© Lloyd

Components of the interstellar medium: % ........ Height .... Temp. ............. Atoms/cc .... State ............. Means of Detection

Molecular clouds: ........................... <1%;........... 80pc; 10—20K; .......... 10^2—10^6; .. molecular; .... Radio and infrared spectral lines

Cold Neutral Medium (CNM): ..... 1—5%; 100—300pc; 50—100K; ............. 20—50; ... neutral atomic; H I 21 cm line absorption

Warm Neutral Medium (WNM): 10—20%; 300—400pc; 6000—10000K; ..... 0.2—0.5; .. neutral atomic; H I 21 cm line emission

Warm Ionized Medium (WIM): . 20—50%; ...... 1000pc; 8000K; .................. 0.2—0.5; ...ionized; ......... Hα emission and pulsar dispersion

H II regions: .................................... <1%; .......... 70pc; 8000K; ................ 10^2—10^4; ionized; ......... Hα emission and pulsar dispersion

Hot Ionized Medium (HIM): ...... 30—70%; 1000—3000pc; 10^6—10^7K; 10^−4—10^−2; ionized (metals also highly ionized); X-ray emission; absorption lines of highly ionized metals