116 Chapter 2 From the Solar System to the Milky Way
The Universe Is Steadily Getting Larger
Whether they believed in the geocentric theory or the heliocentric theory, people in the
16th century thought the universe consisted only of Earth, the Moon, the Sun, planets of
the solar system, and many other stars. The only things moving independently in the sky
were the planets—people thought the other stars were just affixed to the celestial sphere
like lanterns hanging from a wall in the background, forming various constellations.
However, Galileo Galilei felt there were problems with that view of the universe. He
used a telescope he had built himself to gaze at the sky every night, and in 1609, he discov-
ered that the Milky Way was an uncountable aggregation of stars.
Before there were telescopes,
people thought that the Milky Way was
either a cloud or something that was
flowing across the celestial sphere. The
ancient Greek philosopher Democritus
(about 460 BC–370 BC) advocated the
theory that the Milky Way was a col-
lection of faraway stars. Although it is
unclear whether he arrived at this con-
clusion as the result of logical thought
or whether he had extraordinarily
good eyesight, we can guess what his
reasoning must have been: If the Milky
Way were like a gaseous cloud or river,
its position or shape would vary with
time. However, for as long as it has been
observed, its state has not changed,
just like the constellations. Therefore,
it’s natural to believe that it is also a
collection of stars.
Of course, making an inference
is no way to prove a scientific theory,
but Galileo was finally able to observe
that this was the case, approximately
1,200 years after Democritus.
Why Can We See the Milky Way?
If the Milky Way were assumed to be an aggregation of many stars, the next obligation of
scientists should have been to determine its structure. Both Galileo and Kepler must have
been too busy advocating the heliocentric theory, or else they just never thought much
about this point. So let’s try to think about it a little bit ourselves. This will, of course, be
an experiment with no prior knowledge required.
Here’s what we know: Although the Milky Way is a collection of stars, the individual
stars that comprise it don’t shine brightly enough for us to be able to pick them out indi-
vidually with the naked eye. So the Milky Way looks like a cloud to us. This means that the
stars in the Milky Way are either smaller or farther away than the other stars that we can
see from Earth.
The Milky Way

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