Physical Science 8th grade (1).pdf

171 galaxies early civilizations believed that earth

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17.1 Galaxies Early civilizations believed that Earth was the center of the universe. In the 16th century, we became aware that Earth is a small planet orbiting a medium-size star. It was only in the 20th century that we became aware that the sun is one of billions of stars in the Milky Way galaxy, and that there are billions of other galaxies in the universe. In the past 50 years, astronomers have found evidence that the universe is expanding and that it originated 10 billion to 20 billion years ago. In this section you will learn about galaxies and theories about how the universe began. What is a galaxy? The discovery of other galaxies A galaxy is a huge group of stars, dust, gas, and other objects bound together by gravitational forces. The sun, along with an estimated 200 billion other stars, belongs to the Milky Way galaxy . The Milky Way is a typical spiral galaxy (Figure 17.1). From above, it would look like a giant pinwheel, with arms radiating out from the center. Although some stars are in globular clusters above and below the main disk, the majority are arranged in a disk that is more than 100,000 light years across and only 3,000 light years thick. Our sun is 26,000 light years from the center The disk of the Milky Way is a flattened, rotating system that contains young to middle-aged stars, along with gas and dust (Figure 17.2). The sun sits about 26,000 light years from the center of the disk and revolves around the center of the galaxy about once every 250 million years. When you look up at the night sky, you are looking through that disk of the galaxy. On a crystal clear night, you can see a faint band of light stretching across the sky. This is the combined light of billions of stars in the galaxy, so numerous that their light merges.
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357 17.1 G ALAXIES C HAPTER 17: G ALAXIES AND THE U NIVERSE Types of galaxies The discovery of other galaxies At the turn of the 20th century astronomers believed the Milky Way galaxy was the entire universe. As telescopes got better, though, some “smudges” that were thought to be nebulae in the Milky Way were recognized to be whole galaxies far outside our own. The discovery was made in the 1920s by Edwin Hubble, an American astronomer. When he focused a huge telescope on an object thought to be a nebula in the constellation Andromeda, Hubble could see that the “nebula” actually consisted of faint, distant stars. He named the object the Andromeda galaxy. Just since Hubble’s time (1889-1953), astronomers have discovered a large number of galaxies. In fact, many galaxies are detected each year using the famous telescope launched into orbit in 1990: the Hubble Space Telescope, or HST. Galaxy shapes Astronomers classify galaxies according to their shape. Spiral galaxies like the Milky Way consist of a central, dense area surrounded by spiraling arms. Barred spiral galaxies have a bar-shaped structure in the center. Elliptical galaxies look like the central portion of a spiral galaxy without the arms. Lenticular galaxies are lens-shaped with a smooth, even distribution of stars and no central, denser area.
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