![]() ![]() Suppose two stars have identical temperatures and pressures, but the lines of, say, sodium are stronger in one than in the other. Only if the physical conditions in a star’s photosphere are such that lines of an element should (according to calculations) be there can we conclude that the absence of observable spectral lines implies low abundance of the element. As we saw, the temperature and pressure in a star’s atmosphere will determine what types of atoms are able to produce absorption lines. Note that the absence of an element’s spectral lines does not necessarily mean that the element itself is absent. If we see lines of iron in a star’s spectrum, for example, then we know immediately that the star must contain iron. Kalirai (STScI)) Abundances of the ElementsĪbsorption lines of a majority of the known chemical elements have now been identified in the spectra of the Sun and stars. (credit: modification of work by NASA, ESA, A. A giant star with a very-low-pressure photosphere shows very narrow spectral lines (bottom), whereas a smaller star with a higher-pressure photosphere shows much broader spectral lines (top). This figure illustrates one difference in the spectral lines from stars of the same temperature but different pressures. Ionized atoms, as we discussed earlier, have different spectra from atoms that are neutral.įigure 17.9 Spectral Lines. Compared with what happens in the Sun (with its relatively dense photosphere), ionized atoms in a giant star’s photosphere are less likely to pass close enough to electrons to interact and combine with one or more of them, thereby becoming neutral again. But how long atoms stay ionized depends in part on pressure. The ionization of atoms in a star’s outer layers is caused mainly by photons, and the amount of energy carried by photons is determined by temperature. Second, more atoms are ionized in a giant star than in a star like the Sun with the same temperature. Think about it like traffic-collisions are much more likely during rush hour, when the density of cars is high. Collisions will, of course, be more frequent in a higher-density environment. This effect is due to collisions between particles in the star’s photosphere-more collisions lead to broader spectral lines. The difference is large enough that careful study of spectra can tell which of two stars at the same temperature has a higher pressure (and is thus more compressed) and which has a lower pressure (and thus must be extended). First, a star with a lower-pressure photosphere shows narrower spectral lines than a star of the same temperature with a higher-pressure photosphere ( Figure 17.9). ![]() This low pressure affects the spectrum in two ways. As a result, the pressure in a giant star’s photosphere is also low. Because it is so large, a giant star’s atoms are spread over a great volume, which means that the density of particles in the star’s photosphere is low. A giant star has a large, extended photosphere. Suppose you want to determine whether a star is a giant. Luckily for the astronomer, stellar spectra can be used to distinguish giants from run-of-the-mill stars (such as our Sun). Stars of such exaggerated size are called giants. At some periods in their lives, stars can expand to enormous dimensions. Clues to the Size of a StarĪs we shall see in The Stars: A Celestial Census, stars come in a wide variety of sizes. We can also measure its motion toward or away from us and estimate its rotation. From the pressure, we get clues about its size. We can measure its detailed chemical composition as well as the pressure in its atmosphere. Describe the proper motion of a star and how it relates to a star’s space velocityĪnalyzing the spectrum of a star can teach us all kinds of things in addition to its temperature.Explain how astronomers can measure the motion and rotation of a star using the Doppler effect.Understand how astronomers can learn about a star’s radius and composition by studying its spectrum.To observer B, in a direction at right angles to the motion of the source, no effect is observed.By the end of this section, you will be able to: The crests arrive with an increased wavelength and decreased frequency. As a result, the waves are not squeezed together but instead are spread out by the motion of the source. For her, the source is moving away from her location. ![]()
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