(Background) When a prism is placed in the path of light captured by a telescope, the resulting photograph can look almost psychedelic. Each of the stars and other sources of radiation in the picture has had its light split into its component colors, from red to violet. Here, in this strange image, we see a star-forming region (called the Eta Carinae nebula) containing many stars and much loose gas—all of whose light has been colorfully dispersed.
(Inset A) This is the same celestial object, now photographed without the prism. The result is Eta Carinae in its true colors—mostly red, due to vast quantities of hydrogen gas spread across the 900-square-light-year area of this photo.
(Inset B) Looking more carefully at spectra from individual stars, we often see two things: a colorful continuous spectrum extending from red to violet, and a series of thin, dark lines across the spectrum. This is the spectrum of the brightest star in the sky, Sirius A.
(Inset C) This is the slightly different spectrum of Vega, the bright star in the constellation Lyra.
(Inset D) Examining spectra of stars even more closely, we find ever more dark lines. Here, in this spectrum of the star Arcturus, we can see many such lines in the yellow region alone. The tracing below records precise details about these lines that would be difficult or impossible to obtain just by looking at the spectrum. Spectra like these, which tell astronomers virtually all that we know about stars, are the subject of this chapter.
Studying this chapter will enable you to:
Describe the characteristics of continuous, emission, and absorption spectra and the conditions under which each is produced.
Explain the relation between emission and absorption lines and what we can learn from these lines.
Discuss the observations that led scientists to conclude that light has particle as well as wave properties.
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 features in the spectra of those atoms.
Describe the general features of the spectra produced by molecules.
List and explain the kinds of information that can be obtained by analyzing the spectra of astronomical objects.
In the previous chapter we saw how light behaves as a continuous wave and how this description of electromagnetic radiation allows us to begin to decipher the information reaching us from the cosmos in the form of visible and invisible light. However, early in the twentieth century, it became clear that the wave theory of electromagnetic phenomena was incomplete—some aspects of light simply could not be explained purely in wave terms. When radiation interacts with matter on atomic scales, it does so not as a continuous wave but in a jerky, discontinuous way—in fact, as a particle. With this discovery, scientists quickly realized that atoms, too, must behave in a discontinuous way, and the stage was set for a scientific revolution that has affected virtually every area of modern life. In astronomy, the observational and theoretical techniques that enable researchers to determine the nature of distant atoms by the way they emit and absorb radiation are now the indispensable foundation of modern astrophysics.
