A gradual change is
occurring in the heavens above. This change occurs every day, it's just
so minute that most of us never even notice. This shift of the starry
curtain became official with the change of seasons; summer gives way to
autumn on the Autumnal equinox (around September 21). The air will
become cooler, leaves are changing color, but the stars of summer
linger on for just a little while longer.
After dusk in early October go out and look to the top of the sky.
You'll see three bright stars forming the Summer Triangle, the seasons'
dominant asterism. The brightest star, at the upper right as you face
south, is Vega, which is part of the constellation Lyra "the Lyre."
Vega is a white hot star located 26 light-years away. From Vega, direct
your eyes slowly to the east. The first bright star you'll find is
Deneb, the second corner of the Triangle and the tail of Cygnus "the
Swan." Cygnus is also known as the Northern Cross. Looking down to the
south you'll find Altair, the last star of the Triangle. Altair is part
of the head of Aquila "the Eagle." It takes a little more practice and
imagination to see the eagle.
If you view the Summer Triangle from a dark location in the country
you'll notice a peculiar glow. This pearl gray light is caused by
uncounted distant stars in our Milky Way Galaxy. If your skies are
truly dark you should be able to trace the entire summer portion of the
Milky Way. Begin in the northeast, near the "W-shaped" constellation
Cassiopeia and move toward the top of the sky to the Summer Triangle.
Now move toward the southwest where you may come across the
constellation Sagittarius if your horizon is free of trees.
Say goodbye to these stars of summer! They set in the west an average
of four minutes earlier each night. This shift in the stars is caused
by the orbit of Earth around the Sun. It's like having a window in each
wall of a four corner house. As you look out each window, you're
greeted with a new view of the space beyond. The shift of the sky from
summer to autumn is special. Our view turns away from the Milky Way
Galaxy to deep space. No longer are we treated to views of the star
clusters and glowing clouds of gas that populate our Galaxy. Autumn is
a season of galaxies. Before we attempt to locate a galaxy, let's find
two prominent constellations of autumn. The first one is easy.
In early October the constellation of Pegasus "the Winged Horse" is due
south at 11:30 pm. To find Pegasus first look for the Great Square.
That's right! In the summer we have a triangle and in the autumn we
have a square. The stars of the Great Square aren't as brilliant as the
stars of the Summer Triangle, but it's still an easy pattern to
identify. Three stars of the square form part of Pegasus. The northeast
star of the Great Square is the head of Andromeda, the daughter of King
Cepheus and Queen Cassiopeia.
Andromeda appears as two arcs of stars that meet
at the star Alpheratz, which is the head of Andromeda. A very special
object lies in Andromeda. Once the evening sky is Moon (and cloud) free
go out and locate Andromeda. Start with your unaided eye, but be sure
to grab a pair of binoculars. Start from Alpheratz and follow the upper
arc of stars. Move two stars from Alpheratz and then take a hard look
just to the north of this star. What you're looking for is a faint
patch of light. It almost looks like a detached part of the Milky Way.
This couldn't be further from the truth. That faint patch of light is
the Andromeda Galaxy, an "island universe" located 2.5 million
light-years away. It's the most distant object we can see with our
The Andromeda Galaxy is a spiral galaxy like our own. Its diameter is
estimated to be 150,000 light-years, which is 50,000 light-years
greater than our Galaxy. It also contains more stars. The Milky Way
contains an estimated 100-150 billion stars, while Andromeda may
contain 200-300 billion or more stars! Truly our big sister in space!
its great distance from us, the Andromeda Galaxy appears 3 degrees
across in our sky at its fullest extent. This is 6 times the diameter
of the Full Moon!