LEARNING GOALS

	LEARNING GOALS Studying this chapter will enable you to: Discuss the nature of electromagnetic radiation and tell how that radiation transfers energy and information through interstellar space. Describe the major regions of the electromagnetic spectrum. Explain how we can determine an object's temperature by observing the radiation it emits. Describe the characteristics of continuous, emission, and absorption spectra, and the conditions under which each is produced. Specify the basic components of the atom and describe our modern conception of its structure. Explain how electron transitions within atoms produce unique emission and absorption spectra. Describe how the relative motion of a source of radiation and an observer can change the perceived wavelength of the radiation, and explain the importance of this phenomenon to astronomy. Visit astro.prenhall.com/chaisson for additional annotated images, animations and links to related sites for this chapter.
The Ring Nebula in the constellation Lyra is one of the most magnificent objects in our galaxy. Seen here glowing in the light of its own fading radiation, the nebula is actually the expanding atmosphere of a nearly dead star. The dying dwarf star can be seen at the center of the ring of hydrogen-rich gas, shown here in true color. Owing to the nebula's distance of some 5000 light-years, its apparent size is just 1/100th that of the full Moon. It is too dim to see with the naked eye. (STScl)
Astronomical objects are more than just things of beauty in the night sky. Planets, stars, and galaxies are of vital significance if we are to understand fully the big picture—the grand design of the universe. Every object is a source of information about the universe—its temperature, its chemical composition, its state of motion, its past history. The starlight we see tonight began its journey to Earth decades, centuries—even millennia—ago. The faint rays from the most distant galaxies have taken billions of years to reach us. The stars and galaxies in the night sky show us not just the far away but also the long ago. In this chapter, we begin our study of how astronomers extract information from the light emitted by astronomical objects. The observational and theoretical techniques that enable researchers to determine the nature of distant atoms by the way they emit and absorb light are the indispensable foundation of modern astronomy.