The Universe Is Steadily Geing Larger 117
From a common-sense point of view, the theory that small stars are collected together
in one specific location seems improbable. It is hard to logically explain why stars of a cer-
tain size would be gathered together along a band at a certain location.
Therefore, we should probably consider the theory that the stars constituting the Milky
Way are farther away from Earth than the other stars are. From there, we can come up
with some ideas about what the structure of the Milky Way might look like.
We can start by thinking of the solar system as a sphere inside a larger sphere of stars.
The Milky Way galaxy surrounds that sphere of stars near our Sun.
A Disc-Shaped Galactic Model Is the Easiest to
Understand
There were certainly people who believed in this kind of model of the universe, even during
Galileos time. However, if you look carefully, a better idea will reveal itself.
The biggest defect in this model is that the sphere of stars and the Milky Way are sepa-
rated. It is difficult to explain why this would occur.
What happens if we try to attach these two kinds of stars to each other? If we actually
bring the sphere of stars and the Milky Way together to make a disc-shaped structure, a
belt-shaped flow of stars should be visible in the celestial sphere—that is, precisely the cur-
rent model of the galaxy.
Although the structure of the galaxy shown in the following figure became clearer in
the 19th and 20th centuries—nearly 200 years after Galileo lived—it seems that there were
people who believed that the universe was disc shaped even before that.
Here are a few other hypotheses to consider:
• Since Earth and the solar system are revolving, perhaps the universe is also revolving.
• If matter spreads out while rotating, it is likely that the universe will become disc
shaped.
A faulty model of the structure of the Milky way, based only on what we can see from Earth
Sphere of Fixed Stars
Solar System
Milky Way

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