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HUV 5NPF5HUV lMM H$ 5O5 H$ P UUe "  H$ 5PN5H$ lOO HH5QD HHLAR  ` # The Interstellar Medium UV;` % A very large subject X` &  u` ' 1Two components:  Dust  and  Gas  ` (   ` ) Dust ` 0  Around 1% of mass in ISM 6` * < Large particles ( r   wavelength of light ) UV` 4 " Dark and Reflection nebulae 1` +  Complicated makeup N` , Lots of ongoing research k` - 5  ` . Gas UV` 1  The other 99% of mass in ISM ` / "  Atoms  and  Molecules ` 2 "Atoms: Mainly hydrogen and helium  ` 3 Molecules: A long list of stuff '` 5  Emission nebulae (if hot) D` 6  Radio emitters (if cool) HHH5RDHHlUQQ  d5Sl MUUA HH5TS HHLco and  YYU ` $ Interstellar Dust ;` 7 !Around 1% of  mass  in ISM X` 8 ̥  gu` r  J Density n  1/(10 6   m 3 ) = 1/(100  m ) 3 ` s   h : mpA r  l VISIBLE     Dust scatters light  \` ; *Blue light scattered more than red light! y` < 99Interstellar Reddening ` =  m` > cu Ice, silicates, graphite ms:` ? ndEProducts of stellar burning ( 16 O,  12 C,  28 Si) ` @ mi(Absorption spectra give clues to makeup  ` A ( c'` B  Stellar wind cools off D` C   Condensation into dust HH5USHHlRXTT d5VXXHH5WV $HHjt ound 1% X  ` 9 ҥ Interstellar Reddening J ;` D (1!Magnitude and Distance Revisited X` E  u` G 6i$i$J6,H,H6N$N$6HH6(w$(w$8Y`-H;/6H;/6h6$5\Z]Y $Q5]\^YobQZ5^]_YS Z?5_^[Y ?Q? t5`bdYR$ ?sW3QQV\t\tVQQV\t\tVpQpQ`VP\tP\t@V0Q0Q(To W30W3W35a[bYl5ba`Yld5cff KThZ ZtVVf]zdhdh7aj}^l}^l eZlHlH%ۨiJ,!fL,!fL,m:-9yt(-9yt(3kq*9[n,9[n,9r"9vt6Ut t@ztHH5ec ]HHy5f` F Worked Example:  See Zeilik -` Y  J` [ How far away is a G0V star with g` \ ,m V  = 13.0  and  CI  = 1.6? ` ] 3 ` ^ ۹HAppendix 4.3: G0V has  M V  =4.4  and  CI   = 0.60 p` _ ̹ ` ` .Color Excess  CE   = 1.6 - 0.6 = 1.0 ` a 1Visual absorption  A V  = 3.21.0 = 3.2 ` b  `2` c Tm V  -  M V  = 13.0 - 4.4 = 9.6 = 5 log  d  -5 +  A V O` d j  log  d  = 11.4/5 al` e   d   = 191  pc ` f h ` g l'Without reddening, we would have found ,!` i (9.6 = 5 log  d  -5 ,` j U  log  d  = 14.6/5 ` h 5  d   = 831  pc  = 4.4  too big! ` k  7` l 0Note:  We are looking through a dense cloud! T m HWould have expected  A V   (1-2)0.2 = 0.2  to  0.4  mq@ m  from general properties of ISM. HH5fc4: HHlXiee d63gor ii6 0.HH64hg tn HHM.0  `i c ` Z Interstellar Gas =;` n "Nearly all of the mass in the ISM X` o g  u` q L Density n  10 6 / m 3  10 12   n DUST ` t ou&But , it doesnt block your view ` u 9.+  Small particles ` v j+  Quantum absorption only ` w h  x = . Nearly all is hydrogen and helium, but many #@ x %complex molecules have been observed t@` y udCarbon Monoxide (CO) ]` z teFormic acid (HCOOH) 2)z` {  Methane (CH 4 ) ` | er p` }  Hot hydrogen: HII Regions ` ~ $+  Typical around young OB stars ` .+  Generally reddish from H a  lines  ` Not interstellar reddening! (`  -+  May see complete ionization near stars E` Strmgen Sphere HH66igHHlflhh ` od6]j ll 0HH6^kj touHHWb` ulal ` p vOther Ways to Heat bsoC ` Interstellar Gas f` ̥  r ` a Planetary Nebulae ` co0+  Single hot star (White Dwarf) ionizes gas y` id+  Gas is very diffuse ` OH%  Forbidden lines are observed ` (No collisional de-excitation)  "` Re nG ` ~ Supernova Remnants uj` +  Energy provided by ` frpulsar or other compact object ` ` ll&+  Synchrotron radiation important ` te++  Magnetic fields lead to polarization S`  `     These are two very different K @ h mechanisms! HH6`ljHHliokk d6mooalHH6nm atHHsso UU` arCold Hydrogen Gas in the ISM o8` s%Important Data for Galactic Dynamics U`  r` ,Hyperfine Splitting in the Hydrogen Atom: ob`  l = 21  cm radio emission tat`   9Electrons  and  Protons  have spin that makes "H ,them behave like little dipole magnets: ob` B  Energy splitting  D E =  2 m e B p ` ieE  2m e  [(m 0 /2p)(m p / a 0 3 )] `   410 -25  J f`  ` *True Quantum Mechanical calculation finds $` ;D E  = 9.4110 -25  J  =  h (1.42 GHz) A`  =  hc  /  (21 cm) ^`  =  k ( 0.07 K ) {` "ISM is much warmer than this!!! HH6omarHHllrnn d6prrHerHH6qp HH r  ` trCold Molecular Gas ns;` at kX` >Also observe by radio emission (since  D E << kT) u` er s E4But , mechanism is much different than hyperfine @ (msplitting as in hydrogen atom. 0`  ` 1*Instead, use Quantum Mechanical Rotation. ` Tr*Well defined radio lines for molecules. #` D E@` - Examples: ]` z).Carbon Monoxide CO at  n = 115.27  GHz z` -Hydroxyl radical OH at  n = 1.612  GHz ` IS s` n  sHH6rpHHloqq H66smtnHc$6tus c$l66utxsl6lHl$6wy{st k$$6xuys $66yxwsE6isms H$L6zs tg $L4dZ4dZ )Spin up E  = - m e B pucc6{w|s cclQ66|{}s lQ6 llQc6}|~s cQ66~}sOatQ6G QZ$6~zsZ$cH6zs cHsdZsdZ *Spin down E  = + m e B p d5Leftd6Right$d7 ReferencesdDdSdV kdcdgdjdms dpL a vBf a Body. f b   Bulleted\t. f c CellBody. f d  CellHeading. f e V kFootnote. f fT Heading1Body. f gT  Heading2Body. f hT   HeadingRunInBody. f i l yIndented. f j g Numbered.\t. f kE  Numbered1.\tNumbered. f l f TableFootnote. f mT   TableTitleT:Table : .  f nP TitleBody. f o T   TableTitleT:Table : . f p   CellHeading. f q  CellBody. f r   CellFooting. f s  Body. @ t  lHeader. @ u  blaFooter. f w Body. $f x Body. f y Body. 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