118 Chapter 2 From the Solar System to the Milky Way
This second hypothesis is actually easy to understand and not unreasonable—it’s
essentially the same phenomenon you see when someone tosses spinning pizza dough
into the air.
Results of Scientific Observation Also Prove a
Disc-Shaped Universe
Until now, we have guessed at the structure of the universe based on the beliefs of people
from the 17th to 19th centuries. However, one person who attempted to elucidate the
shape of the galaxy from actual observational results was the German-born astronomer
Frederick William Herschel (1738–1822).
His method is extremely easy to understand. From the observable sky, he sampled
areas, which he called “blocks,” each of which was a section of sky equal to the area covered
by approximately 1/4 of the full moon. He used a telescope to count the number of stars in
683 “blocks” in various locations around the night sky. Although this was only 0.1 percent
of the area of the entire sky, this technique is believed to be statistically reliable.
The number of stars that humans can see at night with just the naked eye depends on
a lot of factors—where you are on the planet, what season it is, whether or not you are in
a city, whether or not the Moon is out, pollution, and more. Stars are measured on what’s
called a magnitude scale. Very dim stars have very high magnitudes (the dimmest stars are
magnitude 20) and really bright stars have very low magnitudes (magnitude -26 being the
brightest star we can see, our Sun).
Humans cannot see stars dimmer than magnitude 6. The total number of magnitude
6 or less stars is approximately 8,600. But that is for the entire planet! If you are in New
York, you can’t see stars that are over Japan! So the average night sky for Kanna, Yamane,
and Gloria (or for you and your friends) seems to have approximately 2,000 stars visible to
the naked eye. Herschel was using telescopes that he designed, though. With telescopes,
we can see stars much dimmer than magnitude 6. Although no record remains of the total
number of stars Herschel counted, it was certainly more than 10,000. That was an enor-
mous amount of work.
Diameter is approximately 100,000 light-years
Thickness is approximately
15,000 light-years
Bulge
Galactic disc
Position of the Sun
Length is approximately
28,000 light-years
Central core of the Milky Way galaxy
Model of the galaxy that is currently accepted

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