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.
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