56 ASTRONOMY • FEBRUARY 2022T
Turn your eyes toward the night sky and
you will see a bright, hazy band of light
cutting across the sky.
For millennia, observers speculated
about the Milky Way’s true nature. The
Greeks said the streak of haze in the sky
was milk spurting from the breast of
the goddess Hera, Egyptians thought it
was cows’ milk, and some Aboriginal
Australians thought it was a river f low-
ing through the sky.
Today, we know that we are looking
along the plane of our spiral galaxy, con-
sisting of at least 100 billion stars. But
understanding the shape of the Milky
Way proved elusive up until the 20th
century. The problem is we can’t get a
bird’s eye view of our galaxy because our
solar system is buried within t he ga la x y.
But with the invention of the telescope,
photography, spectroscopy, and radio
astronomy, we have uncovered the shape
and size of our home galaxy — and our
place among the billions of stars that
make up our island universe.The telescope revolution
Before the telescope, there was no clear
understanding of the extent of our gal-
axy. Nearly 25 centuries ago, the Greek
philosopher Democritus proposed that
the Milky Way was filled with stars
that appeared to blend together because
of their great distance. However, 100
years later, Aristotle suggested that the
hazy river of light was an atmospheric
phenomenon. Aristotle’s authority was
accepted for nearly 2,000 years, until two
small pieces of glass finally unseated him.
When Galileo turned his telescope to
the sky in 1609, he made astounding dis-
coveries. He published his observations
in a little book, Sidereus Nuncius, in
March 1610. The Moon was rugged and
imperfect, he declared, and Jupiter had
four companions. Galileo also scanned
the Milky Way and reported, “This spy-
glass has allowed me to discover a multi-
tude of fixed stars never before seen, of
which there are more than ten times as
many as are naturally visible.”To understand the
nature of our galaxy,
astronomers had
to look to distant
island universes.
BY RAYMOND SHUBINSKI
The shape of
the Milky Way
Observing targets like the Orion
Nebula (M42) and the Beehive Cluster
(M44) in the constellation Cancer,
Galileo found myriad stars unobservable
to the naked eye. He thought all the fuzzy
objects in the sky would be resolved into
stars; the astronomer could not know
that each would play a role in giving us
a picture of our galaxy.Spiral nebulae
After achieving overnight fame in 1781
when he discovered Uranus, William
Herschel was swiftly appointed as King
George III’s court astronomer. The king
gave him money to build telescopes,
including his 40-foot-long (12 meters)
telescope with a 48-inch mirror.
With it, Herschel produced perhaps
the first systematic map of the Milky
Way. He started by observing a dense
area of the Milky Way and counting the
number of stars in his field of view. As he
moved away from the plane of the Milky
Way, the number of stars dropped.