The document discusses the history of understanding the Milky Way galaxy. It describes how ancient cultures had different names for the Milky Way. In the 1600s, Galileo first observed the Milky Way through a telescope and realized it was made up of countless stars. In the 1700s, Thomas Wright hypothesized that the Milky Way was a large collection of stars forming a flat disk. Later astronomers like Herschel, Kapteyn, and Shapley made maps of the Milky Way galaxy and helped establish its size and shape, believing it constituted the entire universe until the 1920s.

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THE MILKY WAY UNIVERSE
Cosmic Adventure 3.08

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The Place for the Stars
After the popularization of
Copernicus heliocentric
concept, the question
naturally pop up was:
“Where do the stars
belong?”

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The Milky Way!Photo by European Southern Observatory (ESO)

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The Milky Way
If you look up the sky at night, far away
from other lights such as bright cities,
you’ll be able to see a beautiful streak
of light across the sky. That’s the Milky
Way. It is the groups of stars of which
the sun is one.
There is no written record of how the
Milky Way was noticed. But since it is
there easily picked up by the naked eye,
it must have also been familiar with
our ancestors, long, long time ago.

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The Name of the Milky Way
There were different names and
accounts for the Milky Way in
different cultures.
For example, Ancient Armenian
mythology called the Milky Way
the “Straw Thief’s Way”.
The Cherokee folktale called it the
“Way the Dog Ran Away”.
Peoples in Eastern Asia named this
hazy band of stars the "Silvery
River" of Heaven.
Egyptian mythology described the
Milky Way was as a pool of cow's milk.
Etc. etc.
You can find a great variety of these
names in Wikipedia.
But perhaps the most popular
explanation for the name was
proposed by the ancient Greeks,
although the original records were
lost in history.

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Hera’s Spilled Milk
The legend said that Zeus (Jupiter)
wished to immortalize the infant
Hercules (Heracles). He put the baby
to his sleeping wife Hera's (Juno’s)
breast so the baby could be nursed
secretly. But Hera did not like the
baby, probably because she knew that
it was just another result of Zeus’
affairs. When Hera woke up and tried
to pull away, her breast milk spurted
into the night sky and created the
Milky Way.

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The Formation of Milky Way

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According to Aristotle
According to great Greek
philosopher Aristotle (384-
322 BC), the Milky Way was
the spot “where the celestial
spheres came into contact
with the terrestrial spheres.”
In ancient Greek time, there
was no telescope. It was
hard to say if it was right. It
was only in the early 1600s,
two thousand year later, the
idea began to change

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The Galilean Telescope
One of the greatest things Galileo had
done that the other astronomers did not
was the introduction of the telescope into
astronomy.
The telescope was not invented by Galileo.
It had long been in existence as early as
the 13th century. In 1608, the Dutch
spectacle maker Hans Lippershey began to
make telescope that attracted attention. In
1909, Galileo heard of the invention. He
constructed his own telescope and
improved it to 30 time magnification.

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First Powered Eyes on the Sky
Galileo Galilei began to use his telescope
to observe the heavens in 1609. This was
the first time in history when a human
being set his eye on the universe through
a telescope.
Many of the new things revealed by the
invention were extremely disturbing.
There were craters on a supposedly
perfectly spherical moon; many moons
circling Jupiter. With the ever improving
power of his telescope, Galileo was able
to see that the Milky Way was made up of
countless stars.

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The Milky Disk
In 1750 the English astronomer and
mathematician Thomas Wright
(1711-1786), speculated that the
that the Milky Way was a large
collection of stars held together by
gravity. This huge number of stars
rotate body around each other,
working like the solar system, only
on a much larger scale. The
resulting flat disk of stars can be
seen as a band on the sky from our
point of view on earth.

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Hypothesis of the Universe
In his book “An original
theory or new hypothesis of
the Universe published in
1750, Wright explained the
appearance of the Milky Way
as "an optical effect due to
our immersion in what locally
approximates to a flat layer
of stars“, thus becoming the
first man to define the shape
of the Milky Way.

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Measuring the Milky Way
From 1784 to 1785, the
German-Anglo astronomer
William Herschel (1738-1822)
and his sister Caroline
Herschel (1750-1848), were
trying to survey the Milky Way
for a full map of the stars.
William and his sister Caroline working
on a mirror.

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The Entire Universe
In 1785 the Herschels completed in making a complete map of the Milky
Way. In size, this Milky Way is a huge universe. 100 - 180 kilo light-years
in diameter. 2 kilo light-years in thickness. The sun is the bright dot near
the middle.

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Conelius Kapteyn
From 1901-1922, the Dutch
astronomer, Jacobus Conelius
Kapteyn (1851-1922), made
extensive star counts from
photographic plates.

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~3 𝑘𝑝𝑐
~173 𝑘𝑝𝑐 ≈ 40,000 𝑙𝑖𝑔ℎ𝑡 𝑦𝑒𝑎𝑟𝑠
Kapteyn’s Universe Kapteyn succeeded in making a modern
version of Herschel's Milky Way model with
more accuracy and star counts. The sun is
slightly off the center of the Milky Way.
1 𝑘𝑝𝑐
= 1000 𝑝𝑐
= 1 𝑘𝑖𝑙𝑜𝑝𝑎𝑟𝑠𝑒𝑐

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The Size of our Universe
A light year is the distance
covered by light in one year
and light travels at a speed
of 300,000,000 meters a
second. So the resulted
diameter is just huge
beyond our imagination.
Within this confines, we may
find 200 to 400 billion stars.

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Harlow Shapley
In 1915, the American
astronomer Harlow Shapley
(1885-1972), was working with
the then world’s most powerful
60 inch (1.5 meter) telescope at
Mount Wilson.
Shapley, like most astronomers
of the time, still thought that the
Milky Way was all there was to
the Universe. All other nebulae
were relatively nearby objects
and were merely patches of dust
and gas in the sky.

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Harlow Shapley’s Universe
Shapley succeeded in
measuring and
constructing an overall
picture of the Milky Way
galaxy, showing also the
position of the solar
system. This was
supposed to be the first
complete picture of what
is believed to be our
entire universe.
Our sun is
about here!
About 27,000 light-years from the Galactic centre

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Our Universe
Although familiar, it took
thousands of years for
human to understand what
he was looking at. It was
until 1750 that its shape
was established by Thomas
Wright. Since then up to the
early 20th Century, people
including scientists and
astronomers believed that
the Milky Way in which we
lived, was the only galaxy in
the Universe.

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HUBBLE DISCOVERED A NEW UNIVERSE
To be continued in Cosmic Adventure 3.09