MISSION MILKY WAY Many stars dot the clear night sky Scientists believe there are more than 100 billion trillion in the universe Each of these many stars belongs to a galaxy Billions of Galaxies There are many different galaxies Scientists think there are 100 billion in our universe Along with stars each galaxy contains planets moons and other celestial bodies Right at Home Earth is part of the Milky Way galaxy So are its neighboring planets and the sun But our solar system is just one of many Scientists have found 500 other solar systems in the Milky Way They expect to find billions more
Our galaxy the Milky Way contains about 100 billion stars Big Beyond Belief The Milky Way galaxy is huge Scientists believe it contains 17 billion Earth sized planets Its neighboring galaxy Andromeda is even bigger Andromeda is more than 2 5 million light years from our galaxy But on clear dark nights you can see it with the naked eye
CHAPTER 1 WHAT IS A GALAXY A galaxy is a huge group of stars gas dust and planets held together by gravity There are three main types of galaxies They are spiral elliptical and irregular Most galaxies seen from Earth are spiral galaxies A spiral galaxy has a dense bulge of stars at its center More stars spiral out like long arms This gives the galaxy a pinwheel shape Newer stars are found in a spiral galaxy's outer arms Older stars are closer to the center A barred galaxy is a type of spiral galaxy A bar shaped group of stars spans the center of each barred galaxy More stars spiral out from this bar The Milky Way galaxy is barred Scientists believe about one third of spiral galaxies are barred
All of the stars in a spiral galaxy orbit the galaxy's center
