charpter1 note - Milky Way Galaxy the Sun and all the stars...

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Milky Way Galaxy : the Sun and all the stars we can see with the naked eye make up only a small part of a huge disk-shaped collection of stars. Sun is a star . has about 100 billion stars and is 100,000 light-years in diameter. is one of the two largest among about 40 galaxies in the local group . Galaxy is a great island of stars in space, containing from a few hundred million to a trillion or more stars . All held together by gravity and orbiting a common center Our solar system is located a little over halfway from the galactic center to the edge of the galactic disk. Cluster (or group) of galaxies: a collection of galaxies bound together by gravity . Small collections (up to a few dozen galaxies) are generally called groups , while larger collections are called clusters Galaxy clusters ( ? ? ? ): groups of galaxies with more than a few dozen members Superclusters ( ? ? ? ? ): A gigantic region of space where many individual galaxies and many groups and clusters of galaxies are packed more closely together than elsewhere in the universe. The regions in which galaxies and galaxy clusters are most tightly packed.
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