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Ch 10: Interstellar Medium

Ch 10: Interstellar Medium. Space between stars is not perfect vacuum : very thin gas  ISM (interstellar medium). <n> ~ 1 atom cm -3 lung-full air  500 km cube best human vacuum ~10 12 cm -3 very patchy (clouds+diffuse). ~5% mass of stars . galactic “atmosphere”

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Ch 10: Interstellar Medium

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  1. Ch 10: Interstellar Medium • Space between stars is not perfect vacuum : • very thin gas  ISM (interstellar medium) • <n> ~ 1 atom cm-3 • lung-full air  500 km cube • best human vacuum ~1012 cm-3 • very patchy (clouds+diffuse) • ~5% mass of stars • galactic “atmosphere” • held in place by gravity Full sky - optical

  2. (2) Composition • Usual (cosmic) abundances : 74% H, 24% He, 2% others • Gas : ionized / atomic / molecular all exist • Dust : smoke / smog particles, • size ~1µm, <n> ~ 1 km-3, ~1% of ISM by mass • C, SiO4 , Fe core + water/methane ice mantle

  3. (3) Extinction & Reddening • Extinction : • Dust absorbs & scatters light • ~90% is lost crossing ~1 kpc • distant stars appear dimmer • (cf. smog in LA). IR optical • Reddening : • 95% blue lost ; 85% red lost •  Distant stars appear redder • (cf. setting sun) • Scattering : • Dust scatters blue light more • (cf. blue sky)

  4. (3b) Earth’s examples • Sunset: blue light scattered away • Sky: blue light scattered to us

  5. 220nm feature Graphite “soot” (3c) Dust/gas absorption • extinction curve depends • on dust size & composition • Increases to blue & UV •  objects look redder • spectra of bright distant stars • see weak/narrow absn. lines •  ISM atoms absorbs light • use to analyze ISM properties

  6. (4) Three kinds of nebulae (clouds) • Dark : dust blocks background • Reflection : scatters starlight • – appears blue • Emission : • ionized gas, emission lines • pink (Balmer lines)

  7. (5) Other wavelengths • Infra-red (50 – 100µm) • cold (≤ 100K) dust • warmed by starlight •  thermal emission • e.g. IR “cirrus” • X-ray ~106 K hot gas • from supernova explosions • thermal (& line) emission

  8. (5b) Radio emission • molecular : • vibration & rotation (MHz-GHz frequencies) • e.g. CO, CN, OH, H2O……. CH3CH2OH …. • Note : most common H2 can’t emit, but CO good tracer • (homo/hetero nuclear  can’t/can emit) • 21cm atomic H : electron spin-flip; extremely valuable

  9. (5c) Radio examples 21 cm neutral Hydrogen all sky map long λ  see through dust thin galactic plane Doppler shifts  velocity Full sky – 21 cm optical CO Orion, seen in optical & in carbon monoxide CO line at 1.8 cm. much near “Orion Nebula” star formation region

  10. (6) ISM is inhomogeneous Cloudy/filamentary; in constant motion 4 components in rough pressure balance since Pgas = density x temp  get nT pairs  n↑ T↓

  11. (7) ISM ↔ star cycle ISM plays important role in star life cycle “soil” from which stars grow/arise from and die/return to • Stars born within DMCs, from collapse of dense gas •  dark nebulae • O/B stars ionize remaining gas •  emission & reflection nebulae • Star winds + supernova explosions disperse gas • Star winds + death return gas & dust back to ISM • Galactic rotation + gravity + cooling makes new DMCs • Cycle repeats • Heavy elements gradually build up (currently ~2%)

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