Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
s8pe-41201-ca 11/2/05 8:22 PM MAZER Page 376 We still have much to learn about the universe. Within the last few hundred years, however, new tools and scientific theories have greatly increased our knowledge. In this chapter you will learn about the arrangement of planets and stars. You will also learn about the ways in which astronomers explore and study space. Arrangement of the Universe If you look up at the sky on a clear night, you will see only a tiny fraction of the planets and stars that exist. The number of objects in the universe and the distances between them are greater than most people can imagine. Yet these objects are not spread around randomly. Gravity causes objects in space to be grouped together in different ways. The images on page 377 show some basic structures in the universe. Like a camera lens zooming out, the images provide views of space at different levels of size. 1 Our planet’s diameter is about 13,000 kilometers (8000 mi). This is almost four times the diameter of the Moon, which orbits Earth. An orbit is the path of an object in space as it moves around another object because of gravity. 2 Earth and eight other major planets orbit the Sun. The Sun, the planets, and various smaller bodies make up the solar system. The Sun is about 100 times greater in diameter than Earth. You could fit more than 4000 bodies the size of the Sun between the Sun and the solar system’s outermost planet at its average distance from the Sun. The Sun is one of countless stars in space. Astronomers have detected planets orbiting some of these other stars. 3 Our solar system and the stars you can see with your bare eyes are part of a galaxy called the Milky Way. A galaxy is a group of millions or billions of stars held together by their own gravity. Most stars in the Milky Way are so far away that our galaxy appears to us as a hazy band of light. 4 The Universe reading tip The word orbit can be a noun or a verb. reading tip It is often hard to imagine what distances in space are like. If the solar system were the size of a penny, the length of the Milky Way would be roughly the length of California. Earth Solar System The Milky Way The universe is everything—space and all the matter and energy in it. The Milky Way is just one of many billions of galaxies in the universe. These galaxies extend in all directions. Astronomers study space at each of these different levels. Some focus on planets in the solar system. Other astronomers study distant galaxies. To learn how the universe formed, astronomers even study the smallest particles that make up all matter. check your reading 376 Unit 4: Space Science What is the relationship between the solar system and the Milky Way? PDF