Chapter 28 Stars and the Universe

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722 CHAPTER 28: STARS AND THE UNIVERSE Source of White Light Spectroscope Glass Prism The spectroscope is one of the most important tools that astronomers use. This instrument separates light into its component colors (wavelengths), like the glass prism shown in Figure 28-6. When starlight is passed through a spectroscope, dark lines appear in certain parts of the spectrum. These dark lines are produced when certain wavelengths of light are absorbed by gaseous elements within the outer parts of the star. Each element has its own characteristic absorption lines. Since stars are composed primarily of hydrogen and helium, white light that passes through these elements shows dark lines in the orange, yellow, green, and blue colors that characterize hydrogen and helium. These spectral lines correspond to the energy that electrons absorb when they move to higher energy levels within the atoms. The atoms give off the same colors when the electrons fall to lower or inner energy levels. Each element has a unique set of energy levels. Therefore, these “spectral fingerprints” allow astronomers to identify the composition of distant stars. ACTIVITY 28-4 MAKING A SPECTRUM Long Wavelengths Red Orange Yellow Green Blue Violet Short Wavelengths Figure 28-6 When white light passes through a glass prism, the light separates into the spectrum of colors, or wavelengths, of which it is composed. You can separate sunlight into its spectrum with a glass prism. This works best in a darkened room where windows face the sun. Close the shades so that a narrow slit of direct sunlight enters the room. Place the prism near the narrow opening that admits sunlight. The prism will bend the light beam and separate it into its
colors. You may need to rotate the glass prism to project a visible spectrum. The spectrum can be projected onto a sheet of white paper. The stronger the light and the closer the paper is held to the prism, the brighter the spectrum will be. To increase the size of the spectrum, move the paper screen away from the prism. What two changes in the spectrum do you observe as the paper screen is moved away from the prism? WHAT IS THE STRUCTURE OF THE UNIVERSE? Early astronomers noticed fuzzy objects in the night sky. They called these objects nebulae (singular nebula). The word nebula comes from the Latin word for cloud. Unlike the stars, these objects looked like dim fuzzy patches of light. Nebulae and Galaxies Telescopes revealed that some nebulae are regions of gas and dust where stars are forming. In addition, some nebulae were at greater distances than any known stars. Astronomers eventually realized that some nebulae are huge groups of stars held together by gravity. These objects are called galaxies. The whole Andromeda galaxy is visible as a small, faint patch of light high in the autumn sky. Powerful telescopes revealed that the Andromeda galaxy, like thousands of other galaxies, is a gigantic group of billions of stars. Galaxies are separated by vast distances that contain relatively few stars. Figure 28-7 on page 724 is a typical spiral galaxy. The Milky Way WHAT IS THE STRUCTURE OF THE UNIVERSE? 723 Astronomers realized that all the individual stars visible to us in the night sky are a part of the group called the Milky Way Galaxy. The sun and solar system are part of the Milky
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Chapter 28 Stars and the Universe

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