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The Interstellar Medium (ISM) A general view of the nighttime sky and of the Milky Way: Stars and Nebulae The presence of an interstellar medium Dust Gas Cool Hot General properties: Average temperature about 100K and densities about 1–10 atoms/cm3 , typically atomic Hydrogen (HI) and about 1000 dust particles per cubic kilometer! dust about 1% of the mass of the ISM. Composition of ISM: about 90% hydrogen, about 9% helium; about 1% heavier elements The Dust Dust grains block light, large dark ’rifts’ in Milky way light Dust grains heated by stellar radiation, dust of Milky Way bright in infra-red. Dust grain size about 1 micro-meter (micron), comparable to wavelength of light Dust: obscures reflects scatters blue light more effectively than red light, interstellar reddening alignment of dust grains, interstellar magnetic field What is a dust grain? core: Iron?, Silicates?, Graphite? — all three? mantle: ices of water, methane, ammonia, carbon dioxide The Gas: a. The Hot Component Bright Nebulae in addition to Dark Nebulae Bright Gaseous Nebulae Hot: about 8–10,000K Dense: about 1000 per cm3 Not Large: about 5–10pc Gas Ionized Hydrogen (HII) 1 November 9, 2003 Presence of hot, young O & B stars: much ultra-violet radiation. b. The Cool Component Absorption of starlight: Narrow spectral lines cool clouds between observer and stars A more general probe of the ISM: Emission at 21 cm from atomic hydrogen (HI): electron-proton spins: parallel, antiparallel Spectrum implies: global structure of Milky Way structure of local features The medium pervades all of space. Association of gas and dust A simplistic picture of ISM: HI clouds and HII regions c. The Coldest Component Dense, dark regions — Dark Dust Clouds High density: 105 per cm3 molecular hydrogen H2 . Cold Temperature: about 20K Molecular emissions formation of molecules — cool, low energy environment requires dust grains. Types of molecules Organic molecules; pre-biotic? possible detection of amino acid glycine Giant Molecular Clouds Massive, and sites of Star Formation NGC 7023: GMC large: HII region small bright knot Complex of activity in Orion. the interstellar medium: A medium that is dusty, all pervasive in the Milky Way, with component temperatures that vary from 20K to 10,000K and hotter, and densities from 1 atom per cm3 to 100,000 atoms per cm3 . From this material are the stars born, and to this medium evolved stars return matter. 2 November 9, 2003