Ch11 - Chapter 11 The formation of stars 11.1 Interstellar...

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Chapter 11 The formation of stars 11.1 Interstellar medium ( 星際物質 ) Mostly hydrogen gas of very low density ( ~1 particle cm -3 ). Nebulae ( 星雲 ): Higher density clouds of gas and dust. There are mainly three types: Emission nebula (H II regions) 1 ( 發射星雲 ): An excited cloud of relatively low-density gas (Fig. 11-1). y Hydrogen gas is ionized by ultraviolet radiation from a nearby hot star. y It emits light (mostly red) when the ionized gas recapture electrons. Reflection nebula ( 反射星雲 ): Dust in a gas cloud reflecting light from other stars. Dark nebula ( 黑暗星雲 ): Dark, thick clouds blocking light from other stars (Fig. 11-1). Giant molecular clouds ( 超巨型分子雲 ) typically span over a few hundred pc, containing up to 0.5 million solar masses. A giant molecular cloud is so large that different parts of it have different behaviours, for example, some part behaves like an emission nebula, while some part behaves like a dark nebula. Fig. 11-1 The Horsehead nebula. Dark nebula forms the shape of a “horsehead” (right). Emission nebula is also seen behind the dark nebula. High density (~10 6 particles cm -3 ) and low temperature (~10 K). They contain infrared sources and bright H II regions excited by young and hot stars. 1 H II regions are mainly hydrogen ions ( 氫離子 ) the electrons frequently leave the nuclei by absorbing energy from somewhere. These ions recapture the electrons and thus emit light. H I regions
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Ch11 - Chapter 11 The formation of stars 11.1 Interstellar...

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