Spectral analysis, however, can be quite useful. Look again at Figure 5. The colors the gases emitted when they were heated were the very same colors as those they had absorbed when a continuous source of light was behind them. What if, instead, we heated the same thin gases until they were hot enough to glow with their own light? Figure 4. In fact, the element helium was found first in the Sun from its spectrum and only later identified on Earth. If the surface is smooth and shiny, as with a mirror, the direction of the reflected light beam can be calculated accurately from knowledge of the shape of the reflecting surface. When we have a hot, thin gas, each particular chemical element or compound produces its own characteristic pattern of spectral lines—its spectral signature. Newton found that sunlight, which looks white to us, is actually made up of a mixture of all the colors of the rainbow (Figure 1). If the gas in a container consisted of two elements, then light passing through it was missing the colors (showing dark lines) for both of the elements. is an excellent textbook on many aspects of astrophysics, including spectroscopy. Figure 3. In a typical spectroscopic analysis, a concentration of a few parts per million of a trace element in a material can be detected through its emission spectrum. Passed through an optical dispersion device such as a diffraction grating or … Spectroscopy is the study of the interaction between matter and electromagnetic radiation as a function of the wavelength or frequency of the radiation. It is the precise pattern of wavelengths that makes the signature of each element unique. Rainbows are an excellent illustration of the dispersion of sunlight. What if, instead, we heated the same thin gases until they were hot enough to glow with their own light? To extract this information, however, astronomers must be able to study the amounts of energy we receive at different wavelengths of light in fine detail. Rainbow Refraction: (a) This diagram shows how light from the Sun, which is located behind the observer, can be refracted by raindrops to produce (b) a rainbow. Spectroscopy is a powerful tool in astronomy -- from it, we can often get information about the temperature, density, composition, and important physical processes of an astronomical object. It offers many highly interesting presentations by professional and amateur astronomers, as well as workshops in astrospectroscopy … Thus, a single rainbow always has red on the outside and violet on the inside. The light from an astronomical source can consist of a continuous spectrum, an emission (bright line) spectrum, or an absorption (dark line) spectrum. If the spectrum of the white light from the Sun and stars were simply a continuous rainbow of colors, astronomers would have little interest in the detailed study of a star’s spectrum once they had learned its average surface temperature. Such analysis of spectra is the key to modern astronomy. This discovery was one of the most important steps forward in the history of astronomy. In contrast, absorption spectra occur when passing white light through a cool, thin gas. No two types of atoms or molecules give the same patterns. Liquids and solids can also generate spectral lines or bands, but they are broader and less well defined—and hence, more difficult to interpret. Figure 5. (a) This diagram shows how light from the Sun, which is located behind the observer, can be refracted by raindrops to produce (b) a rainbow. We will discuss astronomical instruments and their uses more fully in Astronomical Instruments. 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