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Unformatted text preview: Chapter 4 Formation and Evolution of Stars The equations of stellar structure, as given in the previous chapter (equa- tions 3.39 through 3.43), do not contain any time dependence. However, although the properties of stars usually change slowly, they must necessar- ily change. Stars have a non-zero luminosity and a finite fuel supply. This implies that they began nuclear fusion at some time in the past, and will exhaust their fuel supply at some time in the future. In this chapter, we will address some of the issues of stellar evolution, starting with the interstellar medium that supplies the raw material of star formation, and ending when a star wrings the last possible joule from nuclear fusion. (Chapter 5 will discuss stellar remnants the dense corpses left over when a star no longer is powered by fusion.) 4.1 Interstellar Dust Stars are formed by gravitational compression of the interstellar medium ; that is, the low density mix of dust and gas that lies in the space between stars. As noted in section 2.5, the dust between stars can be detected because it causes extinction and reddening of starlight. Careful observations of dust yield clues that help us to determine the size, shape, and composition of dust grains. Clue one: Dust causes reddening at both visible and ultraviolet wave- lengths. Figure 4.1 shows the extinction A as a function of wavelength , 69 70 CHAPTER 4. FORMATION AND EVOLUTION OF STARS plotted as a function of 1 / . At all wavelengths > 2200 A, correspond- Figure 4.1: Dust extinction as a function of wavelength, normalized so that A V = 1 magnitude. ing to 1 / < . 00045, the amount of extinction decreases with increasing wavelength. This differential extinction can only happen if the individual dust grains are smaller than the wavelength of light that they are scattering ( d , where d is the length of a dust grain). If interstellar matter were made of pebbles or boulders rather than dust grains, it would absorb all wave- lengths of visible and near ultraviolet light equally. Detailed studies of the extinction as a function of wavelength give an estimate of d 50 2000 A for the size of the dust particles. These minuscule grains are much smaller than the dust particles that you sweep from under the bed; they are more like the particles in cigarette smoke. Clue two: Starlight is polarized by dust grains. A Polaroid filter polarizes light because it contains long polymer molecules that are aligned preferen- tially in one direction. Dust grains polarize light because they are nonspher- ical, and are aligned preferentially in one direction. 1 If dust grains were perfect spheres, they wouldnt cause polarization of light....
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- Winter '06