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. We discuss the dust and gas, and how they are observed, separately. Interstellar 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 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.Grain size, shape, composition. Each dust grain is a solid particle ~1000 times bigger than an atom, containing on average ~109 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 deplet ed , i.e. their abundances are low compared to stars. This is almost certainly because these elements have converted into the solid dust grains. Most astronomers think the grain cores form in the cool winds ejected by dying red giant stars, but some of the depletions, especially the dirty ice coating, probably occurs within the ISM itself. Reddening. Dust grains block light more effectively at small wavelengths. So UV can’t penetrate dust easily, while radio waves can. This different wavelength dependence of scattering and absorption by dust grains is called reddening, because it will take blue light out of a star’s spectrum more efficiently than red light, making the star appear redder. This property of small particles is very general: it is why the sky appears blue and why the sun can look red at sunset or sunrise—the earth’s atmosphere contains lots of “droplets” or “aerosol” particles that behave like dust grains. Infrared dust emission. Dust can also be observed by its infrared (and submillimeter) wavelength emission (don’t confuse this with “reddening”!). Dust grains are typically at temperatures of 10 to 100 degrees K (depending on whether or not they are near a hot luminous star, since the grains are only heated by starlight), so they radiate like little black bodies at that temperature, i.e. a continuous spectrum with most of the energy at large wavelengths. (Figs. 18.13b and 18.21 were obtained this way.)Here are two images of the “Triffid Nebula,” one in the infrared (top) and the other in the visible part of the spectrum (bottom). Notice how the “dark clouds” or lanes so prominent in the visible spectrum (because of extinction by dust) show up as bright regions in the IR image (because the grains emit mostly in the IR). Make sure you understand this! The bright region in the visible image (bottom) is due to emission line and other radiation from gas that is being irradiated by the hot, massive, young stars in this region (you should understand why a massive star must be hot and young). The stars themselves are buried within the dust and show up mostly as the bright spots in the IR image, where they are heating up the dust around them the most.Interstellar Gas Most of the ISM is gas that can be observed in various ways. The densities are very small by Earth standards, only about 1 particle per cubic centimeter on average, but the masses involved in the clouds of gas and dust are very large (ranging from 102 to 106 solar masses) because they are huge (up to 100s of parsecs in size). But there are also “holes” or “voids” where the gas density is much lower than average. The main thing to remember is that the gas is distributed in a very irregular way, probably due to the action of stellar explosions and turbulence, which keeps the gas (and dust) “stirred up.” This same interstellar gas, disturbed as it appears in some regions, is the material that can cool to very low temperatures, form molecular clouds, and collapse to make new stars. We discuss these below. The three primary ways in which we can detect and analyze interstellar gas is: 21 centimeter neutral hydrogen line, spectral lines from numerous interstellar molecules, and emission lines from hot ionized emission nebulae. We now discuss them one by one. 21 centimeter HI radiation. This involves the spin flip transition of an electron in a
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