The Genius of Louis Pasteur

tion. This glow makes it harder to
pick out really dim objects.
Even out in the country, away from
all man-made lights, some nights are
better than others for looking at faint
celestial objects. The biggest culprit
is humidity. Water in the air does not
have to be in cloud form in order to
block out light. On clear but hazy
nights, light from the faintest stars
will be extinguished by water vapor.
Fortunately, in the fall we start to have
cool, clear, and dry nights—perfect
conditions for a look at the sky.
A Giant Dipper
Taken together, the stars in the
constellations Andromeda and Pegasus
can be imagined to form a Giant
Dipper, much larger than the famous
Big Dipper near the Pole Star.
First, you must find the body of the
dipper. These stars are sometimes
called the Great Square in Pegasus.
Look toward the southeast and then
raise your head about 60 degrees.
If you are not accustomed to measuring
angles in degrees, here's a tip:
The width of your outstretched hand
held at arm's length is about 20 degrees.
To find the Great Square,
move your eyes three hand lengths
up from the southeastern horizon.
Each of the sides of the Great Square
is between 15 and 20 degrees, so you
can use your hand measurements
again to make sure you have found
the right stars.
Before finding your way over to
Andromeda, let's look at Pegasus for
a moment. The name comes from
the winged horse of Greek mythology.
For some reason, the horse is
always depicted as being upside
down!
The most noticeable thing about
Pegasus is the lack of stars, compared
to nearby regions of the sky.
This is so because Pegasus lies above
the central plane of our galaxy, which
is the most heavily populated region.
One of the classic tests of determining
how good your vision and
the "seeing" are, is to count the stars
that lie within the Great Square. Most
star charts show 30 or 40 stars, but
you will have to have very good conditions
to see all of these.
Like most relatively bright stars, the
stars that define the corners of the
square have two names. The oldest
name is its Arabic name. Arabic
scholars kept the Greek astronomical
tradition alive during the Dark
Ages. Scientists of the European
Renaissance borrowed the Arabic
star names when they began to renew
their interest in astronomy. The
other star name is usually a letter of
the Greek alphabet combined with
the constellation name. The brightest
star in a constellation gets the first
letter, "alpha," and the others are
assigned letters in descending order
of brightness.
Starting from the northwest corner
of the square, the stars are called
Scheat (Beta Peg), Markab (Alpha
Peg), Algenib (Gamma Peg), and Alpheratz
(Alpha And). This last star is
actually a little inside the Andromeda
constellation, and since it is called
Alpha, you know that it is the brightest
star in Andromeda.
Alpheratz is where Andromeda's
handle joins Pegasus's bowl to form
the Giant Dipper. Unlike the Big
Dipper, the Giant Dipper has two
handles running side by side, two
stars at a time. You will use one pair
of these stars as a pointer to the Great
Galaxy in Andromeda.
The second pair of stars in the handle
are called Beta and Mu Andromedae.
They are about 4 degrees apart
(which is a little less than half your
clenched fist held at arm's length).
Follow an imaginary straight line
north through these two stars another
4 degrees. You will be looking at
the Andromeda Galaxy.
The Andromeda Galaxy
The Andromeda Galaxy is a twin to
our Milky Way Galaxy. Both contain
as many as a trillion stars and both
are spiral galaxies.
Andromeda is inclined 15 degrees
to our line of sight, so we are seeing
it almost edge on. Even though it is
millions of light years away, it is still
surprisingly large in the sky. If our
eyes could see light from the faintest
edges of Andromeda, it would appear
to stretch across 5 degrees, the
length of 10 full moons lined up side
by side! But even under the best
conditions, our eyes can discern only
the brightest 2 degrees of its center.
To the naked eye, Andromeda is a
very faint, fuzzy oval of light.
You will be seeing light that left
Andromeda 2,200,000 years ago.
Since light travels at 186,000 miles a
second, that adds up to too many
miles to work with conveniently, so
astronomers prefer to measure such
large distances by simply referring to
how long the light traveled to reach
us—a light year. The light year is not
a measure of time, but of distance;
one light year equals 5.8786 trillion
miles.
Whether or not you actually see
Andromeda will depend on the
seeing conditions and how carefully
you look. To increase your chances
of success, do the following. Allow
your eyes to adjust to the dark for at
least 30 minutes. If you try to look
when you first come outdoors, you
won't find it.
If after a half hour of allowing your
eyes to get dark adapted, you still do
not see it, try using "averted vision."
Your side or peripheral vision is more
sensitive to faint light than your direct
vision. If you direct your gaze a
little to the side of where you want
to look, you might be able to see that
faint, hazy patch of light.
In the next article, we'll return to
the question of how to tell time by
the stars.
GAUGING DEGREES IN THE SKY
When your arm is outstretched in front
of you, your handspan should measure
about 20 degrees.
The Young Scientist FUSION September-October 1986 55