Radiation can be analyzed with an instrument known as a spectroscope. In its most basic form, this device consists of an opaque barrier with a slit in it (to form a narrow beam of light), a prism (to split the beam into its component colors), and either a detector or a screen (to allow the user to view the resulting spectrum). Figure 2.11 shows such an arrangement.
The spectra encountered in the previous section are examples of continuous spectra. A lightbulb, for instance, emits radiation of all wavelengths (mostly in the visible and near-infrared ranges), with an intensity distribution that is well described by the blackbody curve corresponding to the bulb’s temperature. Viewed through a spectroscope, the spectrum of the light from the bulb would show the familiar rainbow running from red to violet without interruption, as presented in Figure 2.11.
After some experimentation, we would also find that although we could alter the intensity of the lines (for example, by changing the amount of hydrogen in the jar or the strength of the electrical discharge), we could not alter their color (in other words, their frequency or wavelength). This particular pattern of spectral emission lines is a property of the element hydrogen—whenever we perform this experiment, the same characteristic emission spectrum is the result. Other elements yield different emission spectra. Depending on which element is involved, the pattern of lines can be fairly simple or very complex. Always, though, it is unique to that element. The emission spectrum of a gas thus provides a “fingerprint” that allows scientists to deduce its presence by spectroscopic means. Examples of the emission spectra of some common substances are shown in Figure 2.13.
When sunlight is split by a prism, at first glance it appears to produce a continuous spectrum. However, closer scrutiny shows that the solar spectrum is interrupted by a large number of narrow dark lines, as shown in Figure 2.14. These lines represent wavelengths of light that have been removed (absorbed) by gases present either in the outer layers of the Sun or in Earth’s atmosphere. These gaps in the spectrum are called absorption lines. The absorption lines in the solar spectrum are referred to collectively as Fraunhofer lines, after the nineteenth-century German physicist Joseph Fraunhofer, who measured and catalogued more than 600 of them.
At around the time solar absorption lines were discovered, scientists found that absorption lines could also be produced in the laboratory by passing a beam of light from a continuous source through a cool gas, as shown in Figure 2.15. They quickly observed an intriguing connection between emission and absorption lines: The absorption lines associated with a given gas occur at precisely the same wavelengths as the emission lines produced when the gas is heated. Both sets of lines therefore contain the same information about the composition of the gas.
Once astronomers realized that spectral lines are indicators of chemical composition, they set about identifying the observed lines in the Sun’s spectrum. Almost all of the lines observed in the light received from extraterrestrial sources could be attributed to known elements (for example, many of the Fraunhofer lines in sunlight are associated with the element iron). However, some new lines also appeared in the solar spectrum. In 1868 astronomers realized that those lines must correspond to a previously unknown element. It was given the name helium, after the Greek word helios, meaning “Sun.” Only in 1895, almost three decades after its detection in sunlight, was helium discovered on Earth.
The development of spectroscopy is another example of the scientific method in action. (Discovery 1-1) As technology evolved and experimental measurement techniques improved, scientists realized that spectra were unique fingerprints of matter. They then went on to use this knowledge as a means of determining the composition of objects that were otherwise impossible to reach. Yet for all the information that nineteenth-century astronomers could extract from observations of stellar spectra, they still lacked a theory explaining how those spectra arose. Despite their sophisticated spectroscopic equipment, they knew scarcely any more about the physics of stars than did Galileo or Newton. The next step in the circle of scientific progress was an explanation of Kirchhoff’s empirical laws in terms of much more fundamental physical principles, just as Newtonian mechanics explained Kepler’s empirical laws of planetary motion. And just as with Newton’s laws, the new theory—now called quantum mechanics—opened the door to an explosion of scientific understanding far beyond the context in which it was originally conceived.
To understand how spectroscopy can be used to extract detailed information about astronomical objects from the light they emit, we must delve more deeply into the processes that produce line spectra.