spiral structure
in galaxies are many. The two most important
considerations are the brightness of the arms
and the gaps or distance between them. High-
contrast arms are easier to observe in detail with
smaller telescopes. And sometimes a galaxy
with fewer arms — such as a barred spiral, which
often has two — presents clearer arm structure
than a galaxy with many arms.
One such barred spiral is M106, located
roughly midway between Beta (β) Canum
Venticorum and Gamma (γ) Ursae Majoris. Also
designated NGC 4258, this galaxy is inclined
64° from face-on. At about 9th magnitude, the
galaxy may be seen in small instruments. Its
spiral structure is visible in moderate telescopes
and becomes more conspicuous with increasing40 The Jewel Box
If you’re an amateur astronomer planning to travel south
of the equator, one of the best celestial sights you can
point your telescope at is the Jewel Box (NGC 4755). This
collection of stars ranks as one of the sky’s finest open
clusters. But it’s not because of its size (10' in diameter)
or brilliance (magnitude 4.2), nor even the number of
stars it contains (more than 100). The reason the Jewel
Box enthralls observers is its colorful collection of stars.
Most open clusters contain hot, recently formed stars,
which look blue through a telescope to most observers.
The Jewel Box, however, contains more than half a dozen
stars of various shades of blue, yellow, and orange.
Astronomers think NGC 4755, at 14 million years old,
is one of the youngest open clusters. It lies some 6,400
light-years from Earth.
French astronomer Nicolas Louis de Lacaille discov-
ered NGC 4755 with a ½-inch refractor in 1751, when he
traveled to the Cape of Good Hope in South Africa. But
English astronomer Sir John Herschel’s eloquent descrip-
tion — “a superb piece of fancy jewellery” — led to other
astronomers coining the popular name Jewel Box.
NGC 4755 has yet another name: the Kappa (κ)
Crucis Star Cluster. At magnitude 5.9, Kappa Crucis
reigns as the second-brightest star in the cluster. The
only other cluster member that tops it — by an almost
indiscernible 10 percent — is SAO 252069, which glows
at magnitude 5.8.
The Jewel Box lies 1° southeast of magnitude 1.3 Mimosa
(Beta [β] Crucis). Sharp-eyed observers at a dark site will
spot the cluster without optical aid as a glow that looks like
an out-of-focus star.
Most observers viewing NGC 4755 through 4- to 8-inch
telescopes with an eyepiece that provides a wide field
of view (try a magnification around 50x) see the famous
A asterism, which is formed by the stars closest to the
center. Take in the whole cluster, and you’ll see 10 or more
colorful stars, another 20 white ones, and a faint backdrop
of more than 100 other cluster members. — M.B.aperture. Beyond the bar structure is an
extended disk with a lower surface brightness.
M106 is one of five Messier objects in the
small, galaxy-rich constellation of Canes
Venatici. M3 is a globular cluster, while the oth-
ers — M51, M63, and M94 — are spiral galaxies,
each with unique characteristics. M106 is an
SBbc spiral with a generous size of 18.7' by 7.2',
corresponding to 135,000 light-years across
at its distance of 24 million light-years. It is
one-third larger than the estimated size of our
Milky Way.
When you observe this galaxy, look for the
bright, starlike nucleus. With small telescopes,
you will more easily see the core than its other
morphological features. Here, you are looking at
an explosive nucleus powered by a black hole
with some 30 million solar masses.
In the early 1940s, Carl Seyfert studiedgalaxies with spectroscopy. M106 is classified
as a type 2 Seyfert galaxy, due to the narrow
emission lines in its spectrum. Later research
of such so-called active nuclei found they are
similar to quasars, but the galaxies surround-
ing them are more noticeable because the
central black hole’s energy emission is lower.
This galaxy also contains water vapor masers —
regions of excited atoms that emit coherent light
in microwaves instead of visible light like a laser.
Watching the motions of these masers allowed
astronomers to directly determine the distance
to another galaxy, independent of other indica-
tors, for the first time.
While you’re in the neighborhood, check out
NGC 4248, a magnitude 12.5 dusty edge-on spiral
and likely companion to M106 and NGC 4217. The
latter is a more distant edge-on spiral, with a dust
lane and a magnitude of 12.4. — A.G.