101 SKY OBJECTS
100 The Andromeda
Galaxy
What’s the farthest object you can see with only your eyes?
Unless you live under extremely dark skies unspoiled by
light pollution, the answer is the Andromeda Galaxy (M31),
located some 2.5 million light-years away. Traveling across
such a vast, empty gulf reduces the combined light of M31’s
estimated 1 trillion stars to a 4th-magnitude smudge that we
can spot just 1.5° west of Nu (ν) Andromedae.
The early 20th century was a watershed moment for the
Andromeda Galaxy. In 1923, Edwin Hubble calculated the
distance to M31 at 1 million light-years. Although later mea-
surements showed this number was off, Hubble’s estimate
put M31 beyond the known bounds of our galaxy, serving as
the first hint that the Milky Way was not the entire universe.
Classified as a barred spiral galaxy like the Milky Way,
backyard telescopes display Andromeda as a large, oval
smudge of grayish light. Spanning 3°, or six Full Moons
placed side by side, Andromeda will engulf your field of view.
Take your time studying it and you will notice a bright star-
like core marking the center of its disk.
A 4-inch or larger scope shows that the galaxy’s south-
western edge gently fades away, while the northeastern
edge abruptly halts. This latter sharp boundary is due to a
thin lane of opaque dust along the galaxy’s circumference.
A second, dimmer outer dust lane, as well as a dim adjacent
glow, lies beyond, marking a separate spiral arm grasping
out in our direction.
Joining Andromeda are two dwarf elliptical satellite
galaxies, M32 and NGC 205. M32 lies less than half a degree
south of the heart of M31, while NGC 205 is instead just over
half a degree to its northwest. Another distinctive feature in
the field is NGC 206, a large stellar association in M31 con-
taining more than 300 spectral type O and B stars. You’ll find
its soft glow about half a degree southwest of M31’s center.
Today, the Andromeda Galaxy may appear relatively dim
to the naked eye. But it will eventually get much brighter.
In about 5 billion years, it will collide with our Milky Way,
ultimately merging into a single giant elliptical galaxy. — P. H.
A most glorious deep-sky treasure hides
in plain sight in the spring sky — the
Coma star cluster (Melotte 111), which
Ptolemy cataloged as a “nebula” around
A.D. 138. This spangle of seven prominent
naked-eye suns (and twice as many
that are fainter) form the most promi-
nent part of the constellation Coma
Berenices, Berenice’s Hair.
Unlike most open star clusters, which
hug the Milky Way’s spiral arms, we see
the Coma Star Cluster only about 5°
west of the North Galactic Pole — the
point on the celestial sphere’s northern
half at which our galaxy’s axis of rota-
tion is aimed. Shining at magnitude 1.8
and spanning 5° of sky, the Coma star
cluster is one of the largest and bright-
est open star clusters in the sky. At a
distance of 288 light-years, it is also one
of the nearest. The 400- to 600-million-
year-old cluster contains some 270
members ranging from magnitude 5 to
10.5. The cluster's total mass is likely less
than 100 solar masses, and its density is
about one star per 33 cubic light-years.
Its members form a roughly triangular
pattern in the sky that spans 22.5 light-
years of space. Through binoculars,
more than 100 stars can be seen in this
wedge-shaped portion formed by 12,
14, and 17 Comae Berenices. The cluster
contains no fully-evolved giant stars.
Ten reasonably bright galaxies are
within reach of a 4-inch telescope under
a dark sky. They all lie within 2° of the
Coma star cluster: NGC 4225, 4251,4274, 4278, 4283, 4314, 4448, 4494,
4559, and 4565. These all belong to the
great Coma Cluster of galaxies (see #11),
a turbulent array of thousands of galax-
ies (mostly ellipticals and lenticulars)
more than 300 million light-years
distant. It is the nearest massive cluster
of galaxies and is scattered across more
than 20 million light-years of space.
Once you’ve finished inspecting the
Coma star cluster with binoculars and
a telescope, study it with your unaided
eyes to see how many dim stars you can
detect. The cluster contains nine stars
between magnitude 6.0 and 7.4, with
separations ranging from 12" to 31". — S.J.O.101 The Coma star cluster
ALAN DYER
WWW.ASTRONOMY.COM 65ANTONIO FERRETTI/ATTILIO BRUZZONE