Download The Interstellar Medium (ISM) The Dust The Gas: a. The Hot

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)
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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.
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