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Varieties of Galaxies
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 Introduction
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 The Universe Expansion
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| Determine the Expansion |
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| Expansion in Past / Future
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| Utmost End and Center |
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| The Struture |
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| Varieties of Galaxies |
There are various types of galaxies that form the basic unit of a gathering of stars. They are "Disc Galaxy," "Elliptical Galaxy," and "Irregular Galaxy."
Disc galaxies hold 60 percent of the whole galaxies. A disc galaxy has a shape of a thin disc just like a compact disc, and about 100 billion stars are concentrated within it. Our Milky Way galaxy is this shape too. Because the young stars seem to stack in a spiral, it is also called a "spiral galaxy." The reason why it has this shape is because the galaxy itself is rotating. Basically, the stars are attracted by universal gravitation. But the stars that gather rotate as one group, causing centrifugal force on the vertical direction to the rotating axis. Centrifugal force is a force that tries to go away from the center of rotation. If this happens, the galaxies can gather on the same direction as the rotating axis, but not towards the vertical direction to the rotating axis. As the result of this, the galaxies became flatter and flatter, and disc galaxies were formed. The Milky Way galaxy that we see twinkling in the sky is the flat appearance of our galaxy seen from a side.
30 percent of the galaxies are elliptical galaxies. Although the name is "elliptical" galaxy, the shape can be a sphere, lens, or oval. What they have in common is that they do not rotate like the disc galaxies. We have told you that elliptical galaxies do not rotate, so then why do they
not crush without having centrifugal force? This is because the oval galaxies
retain their shape by the random motion of the many stars within them. For example,
if you put a giant rubber band around several people and tell them to move as
they wish even if they crash into each other, they will not divert from their
course. Although they are joined together with a band, you can expect that they
will not, at random, gather at one point and be squashed.
We told you that the disc galaxy retains its shape because of its centrifugal force. Because the galaxy remains as it is, it must mean that this centrifugal force and universal force are in balance. We observe the galaxies, stars and gasses by their light, and we calculate the amount of matter by the amount of light that shines. This also means that we are able to estimate only the mass of matter that is shining. This matter is atoms of hydrogen and helium (Baryons) that are able to radiate and absorb light, and also, the mass consists of protons and neutrons that form the nucleus. But with only the mass of the stars that we could observe, we found that the mass itself cannot produce the gravity in balance with the centrifugal force calculated from the speed of rotation. What this means is that in the universe, other than baryons which we can observe, there is a possibility of the existence of matter with mass that we can't observe. This matter is called the dark matter. (It does not shine, so it is called dark matter.) It is said that the total mass of the dark matter in the universe is more than 10 times the mass of all the shining stars added together.
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