Sky & Telescope - November 2021 - 45

Evenings with the Stars by Fred Schaaf
A Is for
Andromeda
This ancient constellation
leads in more ways than one.
" A
is for Andromeda. " It's a catchy
phrase that suggests that the
constellation comes first - and it does
in a couple of interesting ways. Most
obviously, Andromeda leads the way in
the alphabetical listing of the 88 official
constellations. Let's be glad that the
roll call didn't begin with a faint and
obscure figure like Antlia, the Air Pump.
In constellation-by-constellation star
guides, like the classic Burnham's Celestial
Handbook, it's wonderful to get off
to a smashing start with essays detailing
Andromeda's marvelous galaxy,
color-contrasting double star, and its
many other highlights.
Andromeda also leads the way on
the celestial sphere by virtue of the fact
that its main pattern is dominated by
three 2nd-magnitude stars that stretch
in right ascension from 0h to just past
2h. The only other bright constellation
occupying this right ascension range is
Cassiopeia. But for observers at midnorthern
latitudes, Cassiopeia is circumpolar
and visible all year round, so
it doesn't quite have the strong seasonal
association that Andromeda does.
One practical advantage of being
located near the 0h line is that most star
atlases order their charts from north to
south, starting at 0h right ascension and
proceeding eastward. From the classic
Norton's Star Atlas to the modern Sky
& Telescope's Pocket Sky Atlas, you can
count on finding Andromeda in the first
set of non-circumpolar star maps.
For many skywatchers, the constellation's
name is closely associated with
M31, the famed Andromeda Galaxy.
The object was first described around
AD 964 as " the Little Cloud " by the
Persian astronomical writer Abd alpHEROIC
RESCUE In Greek mythology, Andromeda was chained to a rock, having been sentenced
to death by sea monster (Cetus). Just in the nick of time, the hero Perseus saves her.
Rahman al-Sufi. Until the early part of
the 20th century, astronomers referred
to M31 as the Andromeda Nebula and
thought it was located within the Milky
Way. In reality, the imposing spiral is
even larger and populated with more
stars than our own galaxy. At a distance
of more than 2 million light-years, the
Andromeda Galaxy is the most distant
object readily visible to the naked eye.
How bright is M31? The usual figure
given is around magnitude 3.4. But
because the galaxy is an extended object
whose light is spread over a large area,
it's certainly not as easy to detect as
a star of the same brightness. Most
sources suggest that M31 appears as an
elongated smear of radiance about 1°
wide and 3° long. But talented observer
Walter Scott Houston was, under excellent
conditions, able to trace M31 out
to a length of 4° with his unaided eyes
and 5° with large binoculars.
Let's return to the constellation's
three brightest stars mentioned earlier.
As shown on the Sky Chart presented
on pages 42 and 43, the trio are
arranged in an appealing line. Working
eastward, they are Alpha (α) Andromedae
(Alpheratz or Sirrah), Beta (β)
Andromedae (Mirach), and Gamma (γ)
Andromedae (Alma). Remarkably, each
star is spaced almost exactly 1h of right
ascension from its neighbor: Alpheratz
is at 0h 08m, Mirach at 1h 10m, and
Almak at 2h 04m. Even more amazingly,
their magnitudes are virtually identical,
with Alpheratz and Mirach both
shining at magnitude 2.07 and Almak
at magnitude 2.10. Another interesting
twist is that each star is also at double
the distance of its neighbor. Alpheratz is
97 light-years away, Mirach is 200 lightyears,
and Almak is 400 light-years.
Each star also has at least one notable
trait all its own. Alpheratz performs
double duty by marking the northeast
corner of the Great Square of Pegasus;
Mirach is a striking orange-red star of
spectral type M0; and Almak is a lovely
telescopic double star with component
suns shining gold and blue respectively.
All good reasons to make Andromeda
your first stop on autumn evenings.
¢ As a young boy, FRED SCHAAF read
A for Andromeda, written by astronomer
Fred Hoyle and novelist John Elliot.
sk yand tele scope .o r g * NOVEMBER 2021 45
LIBRARY OF CONGRESS / PUBLIC DOMAIN
http://skyandtelescope.org
Sky & Telescope - November 2021

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