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 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 depleted 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 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 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 21 centimeter HI radiation This involves the spin flip transition of an electron in a neutral hydrogen atom An atom with the electron proton spins parallel has slightly higher energy than e p spins antiparallel Transition to lower energy after excitation by collisions in the cool regions of the ISM gives a spectral line at 21 cm See fig 18 17 This is one of the two major methods for mapping the distribution of gas in our galaxy and others because being in the radio part of the spectrum this emission line easily penetrates interstellar dust It was the first method used to make a map of the gas structure of our own Milky Way galaxy revealing it to have complex structure from spiral like arms to large holes or bubbles and chimneys or fountains where gas is being vented vertically out of the disk of our Galaxy by stellar winds and explosions more later Also because this is a spectral line unlike dust which gives us only continuous absorption we can get radial velocities of the gas motions this way Fig 18 18 The results show that our galaxy has an
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