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עמוד:7
Chapter 3 : Our Place in the Universe Solar System : Our solar system includes the Sun and other bodies , such as planets , moons , asteroids , and comets . The Sun is a star that gives energy ( heat and light ) to our solar system . The Sun's gravity keeps other bodies in orbit around it . This means that the planet or body follows a set path around the Sun . Our solar system includes the Sun and nine known planets : Mercury , Venus , Earth , Mars , Jupiter , Saturn , Uranus , Neptune and Pluto , the farthest planet that orbits the Sun . Universe : The universe contains all matter and energy , such as planets , suns , moons , stars , asteroids and galaxies , including our Milky Way galaxy . No one knows how big the universe is , but astronomers believe that it is still expanding * . How did the universe begin ? Many researchers believe that in the beginning , everything was one tiny , solid point of matter . Then , billions of years ago , this tiny point exploded and created our universe . The idea that our universe started with an explosion is called the Big Bang theory . Earth : The Earth is a planet —a very large , round body that orbits the Sun . It is the third planet from the Sun , and the fifth largest planet in the solar system . Scientists believe that the Earth is the only planet in our solar system that has water on its surface . Water is necessary for life as we know it . The atmosphere surrounds the Earth and is made up of many gases . Galaxy : Galaxies include planetary systems * as well as huge groups , or clusters , of stars . We live in the Milky Way galaxy . Astronomers believe that there are billions of galaxies in our universe , and that each galaxy contains trillions of stars . Because most galaxies are so far away , astronomers must use very powerful telescopes to study them . Weare just beginning to learn about the many galaxies in our universe . Additional activity on 37-38 . * expanding — getting larger . * planetary system —A planetary system has a sun and planets . Only our planetary system is called a solar system .
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