The universe contains at least 100 billion galaxies, each with billions of stars—most of which probably have orbiting planets. Our own galaxy, the Milky Way，consists of about 500 billion stars, including all the ones that we can see in the night sky, as well as large clouds of gas and dust, some of which form new stars. The Milky Way is a flat disk with a central bulge and bright spiral arms. Our Sun is a medium sized star in one of the spiral arms, about two-thirds of the way out from the center. From Earth, we look out across the galaxy’s disk,so the densely packed stars at its center look like a milky band of light across the night sky.

The universe contains at least 100 billion galaxies, each with billions of stars—most of which probably have orbiting planets. Our own galaxy, the Milky Way，consists of about 500 billion stars, including all the ones that we can see in the night sky, as well as large clouds of gas and dust, some of which form new stars. The Milky Way is a flat disk with a central bulge and bright spiral arms. Our Sun is a medium sized star in one of the spiral arms, about two-thirds of the way out from the center. From Earth, we look out across the galaxy’s disk,so the densely packed stars at its center look like a milky band of light across the night sky.

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