interstellar_medium - The Interstellar Medium (ch. 18) The...

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The Interstellar Medium (ch. 18) The interstellar medium (ISM) is all the gas (and about 1% dust) that fills our Galaxy and others. It is the raw material from which stars form, and into which stars eject material when they die, so it is important to understand its properties. Also, remember that these dust grains are what are supposed to collide and grow into terrestrial-like planets! The ISM is distributed in a very irregular manner between the stars, with all sorts of structures that look like “clouds” and “shells” and “holes” and “filaments.” See the many interesting images in your textbook. Most important thing to remember is that : The different types of “clouds”, “regions,” and “nebulae” that you will read about mostly just refer to how this gas+dust is observed, and whether or not it is being heated (and/or ionized) by nearby stars. After you study the material, you should come back to this statement and make sure you understand it. DUST GRAINS Dark clouds . The existence of the ISM was first inferred from photographs of the sky that show numerous large and irregular “holes” between the stars (see Figs. 18.1, 18.2a and 18.8). These are due to blocking (extinction ) of starlight by dust particles , also called grains (like smoke or fog) in regions of the ISM that are denser than normal. We would call these “dust clouds,” or “dark clouds,” (sec. 18.3), but it’s important to realize that the dust and gas are well-mixed, and the dust is just a “tracer” of the gas that is also there (that we can detect using molecular spectral lines—see below). This is well- illustrated on p. 478 of your text. So what may appear as a “dark
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cloud” in a visible wavelength image will appear bright when observed in the infrared or radio, where the dust and gas do most of their emission. Important to realize that extinction is what keeps us from being able to observe things that are very far away in the disk of our galaxy: their light is obscured by the intervening dust.
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Grain size, shape, composition . Each dust grain is a solid particle ~1000 times bigger than an atom, containing on average ~10 9 atoms. These grains are elongated or very irregular in shape (Fig. 18.3); this is known from the way they “polarize” light—don’t worry if you don’t understand details of this—see p.471 for illustration) in shape. We know (how? See p.472, but we’ll discuss in class) they are composed of silicates (like rocks on earth), graphite (or something like it), and maybe iron, with a coating of various ices (“dirty ice”). Origin . When we try to get abundances of the elements in the gas (using spectral lines they emit and absorb) in the ISM, we find that it generally has about the same composition as the sun and other stars (i.e. mostly H, some He, and ~1% heavier elements, consistent with stars forming from this gas), except that some elements are depleted , i.e. their abundances are low compared to
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This note was uploaded on 04/20/2008 for the course AST 301 taught by Professor Harvey during the Fall '07 term at University of Texas at Austin.

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interstellar_medium - The Interstellar Medium (ch. 18) The...

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