3）DarkNebulae暗星云Denseclouds ofgasand dust absorb thelightfromthestarsbehind;致密气体和尘埃吸收背后星光appeardarkinfrontofthebrighterbackground在明亮的背景前显得暗HorseheadNebula
3) Dark Nebulae 暗星云 Barnard 86 Dense clouds of gas and dust absorb the light from the stars behind; 致密气体和尘 埃吸收背后星 光 appear dark in front of the brighter background; 在明亮的背景 前显得暗 Horsehead Nebula
星际红化InterstellarReddeningBluelight isstronglyscatteredand absorbedbyinterstellarcloudsInterstellar蓝光被散射和吸收更厉害cloudRedlightcanmoreeasilypenetratetheStarTelescopecloud,butisstillabsorbedtosomeextent红光被吸收，但仍能穿透PathofbluephotonsPathofredphotonsInfraredradiationishardlyabsorbedatal红外光被很少吸收Barnard 68Interstellar cloudsmake backgroundstarsappearredder星际云使背景星红化（偏红）Infrared红外Visible可见光
Interstellar Reddening 星际红化 Visible可见光 Infrared 红外 Barnard 68 Blue light is strongly scattered and absorbed by interstellar clouds 蓝光被散射和吸收更厉害 Red light can more easily penetrate the cloud, but is still absorbed to some extent 红光被吸收，但仍能穿透 Infrared radiation is hardly absorbed at all 红外光被很少吸 收 Interstellar clouds make background stars appear redder 星际云使背景星 红化（偏红）
StructureoftheISM星际介质结构TheISM occurs intwomaintypesofclouds:星际介质主要有两类：·HI clouds 中性氢云（氢原子）：Cold(T~100K)cloudsofneutral hydrogen(Hl)冷氢云（T~100K)moderate density (n~10-afewhundred atoms/cm3);中等密度（每立方厘米几百个原子）尺度：~100pcHot interstellarmedium热星际介质（氢原子核）：Hot (T ~a few 1000 K), ionized hydrogen (HIl)热电离氢;lowdensity (n~0.1atom/cm3);低密度（每立方米0.1个原子）
Structure of the ISM 星际介质结构 • HI clouds 中性氢云（氢原子）: • Hot interstellar medium 热星际介质（氢原子核）: The ISM occurs in two main types of clouds: 星际介质主要有两类： Cold (T ~ 100 K) clouds of neutral hydrogen (HI); 冷氢云(T ~ 100 K) moderate density (n ~ 10 – a few hundred atoms/cm3 ); 中等密度（每立方厘米几百个原子） 尺度: ~ 100 pc Hot (T ~ a few 1000 K), ionized hydrogen (HII) 热电离氢; low density (n ~ 0.1 atom/cm3 ); 低密度（每立方米0.1个原子）
The Orion NebulaAn Active Star-Forming Region恒星形成区
The Orion Nebula: An Active Star-Forming Region 恒星形成区