1 / 11

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”

raine
Télécharger la présentation

Ch 10: Interstellar Medium

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript


  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%)

More Related