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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

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 composition
(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 extinction reddening
(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)
3b earth s examples
(3b) Earth’s examples
  • Sunset: blue light scattered away
  • Sky: blue light scattered to us
3c dust gas absorption

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
4 three kinds of nebulae clouds
(4) Three kinds of nebulae (clouds)
  • Dark : dust blocks background
  • Reflection : scatters starlight
    • – appears blue
  • Emission :
    • ionized gas, emission lines
    • pink (Balmer lines)
5 other wavelengths
(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
5b radio emission
(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
5c radio examples
(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

6 ism is inhomogeneous
(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↓

7 ism star cycle
(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%)