(Background) The Ring Nebula in the constellation Lyra is one of the most magnificent sights in the nighttime sky. Seen here glowing in the light of its own emitted radiation, the nebula is actually the expanding outer atmosphere of a nearly dead star.
(Inset A) This nearly true color view clearly shows the dying dwarf star at the center of the expanding gas cloud, which is really a three-dimensional shell and not a ring.
(Inset B) False-color images can sometimes enhance certain features. Here some of the fine structure in the shell of gas can be seen more clearly.
(Inset C) Contour images, like this one derived from red light emitted by hydrogen gas in the nebula's shell and star's lingering atmosphere, can be used to map regions of relative brightness.
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.
List the major regions of the electromagnetic spectrum and explain how the properties of Earth's atmosphere affect our ability to make astronomical observations at different wavelengths.
Explain what is meant by the term "blackbody radiation" and describe the basic properties of such radiation.
Tell how we can determine the temperature of an object by observing the radiation that it emits.
Show how the relative motion of a source of radiation and its observer can change the perceived wavelength of the radiation, and explain the importance of this phenomenon to astronomy.
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 our place in the big picture—the "grand design" of the universe. Each object is a source of information about the material aspects of our universe—its state of motion, its temperature, its chemical composition, even its past history. When we look at the stars, the light we see actually 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 the far away and the long ago. In this chapter we begin our study of how astronomers extract information from the light emitted by astronomical objects. These basic concepts of radiation are central to modern astronomy.
