Lecture12 - Interstellar medium Galaxies contain a lot of...

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Interstellar medium Galaxies contain a lot of gas and dust, in addition to stars In galaxies like the Milky Way, most of this material is found in the disk, with the youngest stars The ISM is composed mostly of hydrogen, in molecular, neutral and ionized form ¾ Simple molecules: CO, NH 3 , H 2 O, HCN, OH ¾ Neutral elements: CI, CaI, NaI ¾ Ions: CII, CIII, NII, OII, OIII, OVI, SII, SIII ¾ Cosmic rays (energetic protons, electrons and nuclei) ¾ Magnetic fields (10-30 mGauss) ¾ Dust (solid component) ± Grain sizes: 1nm-10 mm (i.e. similar to visible light); masses 10 -21 -10 -18 kg ± Average mass density of 10 -23 kg/m 3 ± Graphite, SiC, silicates, H 2 , H 2 O Molecular clouds have T~15-50 K and can have masses up to ~10 6 M sun . Dense cloud cores are the site of star formation If a cloud has enough mass, gravity can win out over pressure, and it will collapse.
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Star formation Large, slowly rotating gas clouds begin to collapse ¾ form small, dense regions which continue to grow as long as the internal pressure is less than the gravitational energy of the infalling material ¾ The core heats up, and forms a rapidly rotating, thin disk Outer regions of the disk are less dense and opaque ¾ Thus the outer disk is cooler than the inner disk As protostar gets denser, it cannot radiate away
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Lecture12 - Interstellar medium Galaxies contain a lot of...

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